Migratory Behaviors in Masu Salmon (Oncorhynchus masou) and the Influence of Endocrinological Factors

Arimune Munakata

Department of Biology, Miyagi University of Education, Aoba-ku, Sendai, Miyagi 980-0845, Japan

Abstract

In the freshwater phase of their lifecycle, masu salmon (Oncorhynchus masou) comprise two different phenotypes. A portion of the juveniles (migratory form) exhibit downstream migratory behavior after smoltification. However, some masu salmon (non-migratory form) such as precociously mature males live continuously in their natal rivers throughout their lifetime. The coexistence of migratory and non-migratory forms within the species indicates that this salmon can be effectively used as a model fish to illuminate both inhibitory and stimulatory physiological control mechanisms of migratory behaviors. In masu salmon, it was found that sex steroid hormones inhibit the occurrence of downstream swimming behavior, the initial step in seaward migration. Moreover, after the commencement of downstream migration, sex steroid hormones induced the upstream swimming and subsequent spawning behaviors. These findings indicate that sex steroid hormones influence the occurrence of the downstream and upstream swimming behavior in the resulting rheotaxis fashion (negative and positive, respectively). In contrast to sex steroid hormones, it was also found that cortisol, which is involved substantially in smoltification, stimulates the downstream swimming behavior. These findings indicate that the occurrence of seaward migration is controlled competitively by sex steroid hormones (sexual maturation) and smolt-inducing factors such as cortisol, in masu salmon and potentially other Pacific salmon.

Keywords

cortisol, downstream migration, masu salmon, Oncorhynchus masou, Pacific salmon, sex steroid hormone, spawning, testosterone, upstream migration


Received on April 1, 2011

Accepted on September 22, 2011

Online published on November 20, 2012

e-mail: munakata@staff.miyakyo-u.ac.jp


1. Introduction

Salmonids (family Salmonidae) consist of four genera, Hucho, Salvelinus, Salmo, and Oncorhynchus (Neave 1958; Norden 1961; Murata et al. 1993). These salmonids originally inhabited tributaries from high-through mid-latitude areas in the northern hemisphere (Quinn 2005). It is also known that a large part of salmonids (in quantity: the number of species and biomass) are anadromous, and these fish (i.e., migratory form) regularly exhibit downstream migratory behavior from the rivers to the sea (or lakes), after the occurrence of parr to smolt transformation (i.e., smoltification) (Fig. 1). However, many species capable of anadromy also have phenotypes that are full-time residents of freshwater habitats (i.e., non-migratory forms) and display neither smoltification nor downstream migratory behavior to the sea (Munakata and Kobayashi 2010). Most of the non-migratory forms will live continuously in their natal rivers throughout their lives (Fig. 1). Regardless of these life history types, most salmonids will spawn in freshwater environments, mainly in their natal rivers (Fig. 1) (Quinn 2005). From these phenomena, salmonids are considered to be of freshwater (fluvial) fish origin and their migratory behaviors by and large start from the rivers (Fig. 1).


Fig. 1. Schematic drawing that illustrates the diversity of distance covered by non-migratory and migratory forms for four salmonid genera (shown in the order of the evolutional age). In genus Hucho, most fish live continuously in their natal rivers. In genus Salvelinus and Salmo, some fish migrate to the sea after smoltification. In genus Oncorhynchus, most juveniles perform long distance seaward migration for several years. On the other hand, in masu salmon (O. masou), a portion of the fish perform seaward migration for a year after smoltification, while an equivalent portion of them stay in the rivers similar to genera Hucho, Salvelinus, and Salmo.

page top


Among the four genera of salmonids, two genera Hucho and Salvelinus are considered evolutionally ancient groups, based on the phylogenic analyses (Norden 1961; Murata et al. 1993). Genus Hucho inhabit only the northern Eurasia continent and genus Salvelinus inhabit northern Eurasia and the American continent (Quin 2005). On the other hand, genus Salmo (i.e., Atlantic salmon) and Oncorhynchus (i.e., Pacific salmon), which are considered evolutionally new groups, are widely distributed in the rivers and tributaries around the north Atlantic and Pacific Oceans, respectively (Groot and Margolis 1991).

In regard to migratory behaviors, the majority of fish in the genus Hucho and Salvelinus live continuously in their natal river systems throughout their lifetime, as non-migratory forms (Fig. 1). If at all existent, the proportions of the migratory forms are much smaller, and their temporal and spatial ranges of migratory movements are shorter and narrower, respectively, than those in other salmonids such as Atlantic and Pacific salmon. On the other hand, in Pacific salmon such as pink (O. gorbuscha) and chum (O. keta) salmon, which are considered evolutionally the newest species, most juveniles undergo long distance seaward migration (e.g., from Japanese streams to the Bering Sea) which will continue for several years (Groot and Margolis 1991). Their temporal and spatial ranges of migratory movements are considerably longer and broader than in other salmonid species. Based on these wide differences in migratory patterns among salmonid genera from different evolutional time periods, it is inferred that the proportions of migratory forms increased, and subsequently the temporal and spatial ranges of migration became longer and broader, respectively, through the evolutionary processes (Fig. 1).

In masu salmon (O. masou) (Fig. 2), a Pacific salmon that mainly inhabits Japanese rivers (i.e., western Pacific Ocean), some yearling (1+) fish live continuously in their natal rivers similar to the ancient salmonid genera including genus Hucho and Salvelinus (Machidori and Kato 1984; Kato 1991; Kiso 1995) (Figs. 1, 3). In masu salmon, however, a portion of the 1+ juveniles exhibit downstream migratory behavior after the occurrence of smoltification, as do other Pacific salmon such as pink and chum salmon. In masu salmon, such differentiations in lifecycles regularly occur within the same population from the same rivers, especially in the northern regions of their habitat (e.g., northern Honshu through Hokkaido) (Machidori and Kato 1984; Kato 1991; Kiso 1995).


Fig. 2. Photographs of masu salmon (Oncorhynchus masou). (a) precocious male non-migrants, (b) immature parr non-migrants, (c) pseudo smolt, (d) smolt migrants, and (e) adult smolt migrants that migrated back from the sea.

page top


Taking the lifecycles of masu and other salmonids into consideration, the proportion of non-migratory and migratory forms seem to vary, not only among different salmonid genera, but also within the same genus (e.g., Pacific salmon). Since both the non-migratory and migratory forms appear within the same species, it is hypothesized that the masu salmon possesses both evolutionarily ancestral (i.e., fluvial) and modern (i.e., anadromous) characteristics of migratory behaviors. In Japanese streams, the non-migratory form of masu salmon is called "yamame" meaning mountain girl, and the representative migratory form is called "sakura-masu", meaning cherry blossoms. Why do only a portion of masu salmon juveniles exhibit the ocean-bound migratory behaviors, whereas the rest do not?

In this monograph, an overview of the migratory behaviors, especially the downstream and upstream migratory behaviors, and subsequent spawning behaviors in masu salmon will be presented. Additionally, a theory of hormonal control as a mechanism governing downstream (negative rheotaxis) and upstream (positive rheotaxis) swimming behaviors, major components of downstream and upstream migratory behaviors, and subsequent spawning behaviors in masu and other Pacific and Atlantic salmon will also be presented. Since both non-migratory and migratory forms appear within the same population inhabiting the same river, it was hypothesized that individual masu salmon physiologically control the inhibitory and stimulatory mechanisms of migratory behaviors (Munakata and Kobayashi 2010).

In masu salmon, it is generally known that the non-migratory forms are the precociously mature fish (i.e., precocious males) (Machidori and Kato 1984; Kiso 1995). On the other hand, most of migratory forms are sexually immature male and female smolts, as observed in other Pacific salmon (Quinn 2005). These phenomena thus indicated that "sexual maturation" is one of the key physiological factors that regulate the occurrence of seaward migration. Furthermore, since most of the downstream migrants undergo smoltification before their seaward migration, it was hypothesized that some physiological factors which are closely related to the smoltification stimulatory regulate the occurrence of downstream migratory behavior (Munakata et al. 2007). Thus, previous studies investigated both inhibitory and stimulatory control mechanisms of migratory behaviors in relation to sexual maturation (sex hormones) and smoltification, respectively (Munakata et al. 2000b, 2001a, 2001b, 2007, 2012a, 2012b). New information will be used to reconsider the physiological control mechanisms, roles, and evolutionary processes of the migratory behaviors in masu salmon, and perhaps in entire salmonids. Furthermore, this analysis incorporates not only physiological factors, but also environmental factors that influence the migratory behaviors. It is therefore suggested that the findings have important implications. Especially, these data can serve as new tools for improving our salmon stock-management, focusing specifically on the conservation of their migratory behavior.

page top


2. Roles of sex steroid hormones in the regulation of downstream swimming behavior in masu salmon and other salmonids

2-1. Definition of migratory behaviors in salmonids

Migratory behavior of salmonids regularly consists of downstream migratory behavior (downstream migration) from the river to the sea (or lakes), feeding migratory behavior (feeding migration) in the sea (or lakes), homing migratory behavior (homing migration) from the sea (or lakes) to the mouth of their natal rivers, upstream migratory behavior (upstream migration) from the mouth to the spawning ground in upper reaches in the natal rivers, and spawning behaviors (Munakata and Kobayashi 2010) (see Fig. 3). The downstream migratory behavior, the initial step of seaward migration mainly consists of several specific behaviors, such as schooling behavior, downstream swimming behavior (negative rheotaxis, downstream movement), and salinity preference (seawater adaptation) (Iwata 1995, 1996; Munakata and Kobayashi 2010). Also, the upstream migratory behavior consists of several types of behaviors, such as upstream swimming behavior (positive rheotaxis, upstream movement), and freshwater preference. Among these phenomena, downstream and upstream swimming behaviors, major components of downstream and upstream migratory behaviors, can be observed in an artificial raceway system (see Fig. 5) during their natural downstream and upstream migratory periods (Munakata et al. 2000b, 2001a, 2001b, 2007, 2012a, 2012b). In this monograph, therefore, we mainly investigated the effects of endocrinological (hormonal) factors on the occurrence of downstream and upstream swimming behaviors in the raceway system.


Fig. 3. Lifecycles of masu salmon (Oncorhynchus masou). In masu salmon, some immature juveniles (migratory form) display the downstream migratory behavior after they have transformed from parr to smolt (smoltification). However, some juveniles (non-migratory form) such as precociously mature males (precocious males) will live continuously in their natal rivers throughout their lifetime. The lifecycle (migratory behavior, seaward migration) of migrants consists of downstream migration, feeding, homing, upstream migration, and spawning. On the other hand, the lifecycle of non-migrants consists of downstream movement within a river, stream residence, upstream movement, and spawning.

page top


2-2. Lifecycle of masu salmon

Masu salmon, a Pacific salmon, is broadly distributed in north western Pacific-rim rivers (Kamchatka Peninsula through Kyushu Island) (Machidori and Kato 1984). In addition, several sub-species or sub-types of masu salmon are found in this region: amago salmon (O. masou ishilawae) inhabit streams in southwestern Japan (e.g., southern Honshu, part of Kyushu, and the Shikoku Islands) (Kato 1991); Biwa salmon (O. masou rhodurus) inhabit the tributaries around Lake Biwa (Fujioka et al. 1990); Taiwan salmon (O. masou formosanus), an endangered sub-species, inhabit limited highland rivers of Taiwan (Oshima 1936); and a hybrid type of the Honmasu salmon (O. masu x rhodurus) inhabit tributaries around Lake Chuzenji (Munakata et al. 1999, 2000a). During the spring, yearling (1+) masu salmon juveniles can be classified into two groups, migratory and non-migratory forms. As mentioned above, the representative migratory form that lives in the rivers for 1.5 years regularly begins the seaward migration following the occurrence of smoltification (Kato 1991). On the other hand, representative non-migratory forms such as 1+ precocious males live continuously in their natal rivers throughout their lifetime (Utoh 1976, 1977). In this section, the lifecycles of masu salmon from hatching to the period in which the smolt migrants exhibit downstream migratory behavior will be summarized, with emphasis on environmental factors which induce either the stream residency in non-migrants or smoltification in migrants. Then, an overview of previous and recent studies that have investigated the inhibitory roles of sex steroid hormones, such as testosterone (T), 11-ketotestosterone (11-KT), and estradiol-17β (E2) in the occurrence of smoltification and subsequent downstream swimming behavior in masu and other salmonids will be outlined.

2-3. Early growth after emerging

After emerging from spawning beds (common name: redd) which are located in the main stem or tributaries in upper rivers, underyearling (0+) masu salmon juveniles (3 cm in standard body length) are typically found in shallow areas (e.g., behind large stones, under fallen trees, between roots or stems of emergent plants, etc.) where the flow rate is moderate (Machidori and Kato 1984; Kato 1991). These 0+ juveniles then gradually move to deeper areas, such as the edge or center of the flow in the pools (Kiso 1995). The 0+ juveniles, after emergence, are called "parr", since these fish display large black round spots (i.e., parr marks) on both sides of their body. The parr mark is considered to allow them to be better camouflaged against the background of the rivers, that is, stream beds, rocks, and fallen trees.

The 0+ masu salmon parr mainly forages on drift aquatic insects such as larval Ephemeroptera, Trichoptera, Plecoptera, Chironomidae, and occasionally forages on fallen terrestrial insects (Machidori and Kato 1984; Kato 1991; Kiso 1995). In upper rivers, however, distributions of drifting aquatic and terrestrial insects are generally stratified among the different spaces. This suggests that accessibility to prey items differs considerably among individual 0+ parr. For these reasons, 0+ parr compete frequently with their conspecifics as well as other species (i.e., Japanese charr (Salvelinus fontinalis)) with the same dietary habits for occupying focal foraging areas (i.e., territory) in which they can access substantial drifting prey items (Nakano et al. 1990; Nakano and Furukawa-Tanaka 1994; Nakano 1995). Moreover, to achieve and maintain their focal foraging areas, 0+ parr frequently exhibit territorial aggressiveness against other individuals, and some of the territorial 0+ parr establish themselves to be the dominant fish in each focal foraging space during autumn (Machidori and Kato 1984; Munakata et al. 2000b). On the other hand, juveniles that could not occupy the focal foraging areas become the subordinates in the social order.

2-3A. Wintering downstream movement

During the same period (autumn through winter), a significant proportion of 0+ masu salmon, including non-migrants and migrants, tend to move from their former habitat to the downstream areas (Machidori and Kato 1984; Kato 1991), which is possibly induced by decreased temperature (Giannico and Hinch 2003). These movements, which are called "wintering behavior", result in the augmentation for distribution of masu salmon into broader habitats within the rivers.

2-4. Differentiation in non-migratory forms

In juvenile masu salmon, both non-migratory (i.e., stream resident) and migratory (i.e., smolt) forms commonly originate from dominant and subordinate parr, respectively, and both types can be discriminated by their morphologic characteristics after 0+ summer (Machidori and Kato 1984; Kiso and Matsumiya 1992; Kiso 1995). In regard to the non-migratory forms, the 0+ dominant precocious males regularly become the non-migrants that live continuously in their natal rivers. In 0+ precocious males, standard body length (BL), body weight (BW), depth of body, and testes (gonad weight (GW)) become larger, and the body color becomes darker than that of 0+ immature parr during summer, and these fish subsequently attend to spawning activities in the following autumn (Kato 1991; Kiso 1995). After spawning, testes in 1+ (former 0+) precocious males regress from winter through spring (Munakata et al. 2001a; Fig. 4). However, the size of the testes, expressed by the gonad somatic index (GSI: 100 x GW/BW), in 1+ precocious males is still larger than those of immature male parr (Munakata et al. 2001a; Fig. 4). The 1+ precocious males then begin the maturation process again after spring commences, while most of 1+ immature migrants exhibit downstream migratory behavior following smoltification.


Fig. 4. Changes in body length (BL), body weight (BW), condition factor (CF), gonad somatic index (GSI), plasma levels of testosterone (T), 11-ketotestosterone (11-KT), estradiol-17β (E2), progesterone (P), 17α-progesterone (17α-P), 17,20β-dihydroxy-4-pregnene-3-one (DHP), and pituitary hormone luteinizing hormone (LH) in male and female masu salmon. Differences in mean plasma hormone levels among experimental groups were analyzed by one-way analysis of variance (ANOVA) followed by Scheffe's F-test. * and *** indicates significant difference at P < 0.05 and P < 0.001, respectively. Reprinted from Comp. Biochem. Physiol. Part B, 129, Munakata et al., The involvement of sex steroid hormones in downstream and upstream migratory behavior of masu salmon, 661–669, © 2001a, with permission from Elsevier.

page top


2-5. Smoltification in subordinate juveniles

In contrast to the dominant precocious male non-migrants, most of 0+ subordinates live as immature parr from summer through autumn (Machidori and Kato 1984; Kiso 1990, 1995). In the following winter, some of the 1+ immature fish initiate smoltification, which is completed in the following spring, before the downstream migration begins.

Smoltification regularly consists of a series of morphological, physiological, and behavioral changes, which enable 1+ (former 0+) juveniles to adapt to marine environments (Hoar 1976, 1988; Boeuf 1994; Boeuf et al. 1994; Iwata 1995, 1996; McCormick 2001). In a short span of time, future smolts start to display a silvery body color, and black dorsal and dorsal fin tips, which camouflage them against the colors of the ocean waters (Quinn 2005), similar to other marine migratory fish such as sardines (Engraulis spp.) and saury (Cololabis spp.). The changes in the body colors are supported fundamentally by the accumulation of granules of pigments such as guanine and melanophores on the abdominal and dorsal skins, respectively (Hoar 1988). Masu salmon smolts also display lower condition factor (CF: 100 x BW/BL3) when compared with those values before starting smoltification (Aida et al. 1984; Ikuta et al. 1985, 1987; Munakata et al. 2000b, 2001a). Their osmoregulatory ability is modulated by hormonal factors, such as growth hormone (GH) and cortisol (Hirano 1991; McCormick 2001).

Physiological changes, such as an increase in gill Na+–K+–ATPase activity allow the smolts to adapt to salt water conditions (Boeuf et al. 1989; Iwata et al. 1990).

With regard to behavior, Iwata (1995) suggested that 1+ masu salmon smolts cease to exhibit aggressive behaviors, which support their territorial aggressiveness prior to the smolting period, and these smolts subsequently exhibit a tendency to gather in open spaces even during the daytime hours. According to Hutchison and Iwata (1998), such behavioral changes are caused by the increase of plasma thyroxine, which is considered to be one of the smolt-inducing factors. These behavioral changes seemed to convert the "territorial behavior" into "schooling behavior" in 1+ smolts, as the peak period of their smoltification approached. Subsequently, most 1+ smolts begin downstream swimming behavior throughout the evening (Munakata et al. 2000b), during and after rainfall (Yamauchi et al. 1984, 1985), and snow runoff, etc. in favorable periods during the spring (Iwata 1995, 1996).

In summary, two phenotypes (forms) of masu salmon diverge as juveniles at the age of 1+. The precocious males retain many characteristics of parr, despite undergoing sexual maturation (Utoh 1976, 1977; Machidori and Kato 1984). In contrast, 1+ smolts experience drastic morphological, physiological, and behavioral changes during smoltification (Iwata 1995, 1996). Based on these observations, one might argue that the precocious males are more similar to their fluvial ancestors than smolts. In the next section, we will discuss how sex hormones are involved in the differentiation between the two forms and identify the likely factors regulating these processes.

2-6. Inhibitory roles of sex steroid hormones in the smoltification

In masu salmon, it was previously found that dissection of the testes (castration) of 0+ precocious males, the non-migrants, during autumn induced smoltification in the following (1+) spring, while sham-operated fish remained as precocious males (Aida et al. 1984). Plasma androgen (T + 11-KT) levels (0.12 ng/ml) of 1+ castrated precocious males became lower than those in sham-operated precocious males (1 ng/ml) after surgery. If a portion of the testis was left in the abdominal cavity, those males did not smoltify, just as the sham-operated ones did not. Accordingly, the findings indicate that sexual maturation, more specifically, sex steroid hormones released from the gonads inhibited the occurrence of smoltification in the precocious males. Ikuta et al. (1985, 1987) later confirmed that treatment (oral administration) with exogenous sex steroid hormones, such as methyletestosterone (MT), T, 11-KT, and E2 in 1+ masu salmon smolts in winter through spring impaired some part of the changes associated with smoltification, such as silvery body color, decrease in CF, seawater tolerance capacity, and plasma rises in thyroid hormones. On the other hand, synthetic steroid, 5α-dihydrotestosterone (DHT) which has stronger androgenic effects than T did not exhibit significant inhibitory effects (Ikuta et al. 1987).

The inhibitory effects of sex steroid hormones on the smoltification are also confirmed in other Pacific salmon such as amago salmon (Miwa and Inui 1986). Therefore, it is further hypothesized that inhibitory regulation of smoltification by sex steroid hormones is a common phenomena in a number of Pacific salmon.

2-7. Seasonal changes in plasma sex hormone levels

After the completion of smoltification, most masu and other Pacific salmon smolts subsequently migrate downstream to the sea (Quinn 2005; Munakata and Kobayashi 2010). Therefore, by logical extension, not only the smoltification, but also the downstream migratory behavior is repressed by some of the sex steroid hormones. To investigate which sex steroid hormones are indeed involved in the occurrence of smoltification and downstream swimming behavior, seasonal changes in the plasma levels of sex steroid hormones and pituitary hormone (luteinizing hormone (LH)) were investigated in masu salmon by radioimmunoassays (RIAs), during the period of smoltification and downstream migration (Munakata et al. 2001a; Fig. 4).

2-7A. Males

In 0+ and 1+ males, precocious males (representative non-migrants) appeared and were distinguishable from the immature males of the same age by their larger BL, BW, CF, and GSI values and plasma sex steroid hormone levels (Fig. 4). In 1+ precocious males, values of GSI and plasma levels of T, 11-KT, and 17,20β-dihydroxy-4-pregnene-3-one (DHP) significantly increased from May through September, overlapping the period of the smoltification and downstream migration, while 1+ immature males (smolt and parr) did not exhibit such phenomena (Fig. 4). On the other hand, plasma levels of progesterone (P), 17α-progesterone (17α-P), and LH did not show significant increases in 1+ precocious males.

In 1+ precocious males, it was noteworthy that plasma levels of T began to increase and attained peak levels earlier than did 11-KT and DHP (Fig. 4). Moreover, T maintained high plasma levels extensively from March through September, overlapping the period of their seaward migration.

2-7B. Females

In females, precocious maturation rarely occurred among 0+ and 1+ fish in hatchery-raised strains (Shiribetsu River strain, introduced from Hokkaido in 1980) that were used in our studies (Munakata et al. 2000b, 2001a, 2001b, 2007, 2012a). Consequently, a major part of 1+ immature females exhibited low CF, GSI, and plasma sex hormone levels (Fig. 4).

The females regularly commence apparent sexual maturation at the age of 2+ during the spring through summer period, which coincides with the timing of their upstream migration (Fig. 4). Their GSI and plasma levels of T, E2, 17α-P, DHP, and LH increased in May through October. In addition, most females used in the investigation ovulated around October, similar to the wild populations (Kiso 1995). In 2+ mature females, plasma levels of T and E2 began to increase and attained peak levels earlier than did 17α-P, DHP, and LH (Fig. 4). Moreover, T maintained high plasma levels during a broader period than other sex steroid hormones.

Therefore, in males and females, it seems likely that sex hormones, especially some of the sex steroid hormones, are important factors that regulate (inhibit) the display of downstream swimming behavior as well as smoltification. Furthermore, it was indicated that the sex steroid hormone T commonly increased earlier and more broadly than did other sex steroid hormones in both sexes (Fig. 4). In general, T is considered a reservoir substance for impending conversion to other sex steroid hormones, such as 11-KT, and E2 (male), and E2 (female) (Kagawa et al. 1982a, b). In masu salmon, however, it was also hypothesized that T is one of the more important sex steroid hormones that effectively repress the occurrence of smoltification and the following downstream swimming behavior. This hypothesis was tested in the next set of studies which were conducted in an artificial raceway system (Fig. 5). T and other relevant sex steroid hormones were included in these experiments.


Fig. 5. Schematic drawing of experimental raceway. In order to study the roles of sex steroid hormones in downstream behavior (negative rheotaxis), the fish were transferred into the upper pond (2 × 4 × 0.5 m) of a two-step raceway connected to the lower pond (2 × 8 × 0.5 m) through a fishway (20 cm in diameter by 4 m in length made of a polyvinyl chloride (PVC) half-cut pipe (Munakata et al. 2000b). Spring water was supplied into the upper pond. Flow rate (volume) and velocity of the water in the fishway ranged between 10–20 l/s and 70–85 cm/s, respectively. Water temperature fluctuated between 9–10°C. At the downstream edge of the fishway in the lower pond, a net trap (2 × 0.7 × 0.7 m) was placed to capture fish that moved down from the upper pond. An individual experimental fish was identified as a downstream migrant if it moved from the upper pond into the net trap in the lower pond. In order to investigate the effects of sex steroid hormones on the occurrence of upstream behavior, the experimental fish and net trap were transferred into the lower and upper pond, respectively. The frequency of downstream or upstream migrations is expressed as a percentage of the initial fish numbers. Reprinted with permission from Fish. Sci., 78, Munakata et al., Involvement of sex steroids, luteinizing hormone and thyroid hormones in upstream and downstream migratory behaviors in land-locked sockeye salmon Oncorhynchus nerka, 81–90, Fig. 1, © 2012b, The Japanese Society of Fisheries Science.

page top


2-8. Inhibitory roles of sex steroid hormones in the downstream swimming behavior in masu salmon

I investigated the effects of treatments with exogenous sex steroid hormones, such as T, 11-KT, E2, and DHP, on the occurrence of downstream swimming behavior, in 1+ masu salmon smolts, held in the experimental raceway system (Fig. 5).

2-8A. Roles of T in the downstream swimming behavior

In the raceway, one experiment showed that 89.5% (17 of 19 fish) of the control 1+ masu salmon smolts swam from the upper pond to the lower pond through a fishway (Munakata et al. 2000b; Fig. 6). This contrasts with the behavior of 1+ smolts into which a T 500 μg/fish via a Silastic tube capsule (Dow Corning Corp.: outer diameter 1.95 mm, inner diameter 1.47 mm, length 20 mm) was inserted. These 1+ smolts displayed high plasma T levels (Fig. 6). Under these circumstances, the frequency of downstream swimming behavior in the T 500 μg/fish-treated group was 31.8% (7 of 22 fish). In the non-migrants of the T-treated fish, plasma levels of T were higher than those in migrants, suggesting that higher plasma T levels are important for the suppression of downstream swimming behavior. Since Ikuta et al. (1987) demonstrated that T inhibited natural smoltification, it is further hypothesized that T impairs not only downstream swimming behavior but also seawater preference and schooling behavior in the T-treated 1+ smolts.


Fig. 6. (a) Frequency of migrants and non-migrants, plasma levels of (b) testosterone (T), (c) pituitary contents of luteinizing hormone (LH), (d) plasma levels of LH, (e) thyroxine (T4), and (f) triiodothyronine (T3) in control and T 500 μg-treated 1+ masu salmon smolts. Numbers above columns in (a) indicate the number of migrants and non-migrants. Differences in the frequency of downstream behavior from the control group were analyzed by the χ2-test, using StatView version 4.5 software (Abacus Concepts, Inc., California, USA). ** indicates a significant difference at P < 0.01 from the control group. Differences in mean plasma and pituitary hormone levels among experimental groups were analyzed by one-way analysis of variance (ANOVA) followed by Fisher's PLSD. Differing letters represent significant differences at P < 0.05 among all groups. Reprinted with permission from Zoological Science, 17, Munakata et al., Inhibitory effects of testosterone on downstream migratory behavior in masu salmon, Oncorhynchus masou, 863–870, Fig. 1, © 2000b, Zoological Society of Japan.

page top


2-8B. Effects of T doses on the occurrence of downstream swimming behavior

It was previously demonstrated that T-treatments significantly inhibited the occurrence of downstream swimming behavior in 1+ smolts in a dose dependent manner (Munakata et al. 2000b; Fig. 7). The frequency of downstream swimming behavior in control, T5 μg, T50 μg, T500 μg/fish-treated 1+ smolts, and 1+ precocious male groups were 21.3, 18.2, 6.9, 4.5, and 0%, respectively. Plasma T levels and pituitary LH contents in the T500 μg/fish-treated smolt group were highest among all experimental groups (Fig. 7).


Fig. 7. (a) Frequency of migrants and non-migrants, plasma levels of (b) testosterone (T), (c) pituitary contents of luteinizing hormone (LH), (d) plasma levels of LH, (e) thyroxine (T4), and (f) triiodothyronine (T3) in controls, T 5 μg, T 50 μg, T 500 μg-treated smolts and precociously mature male 1+ masu salmon. Numbers above columns in (a) indicate the number of migrants and non-migrants. Differences in the frequency of downstream behavior from the control group were analyzed by the χ2-test, using StatView version 4.5 software (Abacus Concepts, Inc., California, USA). * indicates a significant difference at P < 0.05 from the control group. Differences in mean plasma and pituitary hormone levels among experimental groups were analyzed by one-way analysis of variance (ANOVA) followed by Fisher's PLSD. Differing letters represent significant differences at P < 0.05 among all groups. Reprinted with permission from Zoological Science, 17, Munakata et al., Inhibitory effects of testosterone on downstream migratory behavior in masu salmon, Oncorhynchus masou, 863–870, Fig. 2, © 2000b, Zoological Society of Japan.

page top


In this study, on the other hand, none of the 1+ precocious males exhibited the downstream swimming behavior in the artificial raceway, whereas their plasma T levels were lower than those in the T500 μg/fish-treated smolt (Fig. 7). Such phenomena are consistent with the findings that most of the 1+ precocious males stay in their natal rivers even though their plasma T levels are not very high.

2-8C. Roles of sex steroid hormones other than T in the downstream swimming behavior

In maturing masu salmon, not only T but also 11-KT (males) and E2 (females) levels increased coincident with the period of downstream migration (Munakata et al. 2001a; Fig. 4). On the other hand, plasma DHP levels increased only during the spawning period in autumn in both sexes. To test the potential roles of sex steroid hormones other than T, the effects of treatments of T, E2, 11-KT, and DHP (500 μg/fish) on the occurrence of downstream swimming behavior were investigated (Munakata et al. 2001a; Fig. 8).


Fig. 8. (a) Frequency of migrants and non-migrants, plasma levels of (b) testosterone (T), (c) estradiol-17β(E2), (d) 11-ketotestosterone (11-KT), (e) 17,20β-dihydroxy-4-pregnene-3-one (DHP), (f) thyroxine (T4), and (g) triiodothyronine (T3) in controls, T, E2, 11-KT, and DHP 500 μg-treated 1+ masu salmon smolts. Numbers above columns in (a) indicate the number of migrants and non-migrants. Differences in the frequency of downstream behavior from the control group were analyzed by the χ2-test, using StatView version 4.5 software (Abacus Concepts, Inc., California, USA). * and *** indicate significant differences at P < 0.05 and P < 0.001, respectively, from the control group. Differences in mean plasma and hormone levels among experimental groups were analyzed by one-way analysis of variance (ANOVA) followed by Fisher's PLSD. Differing letters represent significant differences at P < 0.05 among all groups. Reprinted from Comp. Biochem. Physiol. Part B, 129, Munakata et al., The involvement of sex steroid hormones in downstream and upstream migratory behavior of masu salmon, 661–669, © 2001a, with permission from Elsevier.

page top


It was found that not only T but also E2 and 11-KT 500 μg/fish-treatments resulted in an elevation of each sex steroid hormone and inhibited the occurrence of downstream swimming behavior in 1+ smolts (Fig. 8). Interestingly, DHP 500 μg/fish-treatments did not inhibit the occurrence of downstream swimming behavior.

In the raceway system, it was also demonstrated that all of the forty 1+ masu salmon smolts that were transferred into the upper pond performed downstream swimming behavior within a week (Munakata et al. 2000b; Fig. 9, left columns). By comparison, 60% of T500 μg/fish-treated smolts remained in the upper pond during the same period (Fig. 9, right columns). These phenomena suggest that most 1+ smolts spontaneously exhibit negative rheophilic behavior when flowing water is present, or that some environmental factors are involved in the induction of downstream swimming behavior. Since 1+ smolts seemed to swim downstream spontaneously, it is also hypothesized that T directly inhibits the downstream activity, or inhibits the receptiveness to some environmental factors which induce the downstream swimming behavior.


Fig. 9. (a) Frequency of migrants and non-migrants, plasma levels of (b) testosterone (T), (c) thyroxine (T4), and (d) triiodothyronine (T3) in control and T 500 μg-treated 1+ masu salmon smolts. Numbers above columns in (a) indicate the number of migrants and non-migrants. Differences in mean plasma hormone levels among experimental groups were analyzed by one-way analysis of variance (ANOVA) followed by Fisher's PLSD. * indicates a significant difference at P < 0.05 from migrants. Reprinted with permission from Zoological Science, 17, Munakata et al., Inhibitory effects of testosterone on downstream migratory behavior in masu salmon, Oncorhynchus masou, 863–870, Fig. 3, © 2000b, Zoological Society of Japan.

page top


2-8D. Effects of intra-specific interactions on the downstream swimming behavior

In one of the previous investigations, 1+ precocious male masu salmon, transferred into the upper pond of the raceway, frequently displayed aggressive behaviors towards 1+ smolts (Munakata et al. 2000b, 2012a). Therefore, another acceptable explanation is that the downstream swimming behavior is induced partly by some socio-environmental factors, such as intra-specific interactions from 1+ precocious males.

2-8E. Effects of dosing period of T on the downstream swimming behavior

The dosing period was normally approximately 2 weeks before sex steroid hormone-treated 1+ masu salmon smolts were transferred into the upper pond of the raceway (Munakata et al. 2000b, 2001a). Consequently, some of the fish in the T500 μg/fish-treated 1+ smolts exhibited downstream swimming behavior even when their average plasma T level (13.7 ng/ml) was higher than those of 1+ precocious males (3.9 ng/ml) (Fig. 7). One possible explanation is that multiple sex steroid hormones are necessary to impede downstream swimming behavior. Evidence supporting this supposition is that in 1+ precocious males, plasma levels of not only T but also 11-KT significantly increased (Fig. 4). Moreover, it was demonstrated that treatment with T, E2, and 11-KT 500 μg/fish significantly inhibited the occurrence of downstream swimming behavior in 1+ smolts (Fig. 8).

However, another acceptable explanation is that a continuous release of a sex steroid hormone into the plasma can obstruct the downstream swimming behavior completely. As shown previously (see Subsection 2-4 in detail), 1+ precocious males undergo sexual maturation during the summer of 0+ year-old-life, a half year prior to the downstream migratory period in the river, although their GSI values and plasma androgen levels largely decreased during winter (Aida et al. 1984; Munakata et al. 2000b, 2001a). Considering these factors, it is suggested that chronic release of sex steroid hormones into the plasma is an important factor in the inhibitory regulation of downstream swimming behavior.

2-9. Inhibitory roles of sex steroid hormones in the downstream swimming behavior in other Pacific and Atlantic salmon

Aside from masu salmon, we have discovered that T500 μg/fish-treatments significantly inhibited the occurrence of downstream swimming behavior in 1+ land-locked sockeye salmon (O. nerka) smolts by use of the same raceway (Munakata et al. 2012b, Fig. 10).


Fig. 10. (a) Frequency of migrants and non-migrants, plasma levels of (b) testosterone (T), (c) pituitary contents of luteinizing hormone (LH), (d) plasma levels of LH, (e) thyroxine (T4), and (f) triiodothyronine (T3) in control and T 500 μg-treated smolts, and precociously mature male 1+ sockeye salmon. In Fig. 10c, unit of Y axis in the control group was ng/pituitary, while those of T-treated and precocious male groups was μg/pituitary. Numbers above columns in (a) indicate the number of migrants and non-migrants. Differences in the frequency of downstream behavior from the control group were analyzed by the χ2-test, using StatView version 4.5 software (Abacus Concepts, Inc., California, USA). * indicates a significant difference at P < 0.05 from the control group. Differences in mean plasma and pituitary hormone levels among experimental groups were analyzed by one-way analysis of variance (ANOVA) followed by Fisher's PLSD. Differing letters represent significant differences at P < 0.05 among all groups. Reprinted with permission from Fish. Sci., 78, Munakata et al., Involvement of sex steroids, luteinizing hormone and thyroid hormones in upstream and downstream migratory behaviors in land-locked sockeye salmon Oncorhynchus nerka, 81–90, Fig. 3, © 2012b, The Japanese Society of Fisheries Science.

page top


Furthermore, inhibitory effects of sex steroid hormones upon downstream swimming behavior were also found in Atlantic salmon (Salmo salar) smolts (Berglund et al. 1994). In these smolts, treatment with 11-ketoandrostendione (11-KA) via implantation of Silastic tube capsules inhibited the downstream swimming behavioral activity occurring along the current in the circular round tank. This suggests that inhibitory regulatory mechanisms of downstream swimming behavior by sex steroid hormones are inhered in some Pacific and Atlantic salmon.

page top


3. Roles of sex hormones in the upstream swimming and spawning behaviors in masu salmon and other salmonids

During autumn, most 2+ mature masu salmon sequentially migrate upstream (i.e., upstream swimming behavior) from the sea (or lakes), home to their natal rivers and then spawn in the upper reaches of the catchment basin (Machidori and Kato 1984; Kato 1991; Kiso 1995). Also, during this period, non-migratory, precocious males swim upstream from the mid-to upper-river reaches and consequently spawn with 2+ migrants (see Fig. 3). Since most of these fish exhibit signs of sexual maturation while engaged in these activities, it appears likely that sexual maturation induces or regulates these behaviors. In this section, an overview of the lifecycle of migrants and non-migrants of masu salmon during the periods of homing and upstream migration through spawning will be presented with regard to previous investigations of the stimulatory effects of sex steroid hormones on the occurrence of upstream swimming and spawning behaviors (Munakata et al. 2001a, 2001b, 2002, 2012a).

3-1. Upstream migratory behavior in masu salmon

The feeding migration of 1+ masu salmon smolts in the sea occurs between March and May, after which the major part of the run has entered into either the Pacific Ocean or the Sea of Japan (Machidori and Kato 1984). Most of these smolts are thought to migrate into areas between the Sea of Japan near Hokkaido Island and the Sea of Okhotsk around June. However, it is also thought that a small number of 1+ masu salmon smolts migrate to coastal areas, such as off the Sanriku coast on the Pacific Ocean side of northern Honshu (Kiso 1995). As mentioned in Section 4, such short distance migratory forms are considered to be "coastal migrants" (see Subsection 5-2 in detail). The Sea of Okhotsk is the summer-late autumn feeding ground for most of the 1+ smolts, where they forage on fish, squid, Amphipods, Euphausiids, Decapods, Copepods, and a small number of terrestrial insects (Machidori and Kato 1984; Kato 1991). During this period of time, most masu salmon reach 50 to 60 cm in BL. During winter through spring when the smolt reach the age of 2+, most will head southward towards their spawning ground in natal rivers (i.e., homing migration, upstream migration) (Machidori and Kato 1984; Kato 1991). Values of GSI and plasma levels of sex steroid hormones start to increase in both sexes (Munakata et al. 2001a; Fig. 4). Hence, there is a correlation between the initiation of sexual maturation and the occurrence of homing and upstream migratory behaviors.

In general, most 2+ smolts migrate into their natal rivers and subsequently show upstream swimming behavior during mid spring through early summer. According to information provided by the sports fishing industry, however, it seems that some 2+ masu salmon called the "early run" migrate into their natal rivers (e.g., Kitakami, Mogami, Akagawa, and Omono Rivers) in northern Honshu during late winter through early spring. Also, some of the 2+ smolt migrants, members of the so-called "late run" migrate into small rivers (i.e., Kesen, Hirose, and Natori Rivers) along the Sanriku coast of northeastern Honshu, just before the spawning period (Machidori and Kato 1984; Kiso 1995; Munakata et al. unpublished data). This illustrates that there are variations in timing when the 2+ smolts return to their natal rivers. Moreover, it is also hypothesized that some masu salmon smolts can modulate their osmoregulatory ability coincident with their entry into the natal rivers.

On reaching their natal streams, a significant proportion of masu salmon regularly inhabit deep areas of the rivers, such as pools and the thalweg (center of the flow), usually located downstream from their autumn spawning sites. Because 2+ migrants are now larger (50 to 60 cm in BL), it is probable that they stay in deep areas to hide from potential predators. Furthermore, water temperatures in deeper areas may be modulated and stabilized by spring water upwelling from the river beds. During late summer through autumn, most of the 2+ migrants start to move upstream again, toward their spawning areas (Kato 1991; Kiso 1995; Munakata and Miura, unpublished data). It is thus assumed that the upstream swimming behavior of 2+ migrants can be regularly divided into two steps: 1) movement from the mouth of the river to the areas in which 2+ fish spend the summer, and 2) movement from the latter areas to their spawning areas. It seems that the upstream migratory behavior of the "late run", which is generally found in small streams, coincides with the latter upstream migratory pattern. Most 2+ migrants that reach their spawning sites exhibit high GSI values (Munakata et al. 2001a, 2012a), and subsequently the 2+ males spermiate and 2+ females ovulate (Munakata and Kobayashi 2010).

3-2. Feeding of 2+ masu salmon migrants during the upstream migration

As is typical for semelparous Pacific salmonids, adult masu salmon are thought not to feed after returning to their natal rivers. According to Sano (1947), however, some 2+ masu salmon smolts, which are considered the "early run", in Nishibetsu and Shibetsu Rivers in Hokkaido, occasionally feed. In 2010, it was also discovered that a 2+ masu salmon male migrant caught in the Hirose River, Miyagi Prefecture, that enters Sendai Bay near the Sanriku coast had consumed a number of larval aquatic insects (Munakata and Miura, unpublished data). Interestingly, sport fishermen (i.e., lure, fly, and bait fishing) consistently catch 2+ "early run" smolts in larger rivers, such as the Kitakami, Mogami, Akagawa, Omono Rivers in northern Honshu in late winter through spring, while some fishermen also fish the 2+ masu salon smolts via bait fishing nearshores of Hokkaido coasts.

Thus there are conflicting reports regarding the feeding of the 2+ masu salmon migrants in the rivers. Since most of the 2+ migrants are considered not to feed during summer through autumn, though they seem to feed during winter through spring in both the rivers and offshore sea, one possible hypothesis is that the feeding activity of 2+ migrants is determined by their maturity, but not by the entry into the rivers.

3-3. Lifecycle of non-migratory precocious males

Precocious male non-migrants generally maintain their territories during 0+ winter through 1+ summer in the upper and middle reaches of their natal rivers (Machidori and Kato 1984; Kiso 1995). Subsequently, the 1+ precocious males begin to become sexually competent, resulting in high plasma levels of T and 11-KT after summer (see Fig. 4). The 1+ precocious males then exhibit upstream movements during late summer through autumn the same as 2+ migrants (Kiso 1995; Munakata et al. 2001b).

In contrast to the 2+ migratory smolts, most 1+ precocious males continue to feed on insects or small fish, prior to and during the spawning period (Munakata et al. unpublished data), indicating that the changes in feeding activity do not depend on the maturity of the precocious males. After the occurrence of upstream movements, these fish attend to spawning together with 2+ male and female migrants. The precocious male non-migrants seem to repeat the same phenomena during the ages of 0+ through 2+ (Kiso 1995).

3-4. Spawning behaviors in 2+ migrants

Spawning of masu salmon is observed in natal rivers approximately from August through October (Machidori and Kato 1984; Kiso 1995). The peak period of spawning is generally earlier in northern regions (i.e., Hokkaido) than in southern ones (i.e., Honshu and Kyushu).

During the spawning period, 2+ female migrants start to swim above specific river beds where various sizes of stones and pebbles are located and oxygen rich water indwells, the same as coho salmon (O. kisutch) (Sandercock 1991) (note that pink salmon spawn on river beds where spring water upwells, Heard 1991). Above such a river bed, a 2+ female digs up (i.e., digging behavior) the pebbles and stones to make a spawning bed (i.e., redd: 170 × 80 cm in length and width, with a depth of 12 to 45 cm) by using her body, especially the tail (caudal fin), digging at intervals of 1 to 5 min. While digging, the 2+ female frequently checks the redd's shape (i.e., depth) and substrates by using mainly the pectoral fins (i.e., probing behavior). During and after digging, most 2+ females ovulate to prepare for the oviposition (i.e., egg release) (Munakata and Kobayashi 2010).

Recently, it was discovered that 2+ mature female masu salmon release a sex pheromone, L-kynurenine, which attracts sexually mature males (Yambe et al. 2003, 2006). The urine from 2+ mature females attracts and elicits the male spawning behaviors, such as attending behaviors (see Figs. 17, 18). The timing of its production in the females clearly indicates that this sex pheromone could be a signal to non-specific males indicating sexual maturity, location of the redd, and receptiveness of 2+ females to non-specific males.

After sexually mature 2+ male migrants arrive at the spawning grounds, these males regularly swim around the 2+ digging females and exhibit a series of male spawning behaviors, such as attending and quivering, towards the digging female (Munakata and Kobayashi 2010) (see also Fig. 18). Additionally, 2+ males sometimes exhibit aggressive behaviors towards other male(s) of the same species, to prevent the antagonistic males from showing courtship behavior to the digging female. After the redd is constructed by the female, both the female and male crouch (i.e., crouching behavior) on the accomplished redd, and release eggs (oviposition) and sperm (ejaculation), respectively (Munakata and Kobayashi 2010). Thereafter, the female covers the redd (i.e., covering behavior) with small stones and pebbles by using its caudal fin in a similar manner to the digging behavior. Most of 2+ females and males will repeat such spawning behaviors several times over a few weeks until most ovulated oocytes are released (Machidori and Kato 1984).

3-5. Spawning behaviors in 1+ precocious male non-migrants

Spawning behaviors of precocious male non-migrants are generally different from those in 2+ male migrants (Munakata and Kobayashi 2010). Since the body size of the precocious male non-migrants (10 to 30 cm in BL) is relatively smaller than that of 2+ male migrants (Utoh 1976, 1977), most 0+, 1+, and 2+ precocious males spawn "as sneakers" (Munakata and Kobayashi 2010). Briefly, the sneaker precocious male does not display the specific attending and quivering behaviors towards the nest digging 2+ female. Instead, these males conceal their bodies behind obstacles, such as large rocks, fallen trees, etc., while larger 2+ females and males undertake a series of spawning (pairing) behaviors (Munakata et al. unpublished data). Also, it was observed that some precocious males swim posteriorly to the redd, while 2+ females display digging behavior. These precocious males are called "accessory males". During the spawning period, dominant 2+ male migrants exhibit aggressive behaviors towards other fish including these precocious males. About the time when the 2+ females and males release eggs and sperm, the 0+, 1+, or 2+ precocious male sneaker(s) or accessory males swim onto the redd and immediately release their sperm on the released eggs.

3-6. Spawning behaviors in female non-migrants

Male and female non-migrant masu salmon other than the precocious fish have been reported in some rivers (Kiso 1995). The proportions of these types of non-migrants generally increases towards the southern regions (i.e., Honshu and Kyushu), the same as precocious males. These fish regularly exhibit low GSI values compared with precocious male non-migrants, and their sex steroid hormones remain at low levels. However, their growth rates were relatively higher than those of other immature fish, including the future smolts, during the age of 0+ through 1+ (Kiso 1995). These non-migrants regularly mature and spawn up to twice, during the 1+ and 2+ autumn. Thus 2+ masu salmon spawners are comprised of larger migrants, smaller precocious males and other 2+ parr non-migrants in the rivers. In general, the spawning behaviors of the female and male non-migrants are similar to those of 2+ migrants (Munakata et al. unpublished data).

3-7. Changes in plasma sex hormone levels during the upstream migratory and spawning periods

Thus in masu salmon, the appearance of homing, upstream migratory, and spawning behaviors is closely related to the progress of sexual maturation and an increase in sex hormone levels. To understand which hormonal factors control the occurrence of upstream swimming behaviors and subsequent male and female spawning behaviors, it is required to measure plasma levels of sex steroid hormones and LH before, during, and after the behavioral patterns become manifest. This is covered in the following sections.

3-7A. Males

In 2+ males, values of GSI and plasma levels of T, 11-KT, and DHP increased during the periods of homing migration through spawning (Munakata et al. 2001a; Fig. 4). In 2+ males, moreover, it was noticed that plasma levels of T increased earlier than did 11-KT and DHP, and T retained higher plasma levels extensively during May through September.

Regarding 1+ precocious male non-migrants, such plasma sex steroid hormone elevations coincide with the period when these fish remain in their natal rivers, display upstream movement toward spawning areas, and spawning behaviors (Fig. 4).

3-7B. Females

In 2+ female masu salmon, values of GSI and plasma levels of T, E2, 17α-P, DHP, and LH increased during spring through autumn, overlapping the period of their homing, upstream migration, and spawning (Fig. 4). Among these sex hormones, plasma T and E2 levels began to increase after May and attained peak levels earlier than did other sex hormones, such as 17α-P, DHP, and LH.

3-8. Changes in sex steroid hormone levels before and after the occurrence of upstream swimming behavior in masu salmon

3-8A. Males

We investigated the changes in the plasma levels of sex steroid hormones (T, E2, 11-KT, and DHP) in 2+ masu salmon during the following temporal phases: before the onset of upstream swimming behavior (initial sampling: before transfer into the lower pond of the two-step raceway), during the migratory period (sampled from upstream migrants), and after upstream activity ceased (sampled from non-migrants remaining in the lower pond of the raceway) (Munakata et al. 2012a; Table 1). It was found that all of the 15 2+ males continued to spermiate before and during the onset of upstream movements, indicating that the fish spermiate during the upstream migratory phase. Plasma levels of T, 11-KT, and DHP were considerably higher during September (Table 1), comparable to the levels displayed by hatchery raised 2+ males, as shown in Fig. 4 (Munakata et al. 2001a). Plasma T, E2, and 11-KT levels decreased significantly after the cessation of upstream movement. Even so, plasma T, 11-KT, and DHP maintained higher levels than those of 1+ immature males (Fig. 4).


Table 1. Frequency of migrants and non-migrants, and plasma levels of testosterone (T), estradiol-17β (E2), 11-ketotestosterone (11-KT), 17,20β-dihydroxy-4-pregnene-3-one (DHP), thyroxine (T4), and (g) triiodothyronine (T3) (mean ± SEM) in 2+ male and female masu salmon during upstream migratory period. Differences in mean plasma hormone levels among experimental groups were analyzed by one-way analysis of variance (ANOVA) followed by Fisher's PLSD. Differing letters represent significant differences at P < 0.05 among all groups. —: no sample. Reprinted from Aquaculture, 362–363, Munakata et al., Involvement of sex steroids and thyroid hormones in upstream and downstream behaviors in masu salmon, Oncorhynchus masou, 158–166, © 2012a, with permission from Elsevier.

page top


3-8B. Females

Each of the 10 females at age 2+ that moved upstream in the raceway ovulated while 5 non-migrants that remained in the lower pond did not (Munakata et al. 2012a; Table 1). These phenomena indicate that female masu salmon ovulate during the last phase, or after upstream swimming behavior has ceased. Most of the 10 females moved upstream within 1 week after the experiment began. Interestingly, prior to the experiment, plasma levels of T and E2 in upstream migrants were significantly higher and lower, respectively, than the corresponding levels in non-migrants (Table 1). Furthermore, plasma E2 levels significantly decreased after the females completed their upstream movement. Though the E2 levels decreased, the levels of T, E2, and DHP were considerably higher than those in the 1+ immature females (Fig. 4).

In summary, the plasma levels of sex steroid hormones increased during the upstream migratory and spawning periods, for both 2+ males and females. These findings are supported by the fact that patterns in plasma elevation of sex steroid hormones during the upstream migratory and spawning periods are consistent with those detected in chum salmon, sockeye salmon, chinook salmon (O. tshawytscha), rainbow trout, and Arctic charr (Salvelinus alpinus) engaged in the same activities (Lou et al. 1984; Ueda et al. 1984; Liley et al. 1986; Truscott et al. 1986; Slater et al. 1994; Frantzen et al. 1997).

Considering the changing patterns of sex steroid hormones in masu salmon, sex steroid hormones especially T, E2, 11-KT, and DHP (males), and T, E2, and DHP (females) seem to be important factors that control the occurrence of homing, upstream swimming, and spawning behaviors in 2+ migrants.

In 2+ non-migratory precocious males, moreover, sex steroid hormones such as T and 11-KT seem to influence the occurrence of stream residency, upstream movement from their territory to the spawning ground, and spawning behaviors.

3-9. Stimulatory effects of sex steroid hormones on the upstream swimming behavior in masu salmon

During autumn, 1+ immature parr had low levels of plasma sex steroid hormones and did not exhibit any tendency to move upstream in the artificial raceway. However, when T, E2, and 11-KT 500 μg/fish via SILASCON tubing (Kaneka, Medics Corp.; outer diameter 1.5 mm, inner diameter 1.0 mm, length 30 mm) were implanted into the abdominal cavity of 1+ immature parr, upstream swimming behavior was induced (Munakata et al. 2001a; Fig. 11). Interestingly, it appeared that DHP 500 μg/fish-treatment had little influence on the occurrence of upstream behavior.


Fig. 11. (a) Frequency of migrants and non-migrants, plasma levels of (b) testosterone (T), (c) estradiol-17β (E2), (d) 11-ketotestosterone (11-KT), (e) 17,20β-dihydroxy-4-pregnene-3-one (DHP), (f) thyroxine (T4), and (g) triiodothyronine (T3) in controls, T 500 μg, E2 500 μg, 11-KT 500 μg, and DHP 500 μg-treated 1+ immature masu salmon parr. Numbers above columns in (a) indicate the number of migrants and non-migrants. Differences in the frequency of upstream behavior from the control group were analyzed by the χ2-test, using StatView version 4.5 software (Abacus Concepts, Inc., California, USA). *, **, and *** indicate significant difference at P < 0.05, P < 0.01, and P < 0.001, respectively, from the control group. Differences in mean plasma hormone levels among experimental groups were analyzed by one-way analysis of variance (ANOVA) followed by Fisher's PLSD. Differing letters represent significant differences at P < 0.05 among all groups. Reprinted from Comp. Biochem. Physiol. Part B, 129, Munakata et al., The involvement of sex steroid hormones in downstream and upstream migratory behavior of masu salmon, 661–669, © 2001a, with permission from Elsevier.

page top


In the interest of full disclosure, however, most of the 1+ immature masu salmon used in the research (Shiribetsu River strain) were females, because most males from this stock mature precociously (Munakata et al. 2000b). In this strain, the average male to female ratio of 1+ immature parr was approximately 1:9 (for example, Munakata et al. 2012a). In general, female masu salmon do not have high plasma 11-KT levels (Fig. 4, Table 1). Based on these data, it is proposed that 11-KT is not involved in the regulation of upstream swimming behavior in female masu salmon.

In 1+ precocious males, the control (sham-operated) group after being transferred to the lower pond moved upstream to the upper pond at significantly high frequencies (Munakata et al. 2001b; Fig. 12). To the contrary, castrated 1+ precocious males did not show this tendency. However, upstream swimming behavior was elicited from castrated 1+ precocious males treated with T and 11-KT 500 μg/fish. In contrast, E2 and DHP 500 μg/fish did not induce significant upstream swimming behavior in castrated fish (Fig. 12).


Fig. 12. (a) Frequency of migrants and non-migrants, plasma levels of (b) testosterone (T), (c) estradiol-17β Elsevier Science (USA), with permission from Elsevier. (E2), (d) 11-ketotestosterone (11-KT), (e) 17,20β-dihydroxy-4-pregnene-3-one (DHP), (f) thyroxine (T4), and (g) triiodothyronine (T3) in castrated, castrated + T 500 μg, E2 500 μg, 11-KT 500 μg, and DHP 500 μg/ fish-treated groups, and sham-operated 1+ precocious male masu salmon. Numbers above columns in (a) indicate the number of migrants and non-migrants. Differences in the frequency of upstream behavior from the control group were analyzed by the χ2-test, using StatView version 4.5 software (Abacus Concepts, Inc., California, USA). *, **, and *** indicate a significant difference at P < 0.05, P < 0.01, and P < 0.001, respectively from the control group. Differences in mean plasma hormone levels among experimental groups were analyzed by one-way analysis of variance (ANOVA) followed by Fisher's PLSD. Differing letters represent significant differences at P < 0.05 among all groups. Reprinted from Comp. Biochem. Physiol. Part B, 129, Munakata et al., The involvement of sex steroid hormones in downstream and upstream migratory behavior of masu salmon, 661–669, © 2001a, with permission from Elsevier.

page top


To summarize these experiments, it appears that T and E2 (females), and T and 11-KT (males) play significant roles in inducing the occurrence of upstream swimming behavior in masu salmon. Furthermore, it is also suggested that DHP had no significant effect on the occurrence of the upstream swimming behaviors in either sex (Figs. 11, 12).

3-10. Roles of T in the upstream and downstream swimming behaviors in masu salmon

Since androgen (male sex steroid hormone) T commonly increases plasma levels in both sexes, T is regarded as one of the most important sex steroid hormones in regulating the occurrence of the upstream and downstream swimming behaviors in salmonids, such as masu salmon. In teleosts, however, T is also the precursor converted to other sex steroid hormones such as estrogen E2 (males, females) and androgen 11-KT (males) (Kagawa et al. 1982a, b). For these reasons, it is hypothesized that T itself does not regulate the occurrence of the downstream or upstream swimming behaviors.

3-10A. Females

In order to investigate the potential role(s) of androgen T in the regulation of upstream swimming behavior in females, we examined the effects of implants of T, aromatase inhibitor 1,4,6-androstatrien-3,17-dione (ATD), and the estrogen antagonist tamoxifen 500 μg/fish on the occurrence of upstream swimming behavior in 1+ immature parr using the artificial raceway (Munakata et al. 2012a; Table 2). It was assumed that the upstream swimming behavior in the female was regulated by aromatized E2 but not T. If this hypothesis is correct, it is inferred that administration of ATD and tamoxifen should lower the stimulatory effects of T treatment on the occurrence of upstream swimming behavior. Otherwise, there is a possibility that T itself regulates the occurrence of the upstream swimming behavior without being converted to E2. As a result, ATD and tamoxifen 500 μg/fish-treatment did not decrease the stimulatory effects of T on the occurrence of upstream swimming behavior in 1+ immature female parr (Munakata et al. 2012a; Table 2, Exp 2 and 3).


Table 2. Frequency of migrants and non-migrants, and plasma levels of testosterone (T), estradiol-17β (E2), thyroxine (T4), and (g) triiodothyronine (T3) (mean ± SEM) in 1+ immature masu salmon implanted with T, 1,4,6-androstatriene-3,17-dion (ATD) or tamoxifen 500 μg/fish. Differences in the frequency of upstream behavior from the control group were analyzed by the χ2-test, using StatView version 4.5 software (Abacus Concepts, Inc., California, USA). * indicates significant difference at P < 0.05, from the control group. Differences in mean plasma hormone levels among experimental groups were analyzed by one-way analysis of variance (ANOVA) followed by Fisher's PLSD. Differing letters represent significant differences at P < 0.05 among all groups. —: no sample. Reprinted from Aquaculture, 362–363, Munakata et al., Involvement of sex steroids and thyroid hormones in upstream and downstream behaviors in masu salmon, Oncorhynchus masou, 158–166, © 2012a, with permission from Elsevier.

page top


3-10B. Males

In males, T and 11-KT 500 μg/fish-treatment induced the occurrence of upstream swimming behavior in 1+ castrated precocious males (Munakata et al. 2001b; Fig. 12). Moreover, treatment with estrogen E2 500 μg/fish induced the occurrence of upstream swimming behavior in 1+ intact precocious males (Munakata et al. 2012a; Table 3, Exp 4). Hence, it is speculated that the T, 11-KT, and E2 are potential factors involved in the regulation of upstream swimming behavior in males.


Table 3. Frequency of migrants and non-migrants and, plasma levels of testosterone (T), estradiol-17β (E2), thyroxine (T4), and (g) triiodothyronine (T3) (mean ± SEM) in castrated, castrated + E2 500 μg/fish, sham-operated, sham-operated + E2 500 μg/fish, control, and tamoxifen 500 μg/fish-treated 1+ precocious male masu salmon. Differences in the frequency of upstream behavior from the control group were analyzed by the χ2-test, using StatView version 4.5 software (Abacus Concepts, Inc., California, USA). * indicates significant difference at P < 0.05, from the control group. Differences in mean plasma and pituitary hormone levels among experimental groups were analyzed by one-way analysis of variance (ANOVA) followed by Fisher's PLSD. Differing letters represent significant differences at P < 0.05 among all groups. —: no sample. Reprinted from Aquaculture, 362–363, Munakata et al., Involvement of sex steroids and thyroid hormones in upstream and downstream behaviors in masu salmon, Oncorhynchus masou, 158–166, © 2012a, with permission from Elsevier.

page top


In order to investigate the potential role(s) of androgens, such as T and 11-KT, in the regulation of upstream swimming behavior, the effects of tamoxifen 500 μg/fish-treatment on the occurrence of upstream swimming behavior in 1+ precocious males was investigated. As a result, it was found tamoxifen did not decrease the stimulatory effects of T on the occurrence of upstream swimming behavior (Table 3, Exp 4). Therefore, it is thought that estrogens such as E2, and androgens such as T and 11-KT may regulate the occurrence of upstream swimming behavior in males. Thus in masu salmon, we concluded that both estrogens such as E2 (males and females), and androgens such as T (males and females) and 11-KT (males) are involved in the regulation of upstream swimming behavior. Furthermore, because of the patterns of changes in the plasma levels (Fig. 4), it is concluded that T, especially, is one of the common sex steroid hormones that regulate the occurrence of downstream and upstream swimming behaviors.

In masu salmon, DHP did not exhibit a significant effect on the occurrence of downstream and upstream swimming behavior (Munakata et al. 2001a). DHP is considered to be maturation inducing factor (MIF), which mediate final oocyte maturation (ovulation) and final testicular maturation (spermiation), in most teleosts (Nagahama 1987a, b). Plasma DHP levels regularly increase just prior to the spawning period (Munakata et al. 2001a; Fig. 4), while most salmonids have already arrived at their spawning areas. As Mayer et al. (1994) mentioned, the treatment of DHP via Silastic tube insertion induced the occurrence of male spawning behaviors. These findings indicate that DHP is more important in regulating spawning behaviors than upstream and downstream swimming behaviors.

As demonstrated previously, the frequency of downstream swimming behavior changes in accordance with the treatment dose of T in 1+ immature masu salmon smolts (Munakata et al. 2000b; Fig. 7). In like fashion, the induction of upstream swimming behavior in 1+ immature parr (Fig. 13) and 1+ castrated precocious males (Fig. 14) seemed to be a dose-dependent response (Munakata et al. 2001b). These are also strong indications that the plasma level of T is an important factor regulating the occurrence of either downstream or upstream swimming behaviors.


Fig. 13. (a) Frequency of migrants and non-migrants, plasma levels of (b) testosterone (T), (c) thyroxine (T4), and (d) triiodothyronine (T3) in controls, T 50 μg, T 500 μg, and T 1000 μg-treated 1+ immature masu salmon. Numbers above columns in (a) indicate the number of migrants and non-migrants. Differences in the frequency of upstream behavior from the control group were analyzed by the χ2-test, respectively, using StatView version 4.5 software (Abacus Concepts, Inc., California, USA). * indicates significant difference at P < 0.05, from the control group. Differences in mean plasma hormone levels among experimental groups were analyzed by one-way analysis of variance (ANOVA) followed by Fisher's PLSD. Reprinted from General and Comparative Endocrinology, 122, Munakata et al., The effects of testosterone on upstream migratory behavior in masu salmon, Oncorhynchus masou, 329–340, © 2001b, with permission from Elsevier.

page top


Fig. 14. (a) Frequency of migrants and non-migrants, plasma levels of (b) testosterone (T), (c) thyroxine (T4), and (d) triiodothyronine (T3) in castrated, cast. + T 50 μg, cast. + T 500 μg-treated groups, and sham-operated 1+ precocious male masu salmon. Numbers above columns in (a) indicate the number of migrants and non-migrants. Differences in the frequency of upstream behavior from the control group were analyzed by the χ2-test, using StatView version 4.5 software (Abacus Concepts, Inc., California, USA). * and *** indicate significant difference at P < 0.05 and P < 0.001, respectively from the control group. Differences in mean plasma hormone levels among experimental groups were analyzed by one-way analysis of variance (ANOVA) followed by Fisher's PLSD. Differing letters represent significant differences at P < 0.05 among all groups. Reprinted from General and Comparative Endocrinology, 122, Munakata et al., The effects of testosterone on upstream migratory behavior in masu salmon, Oncorhynchus masou, 329–340, © 2001b, with permission from Elsevier.

page top


It was also detected that the dosing period may be an important factor in initiating the upstream response. When 1+ immature masu salmon were implanted with T 500 μg/fish for approximately 4 months, the frequencies of upstream migrants was considerably higher (43%) than for control individuals (19%) (Munakata et al. 2012a; Table 2, Exp 1). On the other hand, the frequency of upstream migrants given a dosage of T 500 μg/fish for approximately 2 months in different trials were 17.1% (Munakata et al. 2001b; Fig. 13), 22% (Munakata et al. 2001b; Fig. 16), 36% (Munakata et al. 2012a; Table 2, Exp 2), 52% (Munakata et al. 2001b; Fig. 11), and 57% (Munakata et al. 2012a; Table 2, Exp 3). The average frequency of upstream swimming behavior in the later experiments (36.8%) was lower than in the former experiment (43%) (Munakata et al. 2012a; Table 2, Exp 1). Although the inference is not strong, this suggests that the dosing period (duration of plasma sex steroid hormone increase) is an important factor in this process.

In these experiments, major parts of upstream swimming behaviors occurred while there were no significant changes in plasma levels of T being administered through Silastic or SILASCON tubing. This suggests that the continuous release of T may play a role as a "requirement" for the occurrence of the upstream swimming behavior (Munakata and Kobayashi 2010).

3-11. Roles of sex steroid hormones in the upstream swimming behavior in land-locked sockeye salmon

It was determined that T 500 μg/fish treatment significantly induced the occurrence of upstream swimming behaviors in 1+ immature land-locked sockeye salmon in the raceway (Munakata et al. 2012b; Fig. 15). In addition, precocious males of 1+ land-locked sockeye salmon with high plasma T levels also migrated upstream at significantly high frequencies. From this, it is inferred that stimulatory regulation of upstream migratory behavior by sex steroid hormones might be common to multiple Pacific salmon.


Fig. 15. (a) Frequency of migrants and non-migrants, plasma levels of (b) testosterone (T), (c) pituitary contents of luteinizing hormone (LH), (d) plasma levels of LH, (e) thyroxine (T4), and (f) triiodothyronine (T3) in control and T 500 μg/fish-treated 1+ immature fish, and 1+ precociously mature male sockeye salmon. In (c), unit of Y axis in the control group was ng/pituitary, while those of T-treated and precocious male groups was μg/pituitary. Numbers above columns in (a) indicate the number of migrants and non-migrants. N.S. represents no sample. Differences in the frequency of upstream behavior from the control group were analyzed by the Fisher's exact probability test, using StatView version 4.5 software (Abacus Concepts, Inc., California, USA). * and ** indicate a significant difference at P < 0.05 and P < 0.01 from the control group, respectively. Differences in mean plasma and pituitary hormone levels among experimental groups were analyzed by one-way analysis of variance (ANOVA) followed by Fisher's PLSD. Differing letters represent significant differences at P < 0.05 among all groups. Reprinted with permission from Fish. Sci., 78, Munakata et al., Involvement of sex steroids, luteinizing hormone and thyroid hormones in upstream and downstream migratory behaviors in land-locked sockeye salmon Oncorhynchus nerka, 81–90, Fig. 6, © 2012b, The Japanese Society of Fisheries Science.

page top


3-12. Roles of sex hormones other than sex steroid hormones in the upstream swimming behavior in salmonids

In 2+ female masu salmon, all of the 10 upstream migrants ovulated, while 5 non-migrants did not (Munakata et al. 2012a; Table 1). These results suggest that females ovulate during the last phase, or after their upstream swimming behavior ceased. Since spermiation and ovulation are controlled physiologically by pituitary hormones such as LH (Nagahama 1984; Kobayashi et al. 1986, 1988), it was hypothesized that not only sex steroid hormones but also some other sex hormones may be involved in the occurrence of upstream swimming behavior. Investigations into masu salmon (Amano et al. 1992, 1993), coho salmon (Swanson 1991), and rainbow trout (Prat et al. 1996) indicate that LH levels increase with final ovarian maturation, especially during ovulation. These studies support the hypothesis that LH may play a role in the regulation of upstream swimming behavior.

Implants of a gonadotropin-releasing hormone analogue (GnRHa) enhanced the occurrence of homing behavior, the movement from the center of Lake Shikotsu (Hokkaido) to the mouth of the natal rivers in adult land-locked sockeye salmon (Sato et al. 1997). Based on this, it is hypothesized that GnRH directly influences the occurrence of upstream swimming behavior. On the other hand, considering that the GnRH is one important factor which stimulates the secretion of LH from the pituitary gland into the plasma (Amano et al. 1995), it is also inferred that GnRH plays a role in enhancing the occurrence of homing behavior and perhaps also upstream swimming behavior through the release of LH into the plasma.

In masu salmon and land-locked sockeye salmon, however, it was demonstrated that plasma LH levels in T-treated immature fish did not exhibit clear increases over the period of downstream and upstream movement when compared with corresponding values for intact immature fish (Figs. 6, 7, 10, 15, 16). Since one of main roles of LH is to stimulate sexual (gonadal) maturation (Nagahama 1984), there is the possibility that LH influences the upstream swimming behavior through stimulating the synthesis and/or secretion of sex steroid hormones into the plasma. However, the critical role of LH in the regulation of migratory behaviors clearly requires further investigation.


Fig. 16. (a) Frequency of migrants and non-migrants, plasma levels of (b) testosterone (T), (c) pituitary contents of luteinizing hormone (LH), (d) plasma levels of LH, (e) thyroxine (T4), and (f) triiodothyronine (T3) in control and T 500 μg/fish-treated 1+ immature masu salmon. Numbers above columns in (a) indicate the number of migrants and non-migrants. N.S. represents no sample. Differences in the frequency of upstream swimming behavior from the control group were analyzed by the Fisher's exact probability test, using StatView version 4.5 software (Abacus Concepts, Inc., California, USA). ** indicates a significant difference at P < 0.01 from the control group. Differences in mean plasma and pituitary hormone levels among experimental groups were analyzed by one-way analysis of variance (ANOVA) followed by Fisher's PLSD. Differing letters represent significant differences at P < 0.05 among all groups. Reprinted from General and Comparative Endocrinology, 122, Munakata et al., The effects of testosterone on upstream migratory behavior in masu salmon, Oncorhynchus masou, 329–340, © 2001b, with permission from Elsevier.

page top


3-13. Roles of sex steroid hormones in the spawning behavior in masu salmon

Since T was identified as one of the common sex steroid hormones which stimulated the onset of upstream swimming behavior, T may also give rise to spawning behaviors. In order to investigate the stimulatory role of T in the male and female spawning behaviors, the following experiments were conducted. T 500 μg/fish or T 1000 μg/fish dosages were administered to 1+ castrated precocious males or immature females placed in an artificial stream chamber (1.5 × 0.6 m with a water depth of 0.2 m) (Munakata et al. 2002).

3-13A. Roles of T on the spawning behavior in males

In general, the differences in spawning behaviors between "sneakers" (precocial males) and anadromous males (2+ males) have been discussed. However, if a 1+ precocious male and 2+ female are transferred together into an artificial chamber, the 1+ precocious male exhibits male spawning behaviors typically associated with 2+ mature males (see Fig. 18). 1+ precocious males (sham-operated fish) frequently showed attending and quivering behaviors towards the 2+ females (Munakata et al. 2002; Figs. 17, 18). 1+ castrated males did not display such behaviors (Fig. 17). However, the full behavioral array of spawning males (e.g., attending and quivering) can be restored by the administration of T 500 μg/fish.


Fig. 17. Frequency of quivering and attending behaviors in the sham-operated, castrated, and castrated + testosterone (T) 500 μg/fish-treated 1+ precocious male masu salmon. Differences in mean frequencies of quivering and attending behaviors among experimental groups were analyzed by one-way analysis of variance (ANOVA) followed by Fisher's PLSD. Differing letters represent significant differences at P < 0.05 among all groups. Reprinted with permission from Fish. Sci., 68, Munakata et al., Sex steroids control migration of masu salmon, 49–52, Fig. 3, © 2002, The Japanese Society of Fisheries Science.

page top


Fig. 18. Male masu salmon showing (a) attending and (b) quivering behaviors against 2+ mature females performing a series of female spawning behaviors.

page top


3-13B. Roles of T on the spawning behavior in females

In the artificial stream chamber, 2+ ovulated isolated females frequently exhibited digging behaviors on the gravel substrates (Figs. 19, 20). This behavior conforms field observations. Female Pacific salmon generally arrive at spawning sites earlier than do males (Groot and Margolis 1991), and females, but not males, release a pheromone L-kynurenine to attract non-specific mature males (Yambe et al. 2003, 2006).


Fig. 19. Frequency of female digging behavior in the control, testosterone (T) 500 μg/fish treated, and T 1000 μg/fish treated groups. Differences in mean frequency of digging behavior among experimental groups were analyzed by one-way analysis of variance (ANOVA) followed by Fisher's PLSD. Differing letters represent significant differences at P < 0.05 among all groups. Reprinted with permission from Fish. Sci., 68, Munakata et al., Sex steroids control migration of masu salmon, 49–52, Fig. 4, © 2002, The Japanese Society of Fisheries Science.

page top


Fig. 20. Female masu salmon performing digging behavior. (a) 2+ mature females and (b) 1+ immature females treated with testosterone (T) 500 μg/fish.

page top


On the other hand, 1+ immature females did not perform digging behaviors (Fig. 19). The GSI values and plasma sex steroid hormone levels of 1+ immature females were low (Fig. 4), and their ovaries were in the primary growth stage (Kiso 1995). 1+ immature females, however, significantly exhibited digging behaviors when treated with a dose of T 1000 μg/fish (Munakata et al. 2002; Fig. 19).

3-14. Stimulatory effects of sex steroid hormones on the spawning behavior in male rainbow trout

It was demonstrated that treatment of DHP induced the occurrence of quivering and attending behaviors in castrated male rainbow trout towards sexually receptive females (Mayer et al. 1994). Interestingly, administration of 11-ketoandrostendione (11-KA) did not induce male spawning behaviors to any significant degree. This evidence supports the notion that male spawning behaviors in some Pacific salmon are controlled by some sex steroid hormones, potentially T and/or DHP, released into the plasma.

page top


4. Roles of thyroid hormones, cortisol, growth hormone, and environmental factors in the regulation of downstream and upstream swimming behaviors in salmonids

During the period of smoltification and associated downstream migration, salmonid smolts exhibit increases in various types of hormones, such as thyroxine (T4), triiodothyronine (T3), cortisol, GH, and prolactin (Ikuta et al. 1985; Young et al. 1989; Prunet et al. 1989; Hirano 1991; Nagae et al. 1994; Dickhoff et al. 1997; McCormick 2001). Since these hormones are involved in both physiological and morphological changes during smoltification, it is likely that these hormones also play some roles in the regulation of downstream swimming behavior, and perhaps in upstream swimming behavior. In previous studies, drastic increases in plasma levels of T4 (i.e., T4 surge) were discovered during the peak periods of smoltification in several salmonids, such as coho and masu salmon (Grau et al. 1981; Yamauchi et al. 1984, 1985). Our studies on masu and land-locked sockeye salmon also demonstrated that plasma T4 levels in 1+ smolts were higher than those of T-treated 1+ smolts (Munakata et al. 2000b, 2001a, 2012a, 2012b) (Figs. 6–10, Table 4). Furthermore, some 1+ mature and immature masu salmon parr exhibited considerably high plasma T4 levels during the upstream migratory period (Munakata et al. 2001a, 2001b, 2012a, 2012b) (Figs. 11–16, Tables 1–3). Considering these facts, thyroid hormone T4 has been recognized as an important factor involved in the downstream and upstream swimming behaviors.


Table 4. Frequency of upstream and downstream swimming behaviors and, plasma levels of testosterone (T), thyroxine (T4), and (g) triiodothyronine (T3) (mean ± SEM) in controls and T 500 μg/fish-treated 1+ smolts and 1+ precocious male masu salmon. Differences in the frequency of upstream and downstream swimming behavior from the control group were analyzed by the χ2-test, using StatView version 4.5 software (Abacus Concepts, Inc., California, USA). *** indicates significant difference at P < 0.001, from the control group. Differences in mean plasma hormone levels among experimental groups were analyzed by one-way analysis of variance (ANOVA) followed by Fisher's PLSD. Differing letters represent significant differences at P < 0.05 among all groups. Reprinted from Aquaculture, 362–363, Munakata et al., Involvement of sex steroids and thyroid hormones in upstream and downstream behaviors in masu salmon, Oncorhynchus masou, 158–166, © 2012a, with permission from Elsevier.

page top


Plasma levels of cortisol and GH continually increase during the period of downstream migration (Prunet et al. 1989; Nagae et al. 1994; McCormick 2001; Mizuno et al. 2001; Zydlewski et al. 2005). The involvement of cortisol and ovine GH (oGH) treatment on the occurrence of downstream swimming behavior in 1+ and 0+ masu salmon juveniles will be summarized in the following sections.

Environmental factors (i.e., inorganic and organic (e.g., biological) factors) also appear to play an indispensable role in the occurrence of downstream swimming, upstream swimming, and spawning behaviors. In the following section, an overview of the involvement of some environmental factors, such as intra-specific interaction, day-night cycle, etc., in the regulation of downstream swimming, upstream swimming, and spawning behaviors will be presented.

4-1. Roles of thyroid hormones in the downstream and upstream swimming behavior in masu and land-locked sockeye salmon

Grau et al. (1981) and Yamauchi et al. (1985) discovered in coho and masu salmon smolts that plasma T4 levels increased rapidly and significantly (T4 surge) during the peak periods of their smoltification. In our studies, most 1+ masu and land-locked sockeye salmon smolts exhibited high plasma thyroid hormone levels during the downstream migratory period. In masu and land-locked sockeye salmon, and rainbow trout, however, plasma T4 and T3 levels in downstream migrants tended to be lower than those of non-migrants that remained in the upper pond of the raceway (Ewing et al. 1994; Munakata et al. 2000b, 2001a, 2012a, 2012b) (see Figs. 6–10 and Table 4). Based on these observations, it was hypothesized that plasma T4 and T3 levels decrease during or after the initiation of downstream swimming behavior.

In our previous studies, however, the downstream migratory smolts were usually sampled every morning at 09:00, over several weeks or months (Munakata et al. 2000b, 2001a, 2012b) (Figs. 6–10). Therefore, there is a possibility that the sampling delay (i.e., sampling conducted considerably after the downstream swimming behavior) may have influenced the decrease in plasma T4 and T3 levels in downstream migrants.

In the previous study, therefore, the downstream migrants from the lower pond of the three-step raceway (Fig. 21) were sampled within 60 minutes of the occurrence of migration (Table 4). To avoid the disparity in the sampling dates between migrants and non-migrants, six of the 1+ control smolts, T500 μg/fish-treated smolts, and precocious males were each sampled from the middle pond on May 3 during the April 29–May 11 observation period (Munakata et al. 2012a). As a result, there were no clear differences in plasma thyroid hormone levels between downstream migrants and non-migrants (Table 4). Ikuta (1994) reported that plasma T4 levels in 1+ land-locked sockeye salmon smolts of downstream migrants differed: those that were sampled immediately after the onset of downstream swimming behavior tended to have higher plasma concentrations than those of non-migrants, which were sampled simultaneously in the upper pond of the raceway. Therefore, one possible hypothesis is that decreases in plasma thyroid hormone levels occur innately after the start of the downstream swimming behavior.


Fig. 21. Schematic drawing of three-step raceway that consists of upper, middle, and lower ponds. This raceway enables us to quantify upstream and downstream swimming behaviors at the same period. After the experimental fish had been reared in tanks, they were transferred to the middle pond (4 × 2 × 0.5 m) of the raceway. The middle pond was connected to the upper (4 × 2 × 0.5 m) and lower (4 × 2 × 0.5 m) ponds through square holes (50 × 25 cm, thickness 3 cm) which were made on wooden walls. Flow rate (volume) and velocity of the water in the raceway were 10 l/s and 75-85 cm/s. The upstream and downstream migrants are identified when the fish swam from the middle pond to the net traps (2 × 0.7 × 0.7 m) that were located in the upper and lower ponds, respectively. Reprinted from Aquaculture (in press), Munakata et al., Involvement of sex steroids and thyroid hormones in upstream and downstream behaviors in masu salmon, Oncorhynchus masou, © 2012a, with permission from Elsevier.

page top


4-1A. Effects of net trap on plasma thyroid changes during the downstream migratory period in land-locked sockeye salmon

In previous studies (Munakata et al. 2000b, 2001a, 2012a) (Figs. 6–9, Table 4), we regularly used a net trap (2 × 0.7 × 0.7 m) to catch the downstream migrating 1+ masu and land-locked sockeye salmon smolts. Therefore, there is a possibility that captive stress induced by the net trap (Yada et al. 2007) caused a decrease in plasma T4 levels in downstream migrants. Thus in a separate study, so as to avoid captive stress from the net trap, we used a hand dip net to capture the land-locked sockeye salmon downstream migrants directly from a separated area (2 × 4 × 0.5 m) in the lower pond (Fig. 5), a few hours after the onset of downstream swimming behavior. After the downstream swimming behavior ceased, however, plasma T4 levels of downstream migrants became lower than those of non-migrants in the control, T 500 μg/fish-treated, and the precocious male groups (Fig. 10). These results indicate that plasma T4 levels in downstream migrants become lower than those of non-migrants, independent of the use of a net trap.

4-1B. Role of T4 and T3 in the upstream swimming behavior

In ayu, Plecoglossus altivelis, an amphidromous fish, it was found that plasma T4 levels of immature downstream migrants became lower than in the initial levels, likewise masu and land-locked sockeye salmon smolt migrants, and that the levels of upstream migrants became higher than in the initial plasma levels (Tsukamoto et al. 1988). In adult chum salmon, on the other hand, plasma levels of the thyroid hormones in upstream migrants were lower than those of migrants which swim in the coastal sea during their upstream migratory period (Ueda et al. 1984). Based on these findings, it was suggested in some Pacific salmon that the plasma T4 and T3 levels decreased in association with the progression of final gonadal maturation. In a previous study, on the other hand, we used 1+ immature masu and land-locked sockeye salmon as a surrogate for 2+ maturing fish in the upstream migratory period (Munakata et al. 2001a, b). Consequently, plasma T4 and T3 levels in upstream migrants tended to be lower than those of non-migrants in 1+ masu and land-locked sockeye salmon (Munakata et al. 2001a, 2001b, 2012a, 2012b) (Figs. 11–16, Tables 2, 3). Hence, it is suggested that the decrease in thyroid hormones coincided with the occurrence of the upstream swimming behavior. However, the upstream migrants were sampled every morning at 09:00 over several weeks or months in our previous studies (Munakata et al. 2001a, b). Therefore, the sampling delay may have influenced the decrease in plasma thyroid hormone levels in upstream migrants. In a separate experiment (Munakata et al. 2012a; Table 4), the upstream migrating T 500 μg/fish-treated 1+ masu salmon were sampled within 60 minutes of the occurrence of upstream swimming behavior from the upper pond of the three-step raceway. It was found that there were no differences in plasma T4 and T3 levels between upstream migrants and non-migrants. As a result, decreases in plasma T4 and T3 levels may initiate a few hours after the occurrence of the upstream swimming behavior.

To summarize these investigations, thyroid hormones, such as T4 and T3, appear to play some roles in the regulation of downstream and upstream swimming behavior in some anadromous salmonids. Recently, it was demonstrated that T4-treated 0+ coho salmon parr tended to exhibit a higher frequency of downstream swimming behavior than the control parr (Munakata and Schreck, unpublished data). However, it was also previously reported that T4 itself does not induce the occurrence of downstream swimming behavior in salmonids (see a review by Iwata 1995). The role of the thyroid hormones in mediating the downstream and upstream swimming behaviors clearly requires further investigation.

4-2. Roles of cortisol and growth hormone in the downstream swimming behavior in masu salmon

4-2A. Cortisol

In anadromous salmonids, cortisol and GH have been known to regulate the hypo-osmoregulatory ability, during the smoltification period (Hirano 1991; McCormick 2001). In both 1+ smolts and 0+ parr of masu salmon, on the other hand, it was found that treatment of cortisol, but not GH (oGH), caused the occurrence of downstream swimming behavior in the raceway (Munakata et al. 2007; Figs. 22, 23). The frequency of downstream swimming behavior in the cortisol 2 mg/fish-treated group (72%) and oGH 250 μg/fish + cortisol 2 mg/fish-treated group (82%) were significantly higher than those in the control (23%) and oGH 250 μg/fish-treated group (18%) (Fig. 22). The plasma cortisol levels of migrants in the cortisol 2 mg/fish-treated 1+ smolts (Fig. 22) were similar to those levels of naturally occurring 1+ smolts (Nagae et al. 1994; Mizuno et al. 2001).


Fig. 22. Frequency of migrants and non-migrants, plasma levels of oGH and cortisol in the control, oGH 250 μg, cortisol 2 mg, and oGH 250 μg + cortisol 2 mg-treated 1+ masu salmon smolts. Figures beside columns indicate the number of migrants and non-migrants. Differences in the frequency of downstream behavior from the control group were analyzed by the χ2-test, using StatView version 4.5 software (Abacus Concepts, Inc., California, USA). *** indicates a significant difference at P < 0.001 from the control group. Differences in mean plasma hormone levels among experimental groups were analyzed by one-way analysis of variance (ANOVA) followed by Fisher's PLSD. Differing letters represent significant differences at P < 0.05 among all groups. Reprinted from General and Comparative Endocrinology, 150, Munakata et al., Effects of growth hormone and cortisol on the downstream migratory behavior in masu salmon, Oncorhynchus masou, 12–17, © 2007, with permission from Elsevier.

page top


The results indicate that the downstream swimming behavior (negative rheotaxis) is controlled competitively by both sex steroid hormones and smolt inducing factors, such as cortisol.

In 0+ parr, frequency of downstream swimming behavior in the cortisol 2 mg/fish-treated group (82%) and oGH 250 μg/fish + cortisol 2 mg/fish-treated group (90%) were higher than those in the control (18%) and oGH 250 μg/fish-treated group (0%) (Fig. 23). Based on these findings, it can also be hypothesized that cortisol induces the occurrence of downstream swimming behavior not only in 1+ smolts but also in 0+ parr in masu salmon.


Fig. 23. Frequency of migrants and non-migrants, plasma levels of oGH and cortisol in the control, oGH 250 μg, cortisol 2 mg, and oGH 250 μg + cortisol 2 mg-treated 0+ masu salmon parr. Figures beside columns indicate the number of migrants and non-migrants. Differences in the frequency of downstream swimming behavior from the control group were analyzed by the χ2-test, using StatView version 4.5 software (Abacus Concepts, Inc., California, USA). *** indicates a significant difference at P < 0.001 from the control group. Differences in mean plasma hormone levels among experimental groups were analyzed by one-way analysis of variance (ANOVA) followed by Fisher's PLSD. Differing letters represent significant differences at P < 0.05 among all groups. Reprinted from General and Comparative Endocrinology, 150, Munakata et al., Effects of growth hormone and cortisol on the downstream migratory behavior in masu salmon, Oncorhynchus masou, 12–17, © 2007, with permission from Elsevier.

page top


A causal mechanism for how the increases of plasma cortisol levels induce the downstream swimming behavior in 1+ smolts and 0+ parr remains unclear. Since only portions of 1+ masu salmon undergo the smoltification, one possible explanation is that the plasma cortisol levels increase innately as the smoltification process advances. Alternatively, since various types of acute and chronic environmental stimuli, such as inter-and intra-specific interactions (Iwata 1996; Kagawa and Mugiya 2000; Kelsey et al. 2002), fasting (Varnavsky et al. 1995), exposure to low water (Pankhurst and Kraak 2000), fright responses from being chased by a netter (Nichols and Weisbart 1984; Yada et al. 2007), and water quality (Barton et al. 1987; Redding et al. 1987), increased circulating plasma cortisol levels within a few hours in teleosts, another explanation is that some environmental cues directly or indirectly enhance the additional secretion of cortisol into the plasma, and subsequently downstream swimming behavior is induced. Some studies indicated that inter-and intra-specific interactions, among larger and smaller individuals, may be an important factor that not only increased plasma cortisol levels in small subordinates (Kelsey et al. 2002), but also induced movement of smaller salmonid juveniles out of their focal foraging areas (Nakano et al. 1990; Nakano 1995).

Alternatively, as mentioned in Subsection 2-6, it was found that all of the 40 1+ masu salmon smolts transferred into the upper pond of the raceway migrated down within a week (Munakata et al. 2000b; Fig. 9). Based on this, it was hypothesized some other environmental factors induce the plasma cortisol elevations and subsequent downstream swimming behavior. Either downstream or upstream swimming behavior occurs mainly during the evening, through the night, and when it rains (Yamauchi et al. 1985; Munakata et al. 2000b; Munakata et al. unpublished data; Fig. 24). Accordingly, environmental factors, such as photoperiod, temperature, flow rate, and water quality (e.g., turbidity), which exhibit diurnal fluctuations, affect the occurrence of downstream swimming behavior through increases in plasma cortisol.


Fig. 24. (a) Number of fish that exhibited the downstream migratory behavior in the 1+ masu salmon smolts (white column) and the T 500 μg/fish-treated smolts (dark column). (b) date of rainfall during experimental period in May.

page top


4-2B. GH

In anadromous salmonids, GH is considered to be an important factor which regulates the hypo-osmoregulatory ability during smoltification (Hirano 1991; McCormick 2001). Previous studies also demonstrated that treatment of oGH as well as native GH influenced the physiological processes of smoltification and several types of behaviors such as salinity preference, foraging, and anti-predator behaviors (Iwata et al. 1990; Boeuf et al. 1994; Johnsson et al. 1996; Jönsson et al. 1996; Yada et al. 1999). In our previous study, however, most oGH-treated 1+ smolt and 0+ parr did not exhibit downstream swimming behavior during the downstream migratory period (Figs. 22, 23). One possible explanation is that the treatment dose of oGH may have been insufficient, or the treatment period may have been too short to affect the downstream behavior. On the other hand, it is also possible that GH, including oGH, is not involved in the occurrence of downstream swimming behavior in masu salmon.

Recently, Ojima et al. (2009) reported that hypothalamic hormone growth hormone-releasing hormone (GHRH) caused the downstream swimming behavior in 0+ chum salmon fry. The findings thus indicate that GHRH directory modulates the occurrence of downstream movements in chum salmon fry. Furthermore, this also indicates that GH which is stimulated by GHRH plays a role in the downstream swimming behavior. Therefore, it is necessary to investigate the effects of treatment dose and period of both oGH and native GH on downstream swimming behavior.

4-3. Roles of external factors in the downstream swimming, upstream swimming, and spawning behaviors

4-3A. Roles of external factors in downstream swimming behavior

A significant portion of 1+ masu salmon smolts exhibit the downstream movement during favorable periods in the spring. Furthermore, it is believed that the downstream migratory period in the southern regions tends to be earlier than that in the northern regions (Machidori and Kato 1984; Kato 1991; Kiso 1995). This trend clearly indicates that some external (environmental) factors such as photoperiod and temperature, which show seasonal fluctuations and regional differences, may be involved in the occurrence of downstream swimming behavior. As mentioned previously, it is further indicated that some environmental stimulations such as photoperiod and temperature, which exhibit diurnal patterns, may play roles in stimulating this behavior.

According to Yamauchi et al. (1985), 1+ masu salmon smolts exhibited downstream swimming behavior after precipitation occurred. Our previous study also suggested that the number of downstream migrants in the 1+ masu salmon smolts increased when it rained (Fig. 24). Accordingly, it is indicated that rain, snow, and the concomitant increase in flow may trigger the occurrence of downstream swimming behavior (Munakata et al. unpublished data).

As mentioned repeatedly, dominant precocious male parr frequently initiate aggressive behaviors (e.g., attacking, nipping, chasing) towards subordinate fish including phenotypes that will become smolts (Nakano 1995; Kiso 1995; Hutchison and Iwata 1998; Munakata et al. 2000b). In 1+ masu salmon, moreover, the BL and BW of non-migrants, especially the 1+ precocious males, tended to be larger than those of downstream migrants such as 1+ smolts (Munakata et al. 2000b; Table 5). Similarly, BL, BW, and CF in downstream migrants were smaller than those in non-migrants among T500 μg/fish-treated immature smolts (Munakata et al. 2000b; Table 6). Although further investigation is needed, an acceptable rationale is that intra-specific interactions, which depend partly on their body size (growth), play roles in stimulating the downstream behavior in smaller fish.


Table 5. Body length (BL), body weight (BW), and condition factor (CF) in the control, T5 μg/fish-, T 50 μg/fish-, and T 500 μg/fish-treated 1+ smolts and 1+ precocious male masu salmon. Differences in mean BL, BW, and CF among experimental groups were analyzed by one-way analysis of variance (ANOVA) followed by Fisher's PLSD. Differing letters represent significant differences at P < 0.05 among all groups. Reprinted with permission from Zoological Science, 17, Munakata et al., Inhibitory effects of testosterone on downstream migratory behavior in masu salmon, Oncorhynchus masou, 863–870, Table 2, © 2000b, Zoological Society of Japan.

page top


Table 6. Body length (BL), body weight (BW), and condition factor (CF) in the control (Raceway 1) and T500 μg/fish-treated smolts (Raceway 2) in masu salmon (Oncorhynchus masou). Differences in mean BL, BW, and CF among experimental groups were analyzed by one-way analysis of variance (ANOVA) followed by Fisher's PLSD. Differing letters represent significant differences at P < 0.05 among all groups. Reprinted with permission from Zoological Science, 17, Munakata et al., Inhibitory effects of testosterone on downstream migratory behavior in masu salmon, Oncorhynchus masou, 863–870, Table 3, © 2000b, Zoological Society of Japan.

page top


4-3B. Roles of external factors in the upstream swimming behavior

During the upstream migratory period, a significant portion of the sex steroid hormone-treated 1+ immature masu salmon, and 2+ masu and land-locked sockeye salmon moved upstream during and after dusk (Munakata et al. 2001a, 2001b, 2012a, 2012b) (Fig. 25). Thus, we must consider that upstream swimming behavior is triggered or regulated by some environmental factors such as photoperiodicity.


Fig. 25. Eight-day running average of the number of upstream migrants in 2+ male and female sockeye salmon in the raceway. Dark and white bars indicate light dark-periods. This graph shows that the majority of 2+ land-locked sockeye salmon exhibit upstream migratory behavior before, during, and after dusk between 16:00 and 22:00. There was no clear difference between sexes. Reprinted with permission from Fish. Sci., 78, Munakata et al., Involvement of sex steroids, luteinizing hormone and thyroid hormones in upstream and downstream migratory behaviors in land-locked sockeye salmon Oncorhynchus nerka, 81–90, Fig. 4, © 2012b, The Japanese Society of Fisheries Science.

page top


Because plasma sex steroid hormone levels of sex steroid-treated immature fish did not show apparent changes during the upstream migratory period (Munakata et al. 2001a, b; Figs. 11–16, Tables 2–4), sex steroid hormones may play roles in the regulation of upstream behavior as a "requirement" (Munakata and Kobayashi 2010) and sex steroid hormones may modulate receptivity of fish from some environmental stimulations. However, additional research will be required to validate this linkage between the physiological response and the environmental change.

4-3C. Roles of external factors in the spawning behavior

When 2+ ovulated female masu salmon are reared continuously in artificial environments such as in hatchery fiber-reinforced plastic (FRP) tanks, these fish seldom display spawning behavior, including the digging of redds. On the other hand, we discovered that the 2+ ovulated and T 1000 μg/fish-treated 1+ immature females that were transferred into the previously mentioned artificial stream chamber frequently displayed digging behaviors (Fig. 18). The chamber contained both gravel and flowing water, indicating that physical environmental cues may be necessary before a particular spawning behavior is elicited. Satou et al. (1984) demonstrated that spawning behavior in male land-locked sockeye salmon adults was elicited by decoy fish which exhibit movements and oscillations resembling a mature female in spawning condition. This example shows that visual stimulation or oscillation from females can induce spawning behavior in males in the same manner as pheromones, such as L-kynurenine (Yambe et al. 2003, 2006).

page top


5. Conclusion and discussion

5-1. Roles of sex steroid hormones in the occurrence of migratory behaviors in masu salmon

In masu salmon, it was demonstrated that sex steroid hormones, such as T inhibited the occurrence of smoltification and downstream swimming behavior (negative rheotaxis), the initial phase in seaward migration (Aida et al. 1984; Ikuta et al. 1985, 1987; Munakata et al. 2000b, 2001a). On the other hand, treatment with T induced the occurrence of upstream swimming behavior (positive rheotaxis), a component of upstream migration in 1+ immature parr and castrated precocious males (Munakata et al. 2001a, b). Furthermore, T commonly induced spawning behavior in both sexes (Munakata et al. 2002). Therefore, it is concluded that sex steroid hormones, such as T, regulate the occurrence of downstream and upstream swimming behavior, in negative and positive rheotaxis fashions, and that sex steroid hormones negatively control the occurrence of the seaward migration.

Based on these phenomena, we now understand why mature masu salmon that have high levels of plasma sex steroid hormones live continuously in their natal rivers as non-migratory forms (Figs. 3, 26). On the other hand, delay of sexual maturation in the rivers results from low levels of plasma sex steroid hormones in some juveniles, and these fish regularly exhibit downstream swimming behavior along with the river currents, in association with the smoltification (Figs. 3, 26). In comparison, masu salmon that begin to sexually mature and consequently have high plasma sex steroid hormone levels in the sea (or lakes) tend to move against the river current (positive rheotaxis), and home upstream to their natal reach. This behavior is homologous with the upstream movement of precocious non-migrants that live continuously in their natal rivers (Figs. 3, 26). Finally, masu salmon with high levels of plasma sex steroid hormones initiate the display of spawning behavior after they arrive at a potential spawning habitat.


Fig. 26. Diagrammatic representation of the lifecycle of masu salmon (Oncorhynchus masou) (modified the figure by Kiso 1995). There seems to be a sequential diversity in migratory patterns among representative non-migratory (precocious parr) and migratory (smolt) forms. In masu salmon, 1+ parr that occupy focal foraging areas (territory), exhibit precociously sexual maturation, and have high plasma sex steroid levels become representative non-migrants, precocious parr. In contrast, some of the 1+ immature fish that could not gain focal territories become representative migrants, smolt migrants. Their migratory patterns seem to be modulated in an inhibitory fashion by their maturity and/or growth performance (see text for details). Note that the "poor growth fish" which does not differentiate into either precocious parr or smolt migrants during the age of 1+ appears to differentiate into non-migratory parr or migratory smolts in another year, mainly during the age of 2+.

page top


Because sex steroid hormones commonly regulate the occurrence of downstream swimming, upstream swimming, and spawning behaviors, can it be considered that downstream (negative rheotaxis) and upstream migratory behavior (positive rheotaxis) are involved as an obligatory phase before the spawning takes place? More specifically, these migratory behaviors are involved as part of the spawning activities, which take place in the upper river. Because masu salmon use different habitats (rivers and sea) during their lifecycle, it may be that the inhibition of downstream swimming behavior and the stimulation of upstream swimming behavior by the sex steroid hormones are "biological endorsements" that orient sexually mature masu salmon to natal spawning areas, before the initiation of spawning behaviors.

While masu salmon non-migrants such as 1+ precocious males exhibit high plasma sex steroid hormone levels and stay in their natal rivers, representative downstream migratory forms, such as 1+ smolts regularly exhibit increases in plasma T4, T3, cortisol, GH, and prolactin levels during the period of smoltification and through downstream migration (e.g., Dickhoff et al. 1997). Among these hormones, cortisol significantly induced the occurrence of downstream swimming behavior in 1+ smolts (Munakata et al. 2007; Fig. 22). Based on these results, the initiation of seaward (downstream) migration in masu salmon seems to be controlled competitively by sex steroid hormones (sexual maturation in rivers) and cortisol (metamorphosis of smoltification: preparation of marine life).

Since cortisol is an smoltification-inducing factor (Hirano 1991; McCormick 2001), it has been suggested that the levels innately increase with the progression of smoltification. On the other hand, it was inferred that socio-environmental factors, such as intra-, or inter-specific interactions acting as stressors, cause acute and/or chronic plasma cortisol elevations in some teleosts (e.g., salmonids) (Schreck 2000). Previous investigations also discovered that plasma cortisol levels in 1+ masu salmon smolts exhibited considerable fluctuations within one day and among different sampling dates in the Kesen River, in northern Honshu (Munakata et al. unpublished data). These facts suggest that some environmental factors, such as temperature, cause or regulate the occurrence of smoltification and subsequent downstream swimming behavior via elevations of plasma cortisol levels.

If smoltification and downstream swimming behavior in 1+ masu salmon migrants are caused or regulated partly by some environmental factors via plasma cortisol elevations, it is further hypothesized that the differentiation from parr (non-migrants) to smolts (migrants) is influenced by the percipiency of environmental factors and concomitant cortisol elevations. According to Machidori and Kato (1984) and Kiso (1995), it is clear that 0+ precocious males grow faster than immature parr (e.g., smolt migrants) of the same age during 0+ summer, a half year prior to the smoltification period. Because growth and the following sexual maturation of non-migrants appear to be supported partly by their active foraging behaviors and territorial aggressiveness (e.g., Nakano 1995), it is hypothesized that intra-specific interactions, such as territorial aggressiveness and other concomitant phenomena (e.g., hunger, delay of growth, etc.), play key roles in regulating the transformation from non-migrants (parr) to migrants (smolts). As a result, non-migratory forms, such as 1+ precocious males, and migrants, such as 1+ smolts, are eventually reciprocally balanced in some rivers (see Fig. 27).


Fig. 27. Diagrammatic representations of (a) life histories of anadromous, amphidromous, and catadromous fish (modified from Gross 1987) and (b) differences in lifecycle of masu salmon (Oncorhynchus masou) among different regions. In (a) (e.g., northern hemisphere), anadromy and catadromy exceed in northern temperate and southern tropic areas, respectively, while amphidromy are frequent between these areas. In (b), proportion of non-migratory and migratory (e.g., smolts) forms increased in southern (e.g., Kyushu) and northern (e.g., Hokkaido) Japanese streams, respectively, maybe by the differences in productivity among different areas. Also, spawning and downstream migratory period in masu salmon become later in southern and northern Japan, respectively (Machidori and Kato 1984).

page top


5-2. Sub-types of non-migratory and migratory forms

In masu salmon, it has been noted that non-migrants such as precocious male parr and migrants such as 1+ smolts appear in most rivers, and the two forms can be distinguished by their diagnostic characteristics, such as appearance and increasing plasma sex steroid hormone levels (Fig. 4). In masu salmon, however, it has also been recognized that there are phenotypes, which exhibit intermediate migratory patterns between representative non-migrants and migrants (Kiso 1995) (Fig. 26, Table 7). For example, in a significant numbers of rivers, there are so-called "immature parr non-migrants"—some females and a small number of males (Kiso 1995), that live continuously in their natal rivers as do 1+ precocious males (Fig. 26, Table 7). Most of these 1+ non-migrants are considered to be immature parr, based on their appearance (Kiso 1995) and plasma sex steroid hormone levels (Munakata et al. unpublished data).


Table 7. Appearance of frequency and migratory pattern (area) of non-migratory and migratory forms of masu salmon (Oncorhynchus masou) that inhabit streams along Sanriku coast in northern Honshu and its plasma sex steroid hormone levels, gonad somatic index (GSI), and gonadal development and growth stages prior to and during the period of downstream migration, based on Kiso (1995). Shades indicate the potential physiological factors that inhibit or modulate the occurrence of downstream migratory behavior.

page top


In masu salmon, moreover, there are some 1+ downstream migrants, which are identified as smolts by their appearance, but do not travel along migratory routes to the offshore seas, likewise the representative smolt migrants (Machidori and Kato 1984; Kiso 1995) (Fig. 26). For instance, some of these migrants migrate further downstream than the 1+ precocious males and non-migratory 1+ parr. However, the majority of these fish do not enter the sea and instead stay in the mid through lower part of their natal rivers from spring onward, then move upward in the rivers through summer and autumn (Fig. 26). These fish exhibit a silvery body color and low plasma sex steroid hormone levels as do representative smolt migrants, but their body size and CF values tend to be high (Kiso 1995). Therefore, these sub-types of migratory forms are referred to as "pseudo smolts" and "regressive smolts" (Kiso 1995). Although there is no clear difference in the appearance between the "pseudo smolts" and "regressive smolts" during the downstream migratory period, "pseudo smolts" are more likely to reside in the middle to upper reaches of rivers as do the precocious males and the immature parr non-migrants, whereas "regressive smolts" display more distinct downstream migration and will reach the lower part of their natal rivers (Kiso 1995).

Besides such sub-types of migratory forms, there are also other migratory forms considered to be "coastal smolt migrants" that will migrate near the coastal seas between their natal rivers and the Sea of Okhotsk (Machidori and Kato 1984; Kiso 1995). There is also no clear difference in the appearance between the "regressive smolts" and "coastal smolt migrants" (Kiso 1995). Regularly, however, body size (25 to 40 cm in BL) of the "coastal migrants" during the upstream migratory period is larger and smaller than those of "regressive smolts" and "representative smolt migrants", respectively, mainly because of the short migration (less than a year) period in the sea (Kiso 1995).

In masu salmon, there is an additional type of non-migrants in which BL and BW are considerably lower than those of other non-migrants and migrants. According to Kiso (1995), such small sized non-migratory masu salmon can be considered to be a "poor growth fish", which do not differentiate into either precocious parr or smolt migrants during the age of 1+, indicating that not only non-migrants but also smolts migrants need to grow before they initiate the smoltification. Generally, the "poor growth fish" seems to differentiate into non-migratory parr or migratory smolts in another year, mainly at age 2+ (Kiso 1995) (Fig. 26).

Such variations, especially among migratory behaviors, may be influenced heavily by environmental and physiological factors. In Japanese streams, the proportions of "1+ immature parr non-migrants" tend to increase in the southern regions when compared to the northern regions, which is also a trend observed for 1+ precocious males (Fig. 27). In Japan, therefore, these two types of masu salmon are commonly called "yamame" in Japanese. Although the GSI values and plasma sex steroid hormone levels in 1+ immature parr non-migrants are considerably lower than those of the precocious males during spring (Kiso 1995; Munakata et al. unpublished data), the ovarian development stage in 1+ immature female non-migrants is the "yolk vesicle stage", while most 1+ immature female smolt migrants are in the "early peri-nucleolus stage" (Kiso 1995) (Table 7). It is thus indicated that some of the immature non-migrants progress gonadal maturation in rivers, though their sex steroid hormone levels are low.

The existence of such non-migrants indicates that stream residency of non-migrants is regulated not only by high plasma sex steroid hormone levels, but also by other physiological factors. In general, most salmonids exhibit increases in hypothalamic and pituitary hormones such as GnRH, follicle stimulating hormone (FSH), and LH, prior to the elevation in plasma sex steroid hormones to stimulate gonadal maturation after the spring (e.g., Amano et al. 1998; Munakata et al. 2000b). In masu salmon "pseudo smolts" and "regressive smolts", it is also important to note that the GSI values and/or ovarian development stages were slightly higher than those in the 1+ smolt migrants (Table 7). To account for these phenomena, it is hypothesized that some sex hormones other than the sex steroid hormones also play some roles in inhibiting the occurrence of downstream swimming behavior in immature non-migratory forms.

It is generally thought that salmonids initiate gonadal maturation after they have attained sufficient growth (Nordeng 1983; Kiso 1995). Therefore, it appears likely that not only sex hormones but also other hormones such as GH modulate the occurrence of downstream swimming behavior and smoltification, depending on their growth phase. Until now, however, such a hypothesis is far from being established, and which physiological factors are actually involved in the occurrence of downstream swimming behaviors is not fully understood. This topic clearly needs further investigation.

5-3. Variation of migratory behavior in salmonids

In masu salmon, it becomes apparent that some strains exhibit varieties of lifecycle between the representative non-migratory and migratory forms. Also, it is speculated that these varieties of non-migratory and migratory patterns are sequentially regulated by some physiological factors depending on the gonadal maturation and/or growth stages, prior to and during the downstream migratory period (Table 7).

Among the four salmonid genera, there also are varieties of non-migratory and migratory forms that resemble those lifecycles observed in masu salmon (Groot and Margolis 1991; Thorpe 1994; Quinn 2005) (Fig. 1). Considering the hypothesis that salmonids are of a freshwater origin, it is acceptable to think that the evolutionally-ancient genera such as the genus Hucho and Salvelinus remain to show tendency depending on freshwater life, and spawn in the rivers as do the masu salmon non-migrants, whereas evolutionally new genera, such as the genus Salmo and Oncorhynchus evolved to rely more heavily on ocean life (seaward migration), as do the masu salmon migrants (Figs. 3, 26). If the migratory patterns of masu salmon can be considered as an "epitomization" of the variations of salmon migration, it may be hypothesized that the occurrence of migratory behavior for a major part of the salmonids is also controlled by their gonadal maturation and/or growing stages in the rivers, which will require further investigations for validation.

5-4. Driving force of migration from rivers to the sea—Why do salmonids migrate?—

According to Gross (1987), it is thought that the migration (anadromy) of salmonids evolved in relation to the availability of food (or productivity) between the rivers and the sea (Gross 1987) (Fig. 27). Concisely, it is thought that productivity in the sea is higher than that in the rivers in the northern hemisphere regions, whereas contrasting patterns are found in the southern tropic regions. This also indicates that there are gradual variations in the productivities of the rivers and the sea along the latitude gradient within each of the hemispheres.

Actually, in masu salmon, proportions of "precocious male non-migrants" and "immature non-migratory parr" are typically higher in the southern streams (i.e., Kyushu) than those in northern regions (i.e., Hokkaido) (Machidori and Kato 1984) (Fig. 27). On the other hand, the proportion of representative downstream migratory smolts is higher in northern streams than in the southern ones (Machidori and Kato 1984) (Fig. 27). Considering a research hypothesis that the occurrence of non-migrants and migrants (smolts) are modulated by their gonadal maturation and/or growth stages, which are influenced by the productivity of the rivers, such differences in the proportions of non-migrants and migrants seem to be shaped by the gradual changes of productiveness in the rivers throughout different regions.

Again, Gross (1987) has indicated that the seaward migration of salmonids is induced evolutionally by the differences in productivity between the rivers and the sea in high latitude areas of the northern hemisphere. At the mouth of the rivers, however, there is a clear boundary between the fresh and salt (sea) waters, and it is still unclear as to why the ancestral form of salmonids (freshwater origin) crossed over the osmotic boundary and discovered the favorable feeding environments (higher productivity) in the sea.

In this monograph, it was shown that the dominant non-migratory forms of masu salmon regularly occupy focal foraging areas (see Subsection 2-3). Accordingly, it is suggested that most of the downstream migratory behavior in migrants is caused by a reduction in food availability in the natal rivers, as shown by Gross (1987), but more specifically, one of the important and direct factors that cause the downstream migration from the favorable habitat is the intra-specific interactions between the dominant non-migrants and subordinate migrants, depending on their stock density, or other environmental stressors, such as changes in temperature, flow, water quality, and photoperiodicity.

If that is the case, it is easier to understand why some of the migratory forms that could not stay in their focal territories ultimately crossed the boundary between the rivers and the sea. Based on these phenomena, the high productivity of the sea is considered to be less of a causal factor of the migration than a "refuge" for the immature salmonids that perform the downstream migratory behavior.

5-5. Evolution of migratory behavior in salmonids

Most of the juveniles of pink and chum salmon, which are considered evolutionarily new species, exhibit long distance migration, about six months after their hatching (see Fig. 1). Therefore, it is suggested that evolutionarily new species are more likely to depend on marine life when compared to the older salmonid species (Gross 1987). Based on the variations of migratory behavior in salmonids, and the physiological control mechanisms of masu salmon and some other species, however, these patterns in migratory behavior are an extension of the variations of the salmon migrations. Considering these phenomena, it is acceptable to think that the majority of the maturing salmon continue to aspire to spawn in their natal rivers, at the end of their lifecycle. That is, the seaward migration may be undertaken as a subsidiary choice in the lifecycle of most salmonids. Previously, non-migratory forms of masu salmon had been considered to be a "land-locked population", and worse still the precocious male parr within the entire salmonid species were generally considered as a "biological mistake", according to Gross (1987). Based on the migratory patterns and physiological control mechanism of migratory behaviors in masu salmon, however, it is apparent that these types of fish are so-called "reversions" and they represent the ancestral forms of salmonids that matured and spawned in their natal rivers.

5-6. Conservation implications for masu salmon and their habitats

1) Numbers (biomass) of wild masu salmon—both "Yamame" and "Sakura masu" are continuously decreasing in a number of rivers (e.g., Kato 1991).

2) Because the non-migrants and migrants diverge depending on their sexual maturity and growth stages, which are supported by the productivity in their natal rivers, both non-migrant (yamame) and migratory (sakura masu) forms can be increased by improving the productivity of the rivers. In northern streams, carcasses of salmonids which migrate back from the sea are considered to be an important resource which plays a primary role in the increase of productivity in rivers (JoAnna and Richard 2006). Without consideration of such implications, the stock management program of masu salmon would become an insufficient exercise.

3) Migratory behaviors seem to be regulated by both physiological and environmental factors. As a result, it is important to prevent artificial physiological and environmental disruptions that potentially become stressors and influence the migratory behaviors. For example, we need to focus our attentions not only on the direct impacts of dam constructions, but also on the concomitant modifications in flow, temperature, and turbidity, which all have a natural cycle critical to fish.

4) Since most of the 2+ masu salmon migrant smolts that migrate from the sea will stay in the deeper areas of the mid reaches in their natal rivers throughout the summer, where they can avoid predation and sudden changes in water flow and temperature, the conservation of habitat diversity not only in upper reaches (spawning ground), but also in entire rivers is essential.

page top


Acknowledgments

This study was supported partly by research fellowships from the Japan Society for the Promotion of Science and the Saito Houonkai Research Grant. I am grateful to Prof. Katsumi Aida, for providing the opportunity to write this monograph. I thank Dr. Shoji Kitamura, Dr. Kazumasa Ikuta, Dr. Masafumi Amano, Dr. Makito Kobayashi, Dr. Takashi Yada, Dr. Hidenobu Yambe, Dr. Carl Schreck, Dr. Hiram Li, and Dr. David Noakes for their input and for their open discussion of many of these investigations. Mr. Toshio Shikama and Mr. Hidefumi Nakamura helped in part of experiments. I am also grateful to Dr. Hiram Li, Mr. Ralph Lampman, and Mr. Aalon Brock for reading the manuscript and providing useful advice, and Dr. Tsukasa Fukushi, Dr. Nobuharu Goto, Dr. Kimiharu Ishizawa, Dr. Ryusaku Deguchi and Mr. Hiroki Suzuki for their conceptual support during the research. I would also like to thank Mr. Akira Shishido for teaching and helping with the masu salmon sampling in the Kesen River from 2004 through 2009.

page top


References

Aida K, Kato T, Awaji M. Effects of castration on the smoltification of precocious male masu salmon Oncorhynchus masou. Nippon Suisan Gakkaishi 1984; 50: 565–571.

Amano M, Aida K, Okumoto N, Hasegawa Y. Changes in salmon GnRH and chicken GnRH-II contents in the brain and pituitary, and GTH II contents in the pituitary in female masu salmon, Oncorhynchus masou, from hatching through ovulation. Zool. Sci. 1992; 9: 375–386.

Amano M, Aida K, Okumoto N, Hasegawa Y. Changes in levels of GnRH in the brain and pituitary and GTH in the pituitary in female masu salmon, Oncorhynchus masou, from hatching to maturation. Fish. Physiol. Biochem. 1993; 11: 233–240.

Amano M, Hyodo S, Kitamura S, Ikuta K, Suzuki Y, Urano A, Aida K. Salmon GnRH synthesis in the preoptic area and the ventral telencephalon is activated during gonadal maturation in female masu salmon. Gen. Comp. Endocrinol. 1995; 99: 13–21.

Barton BA, Schreck CB. Influence of acclimation temperature on internal and carbohydrate stress responses in juvenile chinook salmon (Oncorhynchus tshawytscha). Aquaculture 1987; 62: 299–310.

Berglund I, Lundqvist H, Fangstan H. Downstream migration of immature salmon (Salmo salar) smolts blocked by implantation of the androgen 11-ketoandrostendione. Aquaculture 1994; 121: 269–276.

Boeuf G. Salmonid smolting: a pre-adaptation to the oceanic environment. In: Rankin GC and Jenson GB (eds.). Fish Ecophysiology. Chapman and Hall. 1994.

Boeuf G, LeBail PY, Prunet P. Growth hormone and thyroid hormones during Atlantic salmon, Salmo salar L., smolting, and after transfer to seawater. Aquaculture 1989; 82: 257–268.

Boeuf G, Marc AM, Prunet P, Bail PYL, Smal J. Stimulation of parr-smolt transformation by hormonal treatment in Atlantic salmon (Salmo salar L.). Aquaculture 1994; 121: 195–208.

Dickhoff WW, Beckman BR, Larsen DA, Duan C, Moriyama S. The role of growth in endocrine regulation of salmon smoltification. Fish Physiol. Biochem. 1997; 17: 231–236.

page top


Ewing RD, Barratt D, Garlock D. Physiological changes related to migration tendency in rainbow trout (Oncorhynchus mykiss). Aquaculture 1994; 121: 277–287.

Frantzen M, Johnsen HK, Mayer I. Gonadal development and sex steroids in a female Arctic charr brood stock. J. Fish Biol. 1997; 51: 697–709.

Fujioka Y, Fushiki S, Tagawa M, Ogasawara T, Hirano T. Downstream migratory behavior and plasma thyroxine levels of Biwa salmon, Oncorhynchus rhodurus. Nippon Suisan Gakkaishi 1990; 56: 1773–1779.

Giannico RG, Hinch SG. The effect of wood and temperature on juvenile coho salmon winter movement, growth, density and survival in side-channels. River Res. Applic. 2003; 19: 219–231.

Grau EG, Dickhoff WW, Nishioka RS, Bern HA, Folmar LC. Lunar phasing of the thyroxine surge preparatory to seaward migration of salmonid Fish. Sci. 1981; 211: 607–609.

Groot C, Margolis L (eds.). Pacific Salmon Life Histories. UBC Press, British Columbia, Vancouver. 1991.

Gross M. Evolution of diadromy in fishes. Am. Fish. Soc. Symp. 1987; 1: 14–25.

Heard RH. Life history of pink salmon (Oncorhynchus Gorbuscha). In: Groot C, Margolis L (eds.). Pacific Salmon Life Histories. UBC Press, British Columbia, Vancouver. 1991; 119–230.

Hirano T. Endocrine control of osmoregulation in migratory fishes. In: Mauchline J, Nemoto T (eds.). Marine Biology: Its Accomplishment and Future Prospect. Hokusen-sha, Japan. 1991, 3–14.

Hoar WS. Smolt transformation: evolution, behavior, and physiology. J. Fish. Res. Bd. Canada. 1976; 33: 1233–1252.

page top


Hoar WS. The physiology of smolting salmonids. In: Hoar WS, Randall DJ (eds.). Fish Physiology, Vol. II B. Academic Press, New York. 1988; 275–343.

Hutchison MJ, Iwata M. Effect of thyroxine on the decrease of aggressive behaviour of four salmonids during the parr-smolt transformation. Aquaculture 1998; 168: 169–175.

Ikuta K. Effects of steroid hormones on migration of salmonid fishes. Bull. Natl. Inst. Aquacult. Suppl. 1994; 2: 23–27.

Ikuta K, Aida K, Okumoto N, Hanyu I. Effects of thyroxine and methyletestosterone on smoltification of masu salmon (Oncorhynchus masou). Aquaculture 1985; 45: 289–303.

Ikuta K, Aida K, Okumoto N, Hanyu I. Effects of sex steroids on the smoltification of masu salmon, Oncorhynchus masou. Gen. Comp. Endocrinol. 1987; 65: 99–110.

Iwata M. Downstream migratory behavior of salmonids and its relationship with cortisol and thyroid hormones: A review. Aquaculture 1995; 135: 131–139.

Iwata M. Downstream migratory behaviors and endocrine control of salmonid fishes. Bull. Natl. Res. Inst. Aquacult. 1996; Suppl. 2: 17–21.

Iwata M, Yamauchi K, Nishioka RS, Lin R, Bern HA. Effects of thyroxine, growth hormone and cortisol on salinity preference of juvenile coho salmon (Oncorhynchus kisutch). Mar. Behav. Physiol. 1990; 17: 191–201.

JoAnna LL, Richard WM. Influence of marine-derived nutrients from spawning salmon on aquatic insect communities in southeast Alaskan streams. Oikos 2006; 113(2): 334–343.

Johnsson JI, Petersson E, Jönsson E, Björnsson BTh, Järvi T. Domestication and growth hormone alter anti-predator behavior and growth patterns in juvenile brown trout, Salmo trutta. Can. J. Fish. Aquat. Sci. 1996; 53(7): 1546–1554.

page top


Jönsson E, Johnsson JI, Björnsson BTh. Growth hormone increases predation exposure of rainbow trout. Proc. Roy. Soc. Lond. Ser. B. 1996; 263: 647–651.

Kagawa H, Young G, Nagahama Y. Estradio-17β production in isolated amago salmon (Oncorhynchus rhodurus) ovarian follicles and its stimulation by gonadotropins. Gen. Comp. Endocrinol. 1982a; 47: 361–365.

Kagawa H, Young G, Adachi S, Nagahama Y. Estradiol-17β production in amago salmon (Oncorhynchus rhodurus) ovarian follicles: role of the thecal and granulosa cells. Gen. Comp. Endocrinol. 1982b; 47: 440–448.

Kagawa N, Mugiya Y. Exposure of goldfish (Carassius auratus) to bluegills (Lepomis macrochirus) enhances expression of stress protein 70 mRNA in the brains and increases plasma cortisol levels. Zool. Sci. 2000; 17: 1061–1066.

Kato F. Life histories of masu and amago salmon (Oncorhynchus masou and Oncorhynchus rhodurus). In: Groot C, Margolis L (eds.). Pacific Salmon Life Histories. UBC Press, British Columbia, Vancouver. 1991; 448–520.

Kelsey DA, Schreck CB, Congleton JL, Davis LE. Effects of juvenile steelhead on juvenile chinook salmon behavior and physiology. Transactions of the American Fisheries Society 2002; 131: 676–689.

Kiso K. Polymorphism of life form in masu salmon (Oncorhynchus masou) in the rivers of southern Sanriku District, Honshu, Japan. Bull. Inst. Zool. Academia Sinica 1990; 29(3): 27–39.

Kiso K. The life history of masu salmon Oncorhynchus masou originated from rivers of the Pacific coast of northern Honshu, Japan. Bull. Natl. Res. Inst. Fish. Sci. 1995; 7: 1–188 (in Japanese with English abstract).

Kiso K, Matsumiya Y. Growth of the fluviatile form masu salmon Oncorhynchus masou in rivers of southern Sanriku District, Honshu, Japan. Nippon Suisan Gakkaishi 1992; 58: 9–13.

Kobayashi M, Aida K, Hanyu I. Gonadotropin surge during spawning in male goldfish. Gen. Comp. Endocrinol. 1986; 62: 70–79.

page top


Kobayashi M, Aida K, Hanyu I. Hormone changes during the ovulatory cycle in goldfish. Gen. Comp. Endocrinol. 1988; 69: 301–307.

Liley NR, Fostier A, Breton B, Tan ES. Endocrine changes associated with spawning behavior and social stimuli in a wild population of rainbow trout (Salmo gaidneri). Gen. Comp. Endocrinol. 1986; 62: 157–167.

Lou SW, Aida K, Hanyu I, Sakai K, Nomura M, Tanaka M, Tazaki S. Endocrine profiles in the female of a twice-annually spawning strain of rainbow trout. Aquaculture 1984; 43: 13–22.

Machidori S, Kato F. Spawning populations and marine life of masu salmon Oncorhynchus masou. Int. North Pacific Fisheries Commission 1984; 43: 1–138.

Mayer I, Liley N, Borg B. Stimulation of spawning behavior in castrated rainbow trout (Oncorhynchus mykiss) by 17α,20β-dihydroxy-4-pregnene-3-one, but not by 11-ketoandrostendione. Hormones and Behavior 1994; 28: 181–190.

McCormick SD. Endocrine control of osmoregulation in teleost fish. American Zool. 2001; 41: 781–794.

Miwa S, Inui Y. Inhibitory effects of of 17α-methyle testosterone and estradiol-17β on smoltification of sterilized amago salmon (Oncorhynchus rhodurus). Aquaculture 1986; 53: 21–39.

Mizuno S, Ura K, Onodera Y, Fukada H, Misaka N, Hara A, Adachi S, Yamauchi K. Changes in transcript levels of gill cortisol receptor during smoltification in wild masu salmon, Oncorhynchus masou. Zool. Sci. 2001; 18: 853–860.

Munakata A, Kobayashi M. Endocrine control of sexual behavior in teleost fish. Gen. Com. Endocrinol. 2010; 165: 456–468.

Munakata A, Amano M, Ikuta K, Kitamura S, Aida K. Growth of wild honmasu salmon parr in a tributary of Lake ChuzenjiGrowth masu salmon, Oncorhynchus masou. Fish. Sci. 1999; 65(6): 965–966.

page top


Munakata A, Björnsson BTh, Jönsson E, Amano M, Ikuta K, Kitamura S, Kurokawa T, Aida K. Post-release adaptation processes of hatchery-reared honmasu salmon parr. J. Fish Biol. 2000a; 56: 163–172.

Munakata A, Amano M, Ikuta K, Kitamura S, Aida K. Inhibitory effects of testosterone on downstream migratory behavior in masu salmon, Oncorhynchus masou. Zool. Sci. 2000b; 17: 863–870.

Munakata A, Amano M, Ikuta K, Kitamura S, Aida K. The involvement of sex steroid hormones in downstream and upstream migratory behavior of masu salmon. Comp. Biochem. Physiol. Part B 2001a; 129: 661–669.

Munakata A, Amano M, Ikuta K, Kitamura S, Aida K. The effects of testosterone on upstream migratory behavior in masu salmon, Oncorhynchus masou. Gen. Comp. Endocrinol. 2001b; 122: 329–340.

Munakata A, Amano M, Ikuta K, Kitamura S, Aida K. Sex steroids control migration of masu salmon. Fish. Sci. 2002; 68 Suppl. 1: 49–52.

Munakata A, Amano M, Ikuta K, Kitamura S, Aida K. Effects of growth hormone and cortisol on the downstream migratory behavior in masu salmon, Oncorhynchus masou. Gen. Comp. Endocrinol. 2007; 150: 12–17.

Munakata A, Amano M, Ikuta K, Kitamura S, Aida K. Involvement of sex steroids and thyroid hormones in upstream and downstream behaviors in masu salmon, Oncorhynchus masou. Aquaculture 2012a; 362–363: 158–166.

Munakata A, Amano M, Ikuta K, Kitamura S, Aida K. Involvement of sex steroids, luteinizing hormone and thyroid hormones in upstream and downstream migratory behaviors in land-locked sockeye salmon Oncorhynchus nerka. Fish. Sci. 2012b; 78: 81–90.

Murata S, Takasaki N, Saitoh M, Okada N. Determination of the phylogenic relationships among Pacific salmonids by using short interspersed elements (SINEs) as temporal landmarks of evolution. Proc. Nat. Acad. Sci. USA. 1993; 90: 6995–6999.

Nagae M, Fuda H, Hara A, Saneyoshi M, Yamauchi K. Changes in serum concentrations of immunoglobulin M (IgM), cortisol and thyroxine (T4) during smoltification in the masu salmon Oncorhynchus masou. Fish. Sci. 1994; 60(2): 241–242.

page top


Nagahama Y. Mechanism of gonadotropin control of steroidogenesis in teleost gonads. In: Gunma Symp. Endocrinol. Center for Academic Publication, Tokyo, Japan. 1984; 21: 167–182.

Nagahama Y. Gonadotropin action on gametogenesis and steroidogenesis in teleost gonads. Zool. Sci. 1987a; 4(2): 209–222.

Nagahama Y. 17α,20β-Dihydroxy-4-pregnen-3-on a teleost maturation-inducing hormone. Develop. Growth and Differ. 1987b; 29(1): 1–12.

Nakano S. Individual differences in resource use, growth and emigration under the influence of a dominance hierarchy in fluvial red-spotted masu salmon in a natural habitat. J. Animal Ecol. 1995; 64: 75–84.

Nakano S, Furukawa-Tanaka T. Intra- and interspecific dominance hyerarchy and variation in foraging tactics of two species of stream-dwelling chars. Ecol. Res. 1994, 9: 9–20.

Nakano S, Kachi T, Nagoshi M. Restricted movement of the fluvial form of red-spotted masu salmon, Oncorhynchus masou rhodurus, in a mountain stream, central Japan. Japanese J. Ichthyol. 1990; 37(2): 158–163.

Neave F. The origin and speciation of Oncorhynchus. Trans. Roy. Soc. Canada 1958; 551: 25–39.

Nichols DJ, Weisbart M. Plasma cortisol concentrations in Atlantic salmon, Salmo salar: Episodic variations, diurnal changes, and short term response to adrenocorticotrophic hormone. Gen. Comp. Endocrinol. 1984; 56: 169–176.

Norden CR. Comparative osteology of representative salmonid fishes, with particular reference to the grayling (Thymallusus arcticus) and its phylogeny. J. Fish. Res. Bd. Can. 1961, 18: 679–791.

Nordeng H. Solution to the "char problem" based on Arctic char (Salvelinus alpinus) in Norway. Can. J. Aquat. Sci. 1983; 40: 1372–1387.

page top


Ojima D, Iwata M. Central administration of growth hormone-releasing hormone triggers downstream movement and schooling behavior of chum salmon (Oncorhynchus keta) fry in an artificial stream. Comp. Biochem. Physiol. Part A. 2009; 152: 293–298.

Oshima M. Ecological study on the masu of the Taiko River. Botany and Zoology 1936; 4: 1–13.

Pankhurst NW, Kraak GVD. Evidence that acute stress inhibits ovarian steroidogenesis in rainbow trout in vivo, through the action of cortisol. Gen. Comp. Endocrinol. 2000; 117: 225–237.

Prat F, Sumpter JP, Tyler CR. Validation of radioimmunoassay for two salmon gonadotropins (GTH I and GTH II) and their plasma concentrations throughout the reproductive cycle in male and female rainbow trout (Oncorhynchus mykiss). Biol. Reproduction. 1996; 54: 1375–1382.

Prunet P, Boeuf G, Bolton JP, Young G. Smoltification and seawater adaptation in Atlantic salmon (Salmo salar): Plasma prolactin, growth hormone, and thyroid hormones. Gen. Comp. Endocrinol. 1989; 74: 355–364.

Quinn, TP. The Behavior and Ecology of Pacific Salmon and Trout. University of Washington Press, Seattle. 2005. 378 pp.

Redding JM, Schreck CB, Everest FH. Physiological effects on coho salmon and steelhead of exposure to suspended solids. Transactions American Fisheries Society 1987; 116: 737–744.

Sandercock FK. Life history of coho salmon (Oncorhynchus kisutch). In: Groot C, Margolis L (eds.). Pacific Salmon Life Histories. UBC Press, British Columbia, Vancouver. 1991; 396–445.

Sano S. Changes in masu salmon during the no-feeding season. Salmon J. 1947; 44: 9–14.

Sato A, Ueda H, Fukaya M, Kaeriyama M, Zohar Y, Urano A, Yamauchi K. Sexual differences in homing profiles and shortening of homing duration by gonadotropin-releasing hormone analog implantation in lacustrine sockeye salmon (Oncorhynchus nerka) in Lake Shikotsu. Zool. Sci. 1997; 14: 1009–1014.

page top


Satou M, Oka Y, Kusunoki M, Matsushima T, Kato M, Fujita I, Ueda K. Telencephalic and preoptic areas integrate sexual behavior in hime salmon (landlocked red salmon, Oncorhynchus nerka): results of electrical brain stimulation experiments. Physiol. Behav. 1984; 33: 441–447.

Schreck CB. Accumulation and long-term effects of stress in fish. In: Moberg G, Mench J (eds.). The Biology of Animal Stress. C.A.B. International Press, Wallingford, U.K. 2000; 147–158.

Slater CH, Schreck CB, Swanson P. Plasma profiles of the sex steroids and gonadotropins in maturing female spring chinook salmon (Oncorhynchus tshawytscha). Comp. Biochem. Physiol. 1994; 109A: 167–175.

Swanson P. Salmon gonadotropins: Reconciling old and new ideas. In: Scott et al. (eds.). Proc. Fourth Int. Symp Reproductive Physiology of Fish. University of East Anglia, Norwich. 1991; 2–7.

Thorpe JE. An alternative view of smolting in salmonid. Aquaculture 1994; 121: 105–113.

Truscott B, Idler DR, So YP, Walsh JM. Maturation steroids and gonadotropin in upstream migratory sockeye salmon. Gen. Comp. Endocrinol. 1986; 62: 99–110.

Tsukamoto K, Aida K, Otake T. Plasma thyroxine concentration and upstream migratory behavior of juvenile ayu. Nippon Suisan Gakkaishi 1988; 54(10): 1687–1693.

Ueda H, Hiroi O, Hara A, Yamauchi K, Nagahama Y. Changes in serum concentrations of steroid hormones, thyroxine, and vitellogenine during spawning migration of the chum salmon, Oncorhynchus keta. Gen. Comp. Endocrinol. 1984; 53: 203–211.

Utoh H. Study of the mechanism of differentiation between the stream resident form and the seaward migratory form in masu salmon, Oncorhynchus masou Brevoort, I. Growth and gonadal maturity of precocious masu salmon parr. Bull. Fac. Fish. Hokkaido Univ. 1976; 26: 321–326 (in Japanese).

Utoh H. Study of the mechanism of differentiation between the stream resident form and the seaward migratory form in masu salmon, Oncorhynchus masou Brevoort, II. Growth and sexual maturity of precocious masu salmon parr (2). Bull. Fac. Fish. Hokkaido Univ. 1977; 28: 66–73 (in Japanese).

page top


Varnavsky VS, Sakamoto T, Hirano T. Effects of premature seawater transfer and fasting on plasma growth hormone levels of yearling coho salmon (Oncorhynchus kisutch) parr. Aquaculture 1995; 135: 141–145.

Yada T, Nagae M, Moriyama S, Azuma T. Effects of prolactin and growth hormone on plasma immunoglobulin M levels of hypophysectomized rainbow trout, Oncorhynchus mykiss. Gen. Comp. Endocrinol. 1999; 115: 46–52.

Yada T, Azuma T, Hyodo S, Hirano T, Grau EG, Schreck CB. Differential expression of corticosteroid receptor genes in rainbow trout (Oncorhynchus mykiss) immune system in response to acute stress. Can. J. Fish Aqua Sci. 2007; 64(10): 1382–1389.

Yamauchi K, Koide N, Adachi S, Nagahama Y. Changes in seawater adaptability and blood thyroxine concentrations during smoltification of the masu salmon, Oncorhynchus masou, and the amago salmon, Oncorhynchus rhodurus. Aquaculture 1984; 42: 247–256.

Yamauchi K, Ban M, Kasahara N, Izumi T, Kojima H, Harako T. Physiological and behavioral changes occurring during smoltification in the masu salmon, Oncorhynchus masou. Aquaculture 1985; 45: 227–235.

Yambe H, Munakata A, Kitamura S, Aida K, Fusetani N. Methyltestosterone induces male sensitivity to both primer and releaser pheromones in the urine of ovulated female masu salmon. Fish Physiol. Biochem. 2003; 28: 279–280.

Yambe H, Kitamura S, Kamio M, Yamada M, Matsunaga S, Fusetani N. L-Kynurenine, and amino acid identified as a sex pheromone in the urine of ovulated female masu salmon. Proc. Natl. Acad. Sci. USA 2006; 103: 15370–15374.

Young G, Björnsson BTh, Prunet, P, Lin, RJ, Bern, HA. Smoltification and seawater adaptation in coho salmon (Oncorhynchus kisutch): Plasma prolactin, growth hormone, thyroid hormones, and cortisol. Gen. Comp. Endocrinol. 1989; 74: 335–345.

Zydlewski GB, Haro A, McCormick D. Evidence for cumulative temperature as an initiation and terminating factor in downstream migratory behavior of Atlantic salmon (Salmo salar) smolts. Can. J. Fish. Aquat. Sci. 2005; 62: 68–78.

page top


List of Figures

Fig. 1. Schematic drawing that illustrates the diversity of distance covered by non-migratory and migratory forms for four salmonid genera (shown in the order of the evolutional age). In genus Hucho, most fish live continuously in their natal rivers. In genus Salvelinus and Salmo, some fish migrate to the sea after smoltification. In genus Oncorhynchus, most juveniles perform long distance seaward migration for several years. On the other hand, in masu salmon (O. masou), a portion of the fish perform seaward migration for a year after smoltification, while an equivalent portion of them stay in the rivers similar to genera Hucho, Salvelinus, and Salmo.

Fig. 2. Photographs of masu salmon (Oncorhynchus masou). (a) precocious male non-migrants, (b) immature parr non-migrants, (c) pseudo smolt, (d) smolt migrants, and (e) adult smolt migrants that migrated back from the sea.

Fig. 3. Lifecycles of masu salmon (Oncorhynchus masou). In masu salmon, some immature juveniles (migratory form) display the downstream migratory behavior after they have transformed from parr to smolt (smoltification). However, some juveniles (non-migratory form) such as precociously mature males (precocious males) will live continuously in their natal rivers throughout their lifetime. The lifecycle (migratory behavior, seaward migration) of migrants consists of downstream migration, feeding, homing, upstream migration, and spawning. On the other hand, the lifecycle of non-migrants consists of downstream movement within a river, stream residence, upstream movement, and spawning.

Fig. 4. Changes in body length (BL), body weight (BW), condition factor (CF), gonad somatic index (GSI), plasma levels of testosterone (T), 11-ketotestosterone (11-KT), estradiol-17β (E2), progesterone (P), 17α-progesterone (17α-P), 17,20β-dihydroxy-4-pregnene-3-one (DHP), and pituitary hormone luteinizing hormone (LH) in male and female masu salmon. Differences in mean plasma hormone levels among experimental groups were analyzed by one-way analysis of variance (ANOVA) followed by Scheffe's F-test. * and *** indicates significant difference at P < 0.05 and P < 0.001, respectively. Reprinted from Comp. Biochem. Physiol. Part B, 129, Munakata et al., The involvement of sex steroid hormones in downstream and upstream migratory behavior of masu salmon, 661–669, © 2001a, with permission from Elsevier.

Fig. 5. Schematic drawing of experimental raceway. In order to study the roles of sex steroid hormones in downstream behavior (negative rheotaxis), the fish were transferred into the upper pond (2 × 4 × 0.5 m) of a two-step raceway connected to the lower pond (2 × 8 × 0.5 m) through a fishway (20 cm in diameter by 4 m in length made of a polyvinyl chloride (PVC) half-cut pipe (Munakata et al. 2000b). Spring water was supplied into the upper pond. Flow rate (volume) and velocity of the water in the fishway ranged between 10–20 l/s and 70–85 cm/s, respectively. Water temperature fluctuated between 9–10°C. At the downstream edge of the fishway in the lower pond, a net trap (2 × 0.7 × 0.7 m) was placed to capture fish that moved down from the upper pond. An individual experimental fish was identified as a downstream migrant if it moved from the upper pond into the net trap in the lower pond. In order to investigate the effects of sex steroid hormones on the occurrence of upstream behavior, the experimental fish and net trap were transferred into the lower and upper pond, respectively. The frequency of downstream or upstream migrations is expressed as a percentage of the initial fish numbers. Reprinted with permission from Fish. Sci., 78, Munakata et al., Involvement of sex steroids, luteinizing hormone and thyroid hormones in upstream and downstream migratory behaviors in land-locked sockeye salmon Oncorhynchus nerka, 81–90, Fig. 1, © 2012b, The Japanese Society of Fisheries Science.

Fig. 6. (a) Frequency of migrants and non-migrants, plasma levels of (b) testosterone (T), (c) pituitary contents of luteinizing hormone (LH), (d) plasma levels of LH, (e) thyroxine (T4), and (f) triiodothyronine (T3) in control and T 500 μg-treated 1+ masu salmon smolts. Numbers above columns in (a) indicate the number of migrants and non-migrants. Differences in the frequency of downstream behavior from the control group were analyzed by the χ2-test, using StatView version 4.5 software (Abacus Concepts, Inc., California, USA). ** indicates a significant difference at P < 0.01 from the control group. Differences in mean plasma and pituitary hormone levels among experimental groups were analyzed by one-way analysis of variance (ANOVA) followed by Fisher's PLSD. Differing letters represent significant differences at P < 0.05 among all groups. Reprinted with permission from Zoological Science, 17, Munakata et al., Inhibitory effects of testosterone on downstream migratory behavior in masu salmon, Oncorhynchus masou, 863–870, Fig. 1, © 2000b, Zoological Society of Japan.

Fig. 7. (a) Frequency of migrants and non-migrants, plasma levels of (b) testosterone (T), (c) pituitary contents of luteinizing hormone (LH), (d) plasma levels of LH, (e) thyroxine (T4), and (f) triiodothyronine (T3) in controls, T 5 μg, T 50 μg, T 500 μg-treated smolts and precociously mature male 1+ masu salmon. Numbers above columns in (a) indicate the number of migrants and non-migrants. Differences in the frequency of downstream behavior from the control group were analyzed by the χ2-test, using StatView version 4.5 software (Abacus Concepts, Inc., California, USA). * indicates a significant difference at P < 0.05 from the control group. Differences in mean plasma and pituitary hormone levels among experimental groups were analyzed by one-way analysis of variance (ANOVA) followed by Fisher's PLSD. Differing letters represent significant differences at P < 0.05 among all groups. Reprinted with permission from Zoological Science, 17, Munakata et al., Inhibitory effects of testosterone on downstream migratory behavior in masu salmon, Oncorhynchus masou, 863–870, Fig. 2, © 2000b, Zoological Society of Japan.

Fig. 8. (a) Frequency of migrants and non-migrants, plasma levels of (b) testosterone (T), (c) estradiol-17β(E2), (d) 11-ketotestosterone (11-KT), (e) 17,20β-dihydroxy-4-pregnene-3-one (DHP), (f) thyroxine (T4), and (g) triiodothyronine (T3) in controls, T, E2, 11-KT, and DHP 500 μg-treated 1+ masu salmon smolts. Numbers above columns in (a) indicate the number of migrants and non-migrants. Differences in the frequency of downstream behavior from the control group were analyzed by the χ2-test, using StatView version 4.5 software (Abacus Concepts, Inc., California, USA). * and *** indicate significant differences at P < 0.05 and P < 0.001, respectively, from the control group. Differences in mean plasma and hormone levels among experimental groups were analyzed by one-way analysis of variance (ANOVA) followed by Fisher's PLSD. Differing letters represent significant differences at P < 0.05 among all groups. Reprinted from Comp. Biochem. Physiol. Part B, 129, Munakata et al., The involvement of sex steroid hormones in downstream and upstream migratory behavior of masu salmon, 661–669, © 2001a, with permission from Elsevier.

Fig. 9. (a) Frequency of migrants and non-migrants, plasma levels of (b) testosterone (T), (c) thyroxine (T4), and (d) triiodothyronine (T3) in control and T 500 μg-treated 1+ masu salmon smolts. Numbers above columns in (a) indicate the number of migrants and non-migrants. Differences in mean plasma hormone levels among experimental groups were analyzed by one-way analysis of variance (ANOVA) followed by Fisher's PLSD. * indicates a significant difference at P < 0.05 from migrants. Reprinted with permission from Zoological Science, 17, Munakata et al., Inhibitory effects of testosterone on downstream migratory behavior in masu salmon, Oncorhynchus masou, 863–870, Fig. 3, © 2000b, Zoological Society of Japan.

Fig. 10. (a) Frequency of migrants and non-migrants, plasma levels of (b) testosterone (T), (c) pituitary contents of luteinizing hormone (LH), (d) plasma levels of LH, (e) thyroxine (T4), and (f) triiodothyronine (T3) in control and T 500 μg-treated smolts, and precociously mature male 1+ sockeye salmon. In Fig. 10c, unit of Y axis in the control group was ng/pituitary, while those of T-treated and precocious male groups was μg/pituitary. Numbers above columns in (a) indicate the number of migrants and non-migrants. Differences in the frequency of downstream behavior from the control group were analyzed by the χ2-test, using StatView version 4.5 software (Abacus Concepts, Inc., California, USA). * indicates a significant difference at P < 0.05 from the control group. Differences in mean plasma and pituitary hormone levels among experimental groups were analyzed by one-way analysis of variance (ANOVA) followed by Fisher's PLSD. Differing letters represent significant differences at P < 0.05 among all groups. Reprinted with permission from Fish. Sci., 78, Munakata et al., Involvement of sex steroids, luteinizing hormone and thyroid hormones in upstream and downstream migratory behaviors in land-locked sockeye salmon Oncorhynchus nerka, 81–90, Fig. 3, © 2012b, The Japanese Society of Fisheries Science.

page top


Fig. 11. (a) Frequency of migrants and non-migrants, plasma levels of (b) testosterone (T), (c) estradiol-17β (E2), (d) 11-ketotestosterone (11-KT), (e) 17,20β-dihydroxy-4-pregnene-3-one (DHP), (f) thyroxine (T4), and (g) triiodothyronine (T3) in controls, T 500 μg, E2 500 μg, 11-KT 500 μg, and DHP 500 μg-treated 1+ immature masu salmon parr. Numbers above columns in (a) indicate the number of migrants and non-migrants. Differences in the frequency of upstream behavior from the control group were analyzed by the χ2-test, using StatView version 4.5 software (Abacus Concepts, Inc., California, USA). *, **, and *** indicate significant difference at P < 0.05, P < 0.01, and P < 0.001, respectively, from the control group. Differences in mean plasma hormone levels among experimental groups were analyzed by one-way analysis of variance (ANOVA) followed by Fisher's PLSD. Differing letters represent significant differences at P < 0.05 among all groups. Reprinted from Comp. Biochem. Physiol. Part B, 129, Munakata et al., The involvement of sex steroid hormones in downstream and upstream migratory behavior of masu salmon, 661–669, © 2001a, with permission from Elsevier.

Fig. 12. (a) Frequency of migrants and non-migrants, plasma levels of (b) testosterone (T), (c) estradiol-17β Elsevier Science (USA), with permission from Elsevier. (E2), (d) 11-ketotestosterone (11-KT), (e) 17,20β-dihydroxy-4-pregnene-3-one (DHP), (f) thyroxine (T4), and (g) triiodothyronine (T3) in castrated, castrated + T 500 μg, E2 500 μg, 11-KT 500 μg, and DHP 500 μg/ fish-treated groups, and sham-operated 1+ precocious male masu salmon. Numbers above columns in (a) indicate the number of migrants and non-migrants. Differences in the frequency of upstream behavior from the control group were analyzed by the χ2-test, using StatView version 4.5 software (Abacus Concepts, Inc., California, USA). *, **, and *** indicate a significant difference at P < 0.05, P < 0.01, and P < 0.001, respectively from the control group. Differences in mean plasma hormone levels among experimental groups were analyzed by one-way analysis of variance (ANOVA) followed by Fisher's PLSD. Differing letters represent significant differences at P < 0.05 among all groups. Reprinted from Comp. Biochem. Physiol. Part B, 129, Munakata et al., The involvement of sex steroid hormones in downstream and upstream migratory behavior of masu salmon, 661–669, © 2001a, with permission from Elsevier.

Fig. 13. (a) Frequency of migrants and non-migrants, plasma levels of (b) testosterone (T), (c) thyroxine (T4), and (d) triiodothyronine (T3) in controls, T 50 μg, T 500 μg, and T 1000 μg-treated 1+ immature masu salmon. Numbers above columns in (a) indicate the number of migrants and non-migrants. Differences in the frequency of upstream behavior from the control group were analyzed by the χ2-test, respectively, using StatView version 4.5 software (Abacus Concepts, Inc., California, USA). * indicates significant difference at P < 0.05, from the control group. Differences in mean plasma hormone levels among experimental groups were analyzed by one-way analysis of variance (ANOVA) followed by Fisher's PLSD. Reprinted from General and Comparative Endocrinology, 122, Munakata et al., The effects of testosterone on upstream migratory behavior in masu salmon, Oncorhynchus masou, 329–340, © 2001b, with permission from Elsevier.

Fig. 14. (a) Frequency of migrants and non-migrants, plasma levels of (b) testosterone (T), (c) thyroxine (T4), and (d) triiodothyronine (T3) in castrated, cast. + T 50 μg, cast. + T 500 μg-treated groups, and sham-operated 1+ precocious male masu salmon. Numbers above columns in (a) indicate the number of migrants and non-migrants. Differences in the frequency of upstream behavior from the control group were analyzed by the χ2-test, using StatView version 4.5 software (Abacus Concepts, Inc., California, USA). * and *** indicate significant difference at P < 0.05 and P < 0.001, respectively from the control group. Differences in mean plasma hormone levels among experimental groups were analyzed by one-way analysis of variance (ANOVA) followed by Fisher's PLSD. Differing letters represent significant differences at P < 0.05 among all groups. Reprinted from General and Comparative Endocrinology, 122, Munakata et al., The effects of testosterone on upstream migratory behavior in masu salmon, Oncorhynchus masou, 329–340, © 2001b, with permission from Elsevier.

Fig. 15. (a) Frequency of migrants and non-migrants, plasma levels of (b) testosterone (T), (c) pituitary contents of luteinizing hormone (LH), (d) plasma levels of LH, (e) thyroxine (T4), and (f) triiodothyronine (T3) in control and T 500 μg/fish-treated 1+ immature fish, and 1+ precociously mature male sockeye salmon. In (c), unit of Y axis in the control group was ng/pituitary, while those of T-treated and precocious male groups was μg/pituitary. Numbers above columns in (a) indicate the number of migrants and non-migrants. N.S. represents no sample. Differences in the frequency of upstream behavior from the control group were analyzed by the Fisher's exact probability test, using StatView version 4.5 software (Abacus Concepts, Inc., California, USA). * and ** indicate a significant difference at P < 0.05 and P < 0.01 from the control group, respectively. Differences in mean plasma and pituitary hormone levels among experimental groups were analyzed by one-way analysis of variance (ANOVA) followed by Fisher's PLSD. Differing letters represent significant differences at P < 0.05 among all groups. Reprinted with permission from Fish. Sci., 78, Munakata et al., Involvement of sex steroids, luteinizing hormone and thyroid hormones in upstream and downstream migratory behaviors in land-locked sockeye salmon Oncorhynchus nerka, 81–90, Fig. 6, © 2012b, The Japanese Society of Fisheries Science.

Fig. 16. (a) Frequency of migrants and non-migrants, plasma levels of (b) testosterone (T), (c) pituitary contents of luteinizing hormone (LH), (d) plasma levels of LH, (e) thyroxine (T4), and (f) triiodothyronine (T3) in control and T 500 μg/fish-treated 1+ immature masu salmon. Numbers above columns in (a) indicate the number of migrants and non-migrants. N.S. represents no sample. Differences in the frequency of upstream swimming behavior from the control group were analyzed by the Fisher's exact probability test, using StatView version 4.5 software (Abacus Concepts, Inc., California, USA). ** indicates a significant difference at P < 0.01 from the control group. Differences in mean plasma and pituitary hormone levels among experimental groups were analyzed by one-way analysis of variance (ANOVA) followed by Fisher's PLSD. Differing letters represent significant differences at P < 0.05 among all groups. Reprinted from General and Comparative Endocrinology, 122, Munakata et al., The effects of testosterone on upstream migratory behavior in masu salmon, Oncorhynchus masou, 329–340, © 2001b, with permission from Elsevier.

Fig. 17. Frequency of quivering and attending behaviors in the sham-operated, castrated, and castrated + testosterone (T) 500 μg/fish-treated 1+ precocious male masu salmon. Differences in mean frequencies of quivering and attending behaviors among experimental groups were analyzed by one-way analysis of variance (ANOVA) followed by Fisher's PLSD. Differing letters represent significant differences at P < 0.05 among all groups. Reprinted with permission from Fish. Sci., 68, Munakata et al., Sex steroids control migration of masu salmon, 49–52, Fig. 3, © 2002, The Japanese Society of Fisheries Science.

Fig. 18. Male masu salmon showing (a) attending and (b) quivering behaviors against 2+ mature females performing a series of female spawning behaviors.

Fig. 19. Frequency of female digging behavior in the control, testosterone (T) 500 μg/fish treated, and T 1000 μg/fish treated groups. Differences in mean frequency of digging behavior among experimental groups were analyzed by one-way analysis of variance (ANOVA) followed by Fisher's PLSD. Differing letters represent significant differences at P < 0.05 among all groups. Reprinted with permission from Fish. Sci., 68, Munakata et al., Sex steroids control migration of masu salmon, 49–52, Fig. 4, © 2002, The Japanese Society of Fisheries Science.

Fig. 20 Female masu salmon performing digging behavior. (a) 2+ mature females and (b) 1+ immature females treated with testosterone (T) 500 μg/fish.

page top


Fig. 21 Schematic drawing of three-step raceway that consists of upper, middle, and lower ponds. This raceway enables us to quantify upstream and downstream swimming behaviors at the same period. After the experimental fish had been reared in tanks, they were transferred to the middle pond (4 × 2 × 0.5 m) of the raceway. The middle pond was connected to the upper (4 × 2 × 0.5 m) and lower (4 × 2 × 0.5 m) ponds through square holes (50 × 25 cm, thickness 3 cm) which were made on wooden walls. Flow rate (volume) and velocity of the water in the raceway were 10 l/s and 75-85 cm/s. The upstream and downstream migrants are identified when the fish swam from the middle pond to the net traps (2 × 0.7 × 0.7 m) that were located in the upper and lower ponds, respectively. Reprinted from Aquaculture (in press), Munakata et al., Involvement of sex steroids and thyroid hormones in upstream and downstream behaviors in masu salmon, Oncorhynchus masou, © 2012a, with permission from Elsevier.

Fig. 22 Frequency of migrants and non-migrants, plasma levels of oGH and cortisol in the control, oGH 250 μg, cortisol 2 mg, and oGH 250 μg + cortisol 2 mg-treated 1+ masu salmon smolts. Figures beside columns indicate the number of migrants and non-migrants. Differences in the frequency of downstream behavior from the control group were analyzed by the χ2-test, using StatView version 4.5 software (Abacus Concepts, Inc., California, USA). *** indicates a significant difference at P < 0.001 from the control group. Differences in mean plasma hormone levels among experimental groups were analyzed by one-way analysis of variance (ANOVA) followed by Fisher's PLSD. Differing letters represent significant differences at P < 0.05 among all groups. Reprinted from General and Comparative Endocrinology, 150, Munakata et al., Effects of growth hormone and cortisol on the downstream migratory behavior in masu salmon, Oncorhynchus masou, 12–17, © 2007, with permission from Elsevier.

Fig. 23 Frequency of migrants and non-migrants, plasma levels of oGH and cortisol in the control, oGH 250 μg, cortisol 2 mg, and oGH 250 μg + cortisol 2 mg-treated 0+ masu salmon parr. Figures beside columns indicate the number of migrants and non-migrants. Differences in the frequency of downstream swimming behavior from the control group were analyzed by the χ2-test, using StatView version 4.5 software (Abacus Concepts, Inc., California, USA). *** indicates a significant difference at P < 0.001 from the control group. Differences in mean plasma hormone levels among experimental groups were analyzed by one-way analysis of variance (ANOVA) followed by Fisher's PLSD. Differing letters represent significant differences at P < 0.05 among all groups. Reprinted from General and Comparative Endocrinology, 150, Munakata et al., Effects of growth hormone and cortisol on the downstream migratory behavior in masu salmon, Oncorhynchus masou, 12–17, © 2007, with permission from Elsevier.

Fig. 24 (a) Number of fish that exhibited the downstream migratory behavior in the 1+ masu salmon smolts (white column) and the T 500 μg/fish-treated smolts (dark column). (b) date of rainfall during experimental period in May.

Fig. 25 Eight-day running average of the number of upstream migrants in 2+ male and female sockeye salmon in the raceway. Dark and white bars indicate light dark-periods. This graph shows that the majority of 2+ land-locked sockeye salmon exhibit upstream migratory behavior before, during, and after dusk between 16:00 and 22:00. There was no clear difference between sexes. Reprinted with permission from Fish. Sci., 78, Munakata et al., Involvement of sex steroids, luteinizing hormone and thyroid hormones in upstream and downstream migratory behaviors in land-locked sockeye salmon Oncorhynchus nerka, 81–90, Fig. 4, © 2012b, The Japanese Society of Fisheries Science.

Fig. 26 Diagrammatic representation of the lifecycle of masu salmon (Oncorhynchus masou) (modified the figure by Kiso 1995). There seems to be a sequential diversity in migratory patterns among representative non-migratory (precocious parr) and migratory (smolt) forms. In masu salmon, 1+ parr that occupy focal foraging areas (territory), exhibit precociously sexual maturation, and have high plasma sex steroid levels become representative non-migrants, precocious parr. In contrast, some of the 1+ immature fish that could not gain focal territories become representative migrants, smolt migrants. Their migratory patterns seem to be modulated in an inhibitory fashion by their maturity and/or growth performance (see text for details). Note that the "poor growth fish" which does not differentiate into either precocious parr or smolt migrants during the age of 1+ appears to differentiate into non-migratory parr or migratory smolts in another year, mainly during the age of 2+.

Fig. 27 Diagrammatic representations of (a) life histories of anadromous, amphidromous, and catadromous fish (modified from Gross 1987) and (b) differences in lifecycle of masu salmon (Oncorhynchus masou) among different regions. In (a) (e.g., northern hemisphere), anadromy and catadromy exceed in northern temperate and southern tropic areas, respectively, while amphidromy are frequent between these areas. In (b), proportion of non-migratory and migratory (e.g., smolts) forms increased in southern (e.g., Kyushu) and northern (e.g., Hokkaido) Japanese streams, respectively, maybe by the differences in productivity among different areas. Also, spawning and downstream migratory period in masu salmon become later in southern and northern Japan, respectively (Machidori and Kato 1984).

page top


Table 1 Frequency of migrants and non-migrants, and plasma levels of testosterone (T), estradiol-17β (E2), 11-ketotestosterone (11-KT), 17,20β-dihydroxy-4-pregnene-3-one (DHP), thyroxine (T4), and (g) triiodothyronine (T3) (mean ± SEM) in 2+ male and female masu salmon during upstream migratory period. Differences in mean plasma hormone levels among experimental groups were analyzed by one-way analysis of variance (ANOVA) followed by Fisher's PLSD. Differing letters represent significant differences at P < 0.05 among all groups. —: no sample. Reprinted from Aquaculture, 362–363, Munakata et al., Involvement of sex steroids and thyroid hormones in upstream and downstream behaviors in masu salmon, Oncorhynchus masou, 158–166, © 2012a, with permission from Elsevier.

Table 2 Frequency of migrants and non-migrants, and plasma levels of testosterone (T), estradiol-17β (E2), thyroxine (T4), and (g) triiodothyronine (T3) (mean ± SEM) in 1+ immature masu salmon implanted with T, 1,4,6-androstatriene-3,17-dion (ATD) or tamoxifen 500 μg/fish. Differences in the frequency of upstream behavior from the control group were analyzed by the χ2-test, using StatView version 4.5 software (Abacus Concepts, Inc., California, USA). * indicates significant difference at P < 0.05, from the control group. Differences in mean plasma hormone levels among experimental groups were analyzed by one-way analysis of variance (ANOVA) followed by Fisher's PLSD. Differing letters represent significant differences at P < 0.05 among all groups. —: no sample. Reprinted from Aquaculture, 362–363, Munakata et al., Involvement of sex steroids and thyroid hormones in upstream and downstream behaviors in masu salmon, Oncorhynchus masou, 158–166, © 2012a, with permission from Elsevier.

Table 3 Frequency of migrants and non-migrants and, plasma levels of testosterone (T), estradiol-17β (E2), thyroxine (T4), and (g) triiodothyronine (T3) (mean ± SEM) in castrated, castrated + E2 500 μg/fish, sham-operated, sham-operated + E2 500 μg/fish, control, and tamoxifen 500 μg/fish-treated 1+ precocious male masu salmon. Differences in the frequency of upstream behavior from the control group were analyzed by the χ2-test, using StatView version 4.5 software (Abacus Concepts, Inc., California, USA). * indicates significant difference at P < 0.05, from the control group. Differences in mean plasma and pituitary hormone levels among experimental groups were analyzed by one-way analysis of variance (ANOVA) followed by Fisher's PLSD. Differing letters represent significant differences at P < 0.05 among all groups. —: no sample. Reprinted from Aquaculture, 362–363, Munakata et al., Involvement of sex steroids and thyroid hormones in upstream and downstream behaviors in masu salmon, Oncorhynchus masou, 158–166, © 2012a, with permission from Elsevier.

Table 4 Frequency of upstream and downstream swimming behaviors and, plasma levels of testosterone (T), thyroxine (T4), and (g) triiodothyronine (T3) (mean ± SEM) in controls and T 500 μg/fish-treated 1+ smolts and 1+ precocious male masu salmon. Differences in the frequency of upstream and downstream swimming behavior from the control group were analyzed by the χ2-test, using StatView version 4.5 software (Abacus Concepts, Inc., California, USA). *** indicates significant difference at P < 0.001, from the control group. Differences in mean plasma hormone levels among experimental groups were analyzed by one-way analysis of variance (ANOVA) followed by Fisher's PLSD. Differing letters represent significant differences at P < 0.05 among all groups. Reprinted from Aquaculture, 362–363, Munakata et al., Involvement of sex steroids and thyroid hormones in upstream and downstream behaviors in masu salmon, Oncorhynchus masou, 158–166, © 2012a, with permission from Elsevier.

Table 5 Body length (BL), body weight (BW), and condition factor (CF) in the control, T5 μg/fish-, T 50 μg/fish-, and T 500 μg/fish-treated 1+ smolts and 1+ precocious male masu salmon. Differences in mean BL, BW, and CF among experimental groups were analyzed by one-way analysis of variance (ANOVA) followed by Fisher's PLSD. Differing letters represent significant differences at P < 0.05 among all groups. Reprinted with permission from Zoological Science, 17, Munakata et al., Inhibitory effects of testosterone on downstream migratory behavior in masu salmon, Oncorhynchus masou, 863–870, Table 2, © 2000b, Zoological Society of Japan.

Table 6 Body length (BL), body weight (BW), and condition factor (CF) in the control (Raceway 1) and T500 μg/fish-treated smolts (Raceway 2) in masu salmon (Oncorhynchus masou). Differences in mean BL, BW, and CF among experimental groups were analyzed by one-way analysis of variance (ANOVA) followed by Fisher's PLSD. Differing letters represent significant differences at P < 0.05 among all groups. Reprinted with permission from Zoological Science, 17, Munakata et al., Inhibitory effects of testosterone on downstream migratory behavior in masu salmon, Oncorhynchus masou, 863–870, Table 3, © 2000b, Zoological Society of Japan.

Table 7 Appearance of frequency and migratory pattern (area) of non-migratory and migratory forms of masu salmon (Oncorhynchus masou) that inhabit streams along Sanriku coast in northern Honshu and its plasma sex steroid hormone levels, gonad somatic index (GSI), and gonadal development and growth stages prior to and during the period of downstream migration, based on Kiso (1995). Shades indicate the potential physiological factors that inhibit or modulate the occurrence of downstream migratory behavior.

page top