The involvement of sex steroid hormones in downstream and upstream migratory behavior of masu salmon

The movement ecology of fishes

Movement of fishes in the aquatic realm is fundamental to their ecology and survival. Movement can be driven by a variety of biological, physiological and environmental factors occurring across all spatial and temporal scales. The intrinsic capacity of movement to impact fish individually (e.g., foraging) with potential knock-on effects throughout the ecosystem (e.g., food web dynamics) has garnered considerable interest in the field of movement ecology. The advancement of technology in recent decades, in combination with ever-growing threats to freshwater and marine systems, has further spurred empirical research and theoretical considerations. Given the rapid expansion within the field of movement ecology and its significant role in informing management and conservation efforts, a contemporary and multidisciplinary review about the various components influencing movement is outstanding. Using an established conceptual framework for movement ecology as a guide (i.e., Nathan et al., 2008: 19052), we synthesized the environmental and individual factors that affect the movement of fishes. Specifically, internal (e.g., energy acquisition, endocrinology, and homeostasis) and external (biotic and abiotic) environmental elements are discussed, as well as the different processes that influence individual-level (or population) decisions, such as navigation cues, motion capacity, propagation characteristics and group behaviours. In addition to environmental drivers and individual movement factors, we also explored how associated strategies help survival by optimizing physiological and other biological states. Next, we identified how movement ecology is increasingly being incorporated into management and conservation by highlighting the inherent benefits that spatio-temporal fish behaviour imbues into policy, regulatory, and remediation planning. Finally, we considered the future of movement ecology by evaluating ongoing technological innovations and both the challenges and opportunities that these advancements create for scientists and managers. As aquatic ecosystems continue to face alarming climate (and other human-driven) issues that impact animal movements, the comprehensive and multidisciplinary assessment of movement ecology will be instrumental in developing plans to guide research and promote sustainability measures for aquatic resources.

Relationships between maturational status and migration behavior of homing chum salmon Oncorhynchus keta in inner bays of the Sanriku coast

The correlations among gonad maturity and various homing behaviors of chum salmon, Oncorhynchus keta, were evaluated using acoustic tracking of tagged fish in Otsuchi Bay, Japan. There was a negative correlation between the time duration from release of tagged fish until river entry and the plasma 17α, 20β-dihydroxy-4-pregnen-3-one (DHP) levels, an indicator of final maturation. Females with high DHP entered the rivers soon after the release, whereas females with low DHP (<10 ng/ml) took a few days to more than one week until river entry. Similar correlation was also found in males. A pattern of river entry correlated with maturational conditions was also observed in fish entering the rivers of neighboring bays. DHP concentrations of fish caught in the rivers were consistently higher. On the other hand, more than half of released salmon departed from the bay regardless of their plasma DHP level, suggesting that maturational status does not force homing adults to enter the most available nearest rivers. Fish entering the rivers experienced ambient temperatures less than 8 °C, which is approximately 5 °C lower than that of the bay. These results indicate that homing salmon hold their position in the bay until just before spawning, which may be attributable to low temperature avoidance. This characteristic type of river entry may be suitable to geographical features and thermal regimes of this region.

Positive Feedback between Behavioral and Hormonal Dynamics Leads to Differentiation of Life-History Tactics

AbstractCompetitive interaction among individuals of a single population may result in the differentiation of two or more distinct life-history tactics. For example, although they exhibit unimodal size distribution, male juveniles of salmonids differentiate into those going down to the ocean to grow and returning to the natal stream after several years to reproduce (migratory tactic) and those staying in the stream and reproducing for multiple years (resident tactic). In this study, we developed a simple mathematical model for the positive feedback between hormonal and behavioral dynamics, with the expectation of establishing multiple discrete clusters of hormone levels leading to differentiation of life-history tactics. The assumptions were that probability of winning in fighting depends both on the body size and hormone level of the two contestants. An individual with a higher hormone level would be more likely to win the competition, which further enhanced hormone production, forming a positive feedback loop between hormone level and fighting ability. If the positive feedback was strong but not excessive, discrete clusters of hormone levels emerged from a continuous distribution. In contrast, no clear clustering structure appeared in the distribution of hormone levels if the probability of winning in fighting was controlled by the body size.

Gonadal steroid fluctuations and reproduction results of European grayling (Thymallus thymallus L.) from broodstock farmed in a recirculation aquaculture system

The initial experiment of this study was conducted to determine whether sex steroid concentrations varied before, during and after the reproductive season of European grayling (Thymallus thymallus L.) farmed broodstock confined in a recirculation aquaculture system (RAS). The results indicated that the plasma sex steroid (testosterone - T, progesterone - P₄ and estradiol - E₂) concentrations varied (P < 0.05) during these reproductive periods. There were greater concentrations of T, P₄ and E₂ before and during the grayling reproductive season indicating there are important functions of these steroid hormones associated with gonadal maturation, gamete production and reproductive behavior during the spawning period. In the second experiment of this study, the effectiveness of European grayling controlled reproduction treatment regimens was analyzed and there were 34.69 % and 68.18 % having ovulations in the 2 and 3-year-old broodfish, respectively. Of the embryos developing to the eyed-egg stage, there were 3.70 % and 6.87 % derived from eggs of 2- and 3-year-old grayling females, respectively. Proportions of embryos developing from the eyed-egg stage to hatching were 38.1 % and 52.1 % from eggs of 2- and 3-year-old grayling females, respectively, when there was culturing in a RAS. The results indicate that with grayling broodfish there is greater efficacy in induction of reproduction when there is imposing of the controlled reproduction treatment regimen on 3- rather than 2-year-old broodfish.

Acoustic signals produced by Nile tilapia Oreochromis niloticus and black-chinned tilapia Sarotherodon melanotheron during intra- and interspecific pairings

We characterised, for the first-time, the sound production of black-chinned tilapia Sarotherodon melanotheron and show differences with that of Nile tilapia Oreochromis niloticus in a hybridization pairing context. Although both species were able to produce drum sounds, they showed different acoustic features. Drum sounds were produced in aggressive (chasing or lateral attack) and non-aggressive (courtship) contexts by O. niloticus but only in aggressive situations (fleeing or avoidance) by S. melanotheron. The second type of sounds produced by O. niloticus were grunts, produced in both aggressive (chasing and after biting) and non-aggressive contexts (nest building). The second type of sound produced by S. melanotheron was a rolling sound, produced only during courtship. Each species was able to produce common sounds (drum) and species-specific sounds (grunts and rolling). This implies that species can communicate without being able to understand each other because the sounds emitted may probably have different significance. Drumming corresponded only to aggressivity in S. melanotheron, whereas this was not true for O. niloticus. 11-ketotestosterone (11-kt) levels were significantly higher in male O. niloticus than male S. melanotheron, but there was no significant correlation between 11-kt or estradiol concentrations and the number of sounds produced in aggressive or non-aggressive behavioural contexts in either species. During interspecies interactions, O. niloticus drum sounds are likely considered to be aggressive by S. melanotheron and could potentially constitute a reproductive barrier between the two species.

Reduced lactate dehydrogenase activity in the heart and suppressed sex hormone levels are associated with female-biased mortality during thermal stress in Pacific salmon

Female-biased mortality has been repeatedly reported in Pacific salmon during their upriver migration in both field studies and laboratory holding experiments, especially in the presence of multiple environmental stressors, including thermal stress. Here, we used coho salmon (Oncorhynchus kisutch) to test whether females exposed to elevated water temperatures (18°C) (i) suppress circulating sex hormones (testosterone, 11-ketotestosterone and estradiol), owing to elevated cortisol levels, (ii) have higher activities of enzymes supporting anaerobic metabolism (e.g. lactate dehydrogenase, LDH), (iii) have lower activities of enzymes driving oxidative metabolism (e.g. citrate synthase, CS) in skeletal and cardiac muscle, and (iv) have more oxidative stress damage and reduced capacity for antioxidant defense [lower catalase (CAT) activity]. We found no evidence that a higher susceptibility to oxidative stress contributes to female-biased mortality at warm temperatures. We did, however, find that females had significantly lower cardiac LDH and that 18°C significantly reduced plasma levels of testosterone and estradiol, especially in females. We also found that relative gonad size was significantly lower in the 18°C treatment regardless of sex, whereas relative liver size was significantly lower in females held at 18°C. Further, relative spleen size was significantly elevated in the 18°C treatments across both sexes, with larger warm-induced increases in females. Our results suggest that males may better tolerate bouts of cardiac hypoxia at high temperature, and that thermal stress may also disrupt testosterone- and estradiol-mediated protein catabolism, and the immune response (larger spleens), in migratory female salmon.

Disruptive effects of nonylphenol on reproductive hormones, antioxidant enzymes, and histology of liver, kidney and gonads in Caspian trout smolts

The endocrine-disrupting effect of pollutants such as alkylphenols has been considered a major concern during recent years. Although the effects of nonylphenol on the reproductive hormones of fish have been investigated in several studies, the effect of this endocrine disruptor on reproductive hormones of immature fish and salmonid smolts has been less addressed. The present work studied the effects of 1, 10 and 100 μg/l concentrations of nonylphenol on the levels of plasma reproductive hormones and liver antioxidant enzymes as well as on histopathology of reproductive and non-reproductive organs of male and female Caspian brown trout (Salmo trutta caspius) smolts after 21 days of exposure. The results of the present study showed that environmentally relevant concentrations of nonylphenol affected plasma levels of sex steroids; gonadotropins, phosphorus, estradiol to testosterone ratio, and also caused histopathological lesions in liver, kidney and testis tissues of immature Caspian brown trout during smolting. Nonylphenol significantly increased the levels of estradiol in plasma of both male and female smolts exposed to nonylphenol compared with the control groups. Exposure to nonylphenol decreased testosterone and FSH levels in both genders. It has also increased plasma levels of LH in females but did not affect LH levels in male fish. Liver SOD and CAT content was decreased in nonylphenol-exposed smolts. Therefore, the release of this economically valuable and endangered species into the rivers contaminated with nonylphenol should be avoided as it can have significant effects on the development and reproductive function of smolts.

Involvement of 11-ketotestosterone in hooknose formation in male pink salmon (Oncorhynchus gorbuscha) jaws

Mature male Pacific salmon (Oncorhynchus spp.) develop a hooknose, as a secondary male sexual characteristic, during the spawning period. It is likely that androgens regulate hooknose formation. However, endocrinological and histochemical details about the relationship between androgens and hooknose formation are poorly understood. In this study, we performed assays of serum androgens, detection of androgen receptor (AR) in hooknose tissues, external morphological measurement of hooknose-related lengths, and microscopic observation of hooknose tissues of pink salmon (O. gorbuscha) at different stages of sexual maturation. Expression of the arβ gene was detected in hooknose tissues of males but not females. The elongation of these tissues was mediated directly via androgens. Serum 11-ketotestosterone (11-KT) concentrations indicated a significant positive correlation with both jaw lengths during sexual maturation of males. In the upper jaw, cartilage tissue developed during hooknose formation, and AR-immunoreactive chondrocytes were located in the rostal-vetral regions of hooknose cartilage in maturing male. The chondrocytes in maturing males before entering into rivers exhibited rich-cytoplasm with high cell activity than at other sexual development stages. On the other hand, in the lower jaw, the development of the spongiosa-like bone meshworks. AR-immunoreactivity was detected in a proportion of the osteocytes and osteoblast-like cells in the spongiosa-like bone meshworks. These results indicate that hooknose formation in pink salmon, which is associated with the buildup of a structure with sufficient strength that it can be used to attack other males on the spawning ground, is regulated by 11-KT.

Evaluation of seasonal and daily changes of plasma thyroxine and cortisol levels in wild masu salmon Oncorhynchus masou, sampled by a Japanese fishing method

A new fish sampling method was developed using a Japanese bait fishing rod (8-9 m carbon rod and a nylon line with a small fine wire single hook), which is considered to catch wild salmonid juveniles with low sampling stress. Using this method, seasonal and daily changes of plasma thyroxine (T4 ) and cortisol levels were examined in wild parr, pre-smolts and smolts of masu salmon Oncorhynchus masou in contiguous locations in a coastal river (Kesen River; 44 km) in northern Honshu Island, Japan, overlapping the period of smoltification and seaward migration from August to March. Plasma T4 and cortisol were low in 0+ and 1+ year parr caught in August and September. In March, some yearling (1+ year) fish, which were judged as pre-smolts, and smolts appeared mainly in mid and lower reaches, while parr (0+ and 1+ year parr) continued to appear in the upper and mid reaches. In March, 1+ year pre-smolts and smolts showed high plasma T4 levels while the levels of 1+ year parr were low. During March 2008-2010, plasma T4 levels of 1+ year pre-smolts and smolts had high levels from early to mid-March, whereas plasma cortisol levels of 1+ year smolts were low in early March and increased towards mid-March. Based on these data, plasma cortisol increases probably occur following the increases of plasma T4 levels to lead the 1+ year O. masou to the completion of smoltification and initiation of seaward migration.

Oestradiol and prostaglandin F2α regulate sexual displays in females of a sex-role reversed fish

The mechanisms regulating sexual behaviours in female vertebrates are still poorly understood, mainly because in most species sexual displays in females are more subtle and less frequent than displays in males. In a sex-role reversed population of a teleost fish, the peacock blenny Salaria pavo, an external fertilizer, females are the courting sex and their sexual displays are conspicuous and unambiguous. We took advantage of this to investigate the role of ovarian-synthesized hormones in the induction of sexual displays in females. In particular, the effects of the sex steroids oestradiol (E2) and testosterone (T) and of the prostaglandin F2α (PGF2α) were tested. Females were ovariectomized and their sexual behaviour tested 7 days (sex steroids and PGF2α) and 14 days (sex steroids) after ovariectomy by presenting females to an established nesting male. Ovariectomy reduced the expression of sexual behaviours, although a significant proportion of females still courted the male 14 days after the ovary removal. Administration of PGF2α to ovariectomized females recovered the frequency of approaches to the male's nest and of courtship displays towards the nesting male. However, E2 also had a positive effect on sexual behaviour, particularly on the frequency of approaches to the male's nest. T administration failed to recover sexual behaviours in ovariectomized females. These results suggest that the increase in E2 levels postulated to occur during the breeding season facilitates female mate-searching and assessment behaviours, whereas PGF2α acts as a short-latency endogenous signal informing the brain that oocytes are mature and ready to be spawned. In the light of these results, the classical view for female fishes, that sex steroids maintain sexual behaviour in internal fertilizers and that prostaglandins activate spawning behaviours in external fertilizers, needs to be reviewed.

Effect of short-term decrease in water temperature on body temperature and involvement of testosterone in steelhead and rainbow trout, Oncorhynchus mykiss

The Pacific salmonid species Oncorhynchus mykiss is separated into a migratory form (steelhead trout) and a non-migratory form (rainbow trout). A decrease in water temperature is likely a cue triggering downstream behavior in the migratory form, and testosterone inhibits onset of this behavior. To elucidate differences in sensitivity to water temperature decreases between the migratory and non-migratory forms and effect of testosterone on the sensitivity, we examined two experiments. In experiment 1, we compared changes in body temperature during a short-term decrease in water temperature between both live and dead steelhead and rainbow trout. In experiment 2, we investigated effects of testosterone on body temperature decrease in steelhead trout. Water temperature was decreased by 3°C in 30min. The body temperature of the steelhead decreased faster than that of the rainbow trout. In contrast, there was no significant difference in the decrease in body temperature between dead steelhead and rainbow trout specimens. The body temperature of the testosterone-treated steelhead trout decreased more slowly than that of control fish. Our results suggest that the migratory form is more sensitive to decreases in water temperature than the non-migratory form. Moreover, testosterone might play an inhibitory role in sensitivity to such decreases.

Physiological mechanisms of imprinting and homing migration in Pacific salmon Oncorhynchus spp

After several years of feeding at sea, salmonids have an amazing ability to migrate long distances from the open ocean to their natal stream to spawn. Three different research approaches from behavioural to molecular biological studies have been used to elucidate the physiological mechanisms underpinning salmonid imprinting and homing migration. The study was based on four anadromous Pacific salmon Oncorhynchus spp., pink salmon Oncorhynchus gorbuscha, chum salmon Oncorhynchus keta, sockeye salmon Oncorhynchus nerka and masu salmon Oncorhynchus masou, migrating from the North Pacific Ocean to the coast of Hokkaido, Japan, as well as lacustrine O. nerka and O. masou in Lake Toya, Hokkaido, where the lake serves as the model oceanic system. Behavioural studies using biotelemetry techniques showed swimming profiles from the Bering Sea to the coast of Hokkaido in O. keta as well as homing behaviours of lacustrine O. nerka and O. masou in Lake Toya. Endocrinological studies on hormone profiles in the brain-pituitary-gonad axis of O. keta, and lacustrine O. nerka identified the hormonal changes during homing migration. Neurophysiological studies revealed crucial roles of olfactory functions on imprinting and homing during downstream and upstream migration, respectively. These findings are discussed in relation to the physiological mechanisms of imprinting and homing migration in anadromous and lacustrine salmonids.

Physiological condition differentially affects the behavior and survival of two populations of sockeye salmon during their freshwater spawning migration

Recently, a segment of the Adams-Shuswap sockeye salmon (Oncorhynchus nerka) population initiated freshwater migration several weeks earlier than historically recorded, resulting in high mortality rates. The comigrating Chilko population maintained their historic river entry timing and did not experience elevated mortality. To test the hypothesis that population-specific differences in physiological condition would differentially influence behavior and survival when exposed to fisheries capture stress, we physiologically sampled individuals from both populations at the onset of the freshwater phase of their reproductive migration and tracked the remainder of their migrations using radio telemetry. Adams-Shuswap individuals had slower migration rates and were less likely to reach natal subwatersheds relative to Chilko individuals. Metabolic and osmoregulatory impairment was related to mortality for Adams-Shuswap individuals but not for Chilko individuals. Similarly, physiological condition correlated with migration rate for Adams-Shuswap but not Chilko fish. Survival to natal subwatersheds was 1.9 times higher for Chilko relative to Adams-Shuswap, a result that did not emerge until individuals approached natal subwatersheds several days after the stressor was applied. We conclude that physiological condition differentially affects the behavior and survival of these two populations, which may be a consequence of the early-entry phenomenon by a segment of the Adams-Shuswap population.

Mechanisms influencing the timing and success of reproductive migration in a capital breeding semelparous fish species, the sockeye salmon

Two populations of homing sockeye salmon (Oncorhynchus nerka; Adams and Chilko) were intercepted in the marine approaches around the northern and southern ends of Vancouver Island (British Columbia, Canada) en route to a natal river. More than 500 salmon were nonlethally biopsied for blood plasma, gill filament tips, and gross somatic energy (GSE) and were released with either acoustic or radio transmitters. At the time of capture, GSE, body length, and circulating testosterone ([T]) differed between populations, differences that reflected known life-history variations. Within-population analyses showed that in Adams sockeye salmon, plasma glucose ([glu]), lactate ([lactate]), and ion concentrations were higher in the northern approach than in the southern approach, suggesting that the former was more stressful. GSE, [T], and gill Na(+),K(+)-ATPase activities also differed between the two locales, and each varied significantly with Julian date, suggesting seasonality. Despite these relative geographic differences, the timing of river entry and the ability to reach spawning areas were strongly correlated with energetic, reproductive, and osmoregulatory state. Salmon that delayed river entry and reached spawning areas had relatively high GSE and low [T] and gill ATPase. In contrast, salmon that entered the river directly but that ultimately failed to reach spawning areas had lower GSE and higher [T] and gill ATPase, and they also swam at significantly faster rates (failed fish approximately 20.0 km d(-1) vs. successful fish approximately 15.5 km d(-1)). Physiologically, salmon that did not enter the river at all but that presumably died in the marine environment exhibited high stress (plasma [glu] and [lactate]) and ionoregulatory measures (plasma [Na(+)], [Cl(-)], osmolality).

Endocrine control of sexual behavior in teleost fish

Sexual behavior is one of the most profound events during the life cycle of animals that reproduce sexually. After completion of gonadal development that is mediated by various hormones, oviparous teleosts perform a suite of behaviors, often termed as spawning behavior. This is particularly important for teleosts that have their gametes fertilized externally as the behavior patterns ensures the close proximity of both sexes for gamete release, fusion and ultimately the production of offspring. As in other vertebrates, sexual behavior of fish is also under the control of hormones. Testicular androgen is a requirement for male sexual behavior to occur in most fish species that have been studied. Unlike tetrapods, however, ovarian estrogen does not appear to be essential for the occurrence of female sexual behavior for fish that have their gametes fertilized externally. Prostaglandins produced in the ovary after ovulation act as a trigger in some teleosts to induce female sexual behavior. Potentiating effects of gonadotropin-releasing hormone in the brain on sexual behavior are reported in some species. Under endocrine regulation, male and female fish exhibit gender-typical behavior during spawning, but in some fish species there is also some plasticity in their sexual behavior. Sex changing fish can perform both male-typical and female-typical sexual behaviors during their lifetime and this sexual plasticity can also be observed in non-sex changing fish when undergoing hormonal treatment. Although the neuroanatomical basis is not clear in fish, results of field and laboratory observations suggest that some teleosts possess a sexually bipotential brain which can regulate two types of behaviors unlike most other vertebrates which have a discrete sex differentiation of their brain and can only perform gender-typical sexual behavior.

Changes in the plasma levels of insulin-like growth factor-I from the onset of spawning migration through upstream migration in chum salmon

An increase in activity of the pituitary-gonadal axis (PG-axis) and gonadal development are essential for the onset of spawning migration of chum salmon from the Bering Sea. In the Bering Sea, fish with larger body sizes initiated gonadal development and commenced spawning migration to the natal river by the end of summer. We thus hypothesized that insulin-like growth factor-I (IGF-I), a somatotropic signal that interacts with the PG-axis, can be one of such factors responsible for the onset of migration, and examined changes in plasma levels and hepatic expression of IGF-I gene in oceanic and homing chum salmon in 2001-2003. The plasma IGF-I levels and corresponding body sizes in maturing adults, which had developing gonads, were significantly higher than those in immature fish in all years examined. Such increase in the plasma IGF-I levels in maturing fish was observed even in the Gulf of Alaska during February 2006, while coincident increase was not observed in the hepatic amounts of IGF-I mRNA. In autumn, the plasma IGF-I levels in homing adults decreased during upstream migration in the Ishikari River-Ishikari bay water system in Hokkaido, Japan. In conclusion, the plasma IGF-I levels increased with gonadal development when chum salmon migrated from the winter Gulf of Alaska to the summer Bering Sea. Circulating IGF-I may interact with the PG-axis and promote gonadal development that is inseparable from the onset of spawning migration. Circulating IGF-I levels were thereafter lowered in accordance with final maturation during upstream migration in the breeding season.

Gene-expression signatures of Atlantic salmon's plastic life cycle

How genomic expression differs as a function of life history variation is largely unknown. Atlantic salmon exhibits extreme alternative life histories. We defined the gene-expression signatures of wild-caught salmon at two different life stages by comparing the brain expression profiles of mature sneaker males and immature males, and early migrants and late migrants. In addition to life-stage-specific signatures, we discovered a surprisingly large gene set that was differentially regulated-at similar magnitudes, yet in opposite direction-in both life history transitions. We suggest that this co-variation is not a consequence of many independent cellular and molecular switches in the same direction but rather represents the molecular equivalent of a physiological shift orchestrated by one or very few master regulators.

Activity of the pituitary-gonadal axis is increased prior to the onset of spawning migration of chum salmon

The activity of the pituitary-gonadal axis (PG axis) in pre-migratory and homing chum salmon was examined because endocrine mechanisms underlying the onset of spawning migration remain unknown. Pre-migratory fish were caught in the central Bering Sea in June, July and September 2001, 2002 and 2003, and in the Gulf of Alaska in February 2006. They were classified into immature and maturing adults on the basis of gonadal development. The maturing adults commenced spawning migration to coastal areas by the end of summer, because almost all fish in the Bering Sea were immature in September. In the pituitaries of maturing adults, the copy numbers of FSHbeta mRNA and the FSH content were 2.5- to 100-fold those of the immature fish. Similarly, the amounts of LHbeta mRNA and LH content in the maturing adults were 100- to 1000-fold those of immature fish. The plasma levels of testosterone, 11-ketotestosterone and estradiol were higher than 10 nmol l(-1) in maturing adults, but lower than 1.0 nmol l(-1) in immature fish. The increase in the activity of the PG-axis components had already initiated in the maturing adults while they were still in the Gulf of Alaska in winter. In the homing adults, the pituitary contents and the plasma levels of gonadotropins and plasma sex steroid hormones peaked during upstream migration from the coast to the natal hatchery. The present results thus indicate that the seasonal increase in the activity of the PG axis is an important endocrine event that is inseparable from initiation of spawning migration of chum salmon.

Exposure to high temperature influences the behaviour, physiology, and survival of sockeye salmon during spawning migration

Since 1996, some populations of Fraser River sockeye salmon ( Oncorhynchus nerka Walbaum in Artedi, 1792) have begun spawning migrations weeks earlier than normal, and most perish en route as a result. We suspect that a high midsummer river temperature is the principal cause of mortality. We intercepted 100 sockeye during normal migration near a spawning stream and measured somatic energy and aspects of plasma biochemistry. Fish were then held at either 10 or 18 °C for 24 days. Before release, fish were biopsied again and implanted with acoustic transmitters. A group of biopsied but untreated control salmon were released at the same time. Sixty-two percent (8 of 13) of control salmon and 68% (21 of 31) of 10 °C salmon reached spawning areas. The 18 °C-treated fish were half as successful (35%; 6 of 17). During the holding period, mortality was 2 times higher and levels of Parvicapsula minibicornis (Kent, Whitaker and Dawe, 1997) infection were higher in the 18 °C-treated group than in the 10 °C-treated group. The only physiological difference between treatments was a change in gill Na+,K+-ATPase activity. This drop correlated negatively with travel times for the 18 °C-treated males. Reproductive-hormone levels and stress measures did not differ between treatments but showed significant correlations with individual travel times.

Sex steroids and their receptors in lampreys

The use of steroids and their receptors as ligand-gated transcription factors is thought to be an important step in vertebrate evolution. The lamprey is the earliest-evolving vertebrate to date in which sex steroids and their receptors have been demonstrated to have hormonal roles similar to those found in jawed vertebrates. Sex steroids and their receptors have been examined in several lamprey species, and the majority of studies have focused on the sea lamprey, Petromyzon marinus. While classical steroids appear to be present in lampreys, their function, concentrations, and synthesis have not been determined conclusively. The only classical steroid that is thought to act as a hormone in both males and females is estradiol. Recent research has established that lampreys produce and circulate 15alpha-hydroxylated steroids, and that these steroids respond to upstream stimulation within the hypothalamic-pituitary-gonadal axis. In particular, 15alpha-hydroxyprogesterone is highly sensitive and responds in great magnitude to stimulation, and is likely a hormone. Lampreys also appear to use androstenedione, a precursor to vertebrate androgens, as their main androgen, and a receptor for androstenedione has recently been described. Non-classical steroids are prevalent in many aquatic vertebrates, and the non-classical steroids found in the sea lamprey may represent an evolutionary artifact, or alternatively may be a way to avoid endocrine disruption when ingesting the body fluids of host fish. The lamprey will continue to be an interesting model for examining the evolution of steroid hormones, steroid receptors, and steroid function.

Expression of hormone genes and osmoregulation in homing chum salmon: a minireview

Pacific salmon migrate from ocean through the natal river for spawning. Information on expression of genes encoding osmoregulatory hormones and migratory behavior is important for understanding of molecular events that underlie osmoregulation of homing salmon. In the present article, regulation of gene expression for osmoregulatory hormones in pre-spawning salmon was briefly reviewed with special reference to neurohypophysial hormone, vasotocin (VT), and pituitary hormones, growth hormone (GH) and prolactin (PRL). Thereafter, we introduced recent data on migratory behavior from SW to FW environment. In pre-spawning chum salmon, the hypothalamic VT mRNA levels increased in the males, while decreased in the females with loss of salinity tolerance when they were kept in SW. The amounts of GH mRNA in the pituitary decreased during ocean migration prior to entrance into FW. Hypo-osmotic stimulation by SW-to-FW transfer did not significantly affect the amount of PRL mRNA, but it was elevated in both SW and FW environments along with progress in final maturation. Behaviorally, homing chum salmon continued vertical movement between SW and FW layers in the mouth of the natal river for about 12h prior to upstream migration. Pre-spawning chum salmon in an aquarium, which allowed fish free access to SW and FW, showed that individuals with the lower plasma testosterone (T) and higher estradiol-17beta (E2) levels spent longer time in FW when compared with the SW fish. Taken together, neuroendocrine mechanisms that underlie salt and water homeostasis and migratory behavior from SW to FW may be under the control of the hypothalamus-pituitary-gonadal axis in pre-spawning salmon.

Effects of growth hormone and cortisol on the downstream migratory behavior in masu salmon, Oncorhynchus masou

The effects of ovine growth hormone (oGH) and cortisol on downstream migratory behavior in yearling (1(+)) smolts and underyearling (0(+)) parr of masu salmon, Oncorhynchus masou, were examined during the downstream migratory period in spring using artificial raceways. In May, each of 22 1(+) smolts and 0(+) parr were implanted with cholesterol pellets containing 250 microg of oGH and/or 2 mg of cortisol. Their downstream migratory behavior was subsequently observed in artificial raceways, along with control groups 4-23 days after implantation. In 1(+) smolts, the frequency of downstream migratory behavior was 23%, 18%, 72%, and 82% in the control, oGH, cortisol, and oGH+cortisol-treated groups, respectively. The frequency was significantly higher in the cortisol and oGH+cortisol-treated groups than in the control and oGH-treated groups. In 0(+) parr, the frequency of downstream migratory behavior in the cortisol (82%) and cortisol+oGH-treated (90%) groups was significantly higher than in the control (18%) and oGH-treated (0%) groups. These results indicate that cortisol is an important endocrine factor inducing downstream migratory behavior in both 1(+) smolt and 0(+) parr of masu salmon.

Effect of waterborne exposure to 4-tert-octylphenol and 17beta-estradiol on smoltification and downstream migration in Atlantic salmon, Salmo salar

Groups of Atlantic salmon parr (November, Exp. 1) or pre-smolts (March, Exp. 2) were exposed to estradiol-17beta (E2 conc.: nominal 500 ngl(-1)/actual 8-16 ngl(-1)) and two doses of tert-octylphenol (OP: nominal 25 microgl(-1)/actual 4.5-6.5 microgl(-1) and OP: nominal 100 microgl(-1)/actual 10-30 microgl(-1)) for 26 days in fresh water, and the effects on physiological and behavioural aspects of parr-smolt transformation were investigated. Vitellogenesis was induced by all treatments, as indicated by elevated levels of plasma vitellogenin (Vtg) and hepatosomatic index. Elevated Vtg levels were still found in OP-100 and E2-treated fish 4-5 months after cessation of treatment, indicating a slow clearance of Vtg from circulation. Smolting was compromised by E2 and OP-100 treatment as judged by reduced gill Na(+), K(+)-ATPase activity and impaired ability to regulate plasma osmolality and muscle water content in 24-h sea water (SW) challenge tests during the period of smolting. Downstream migratory behaviour was monitored from late April to July (Exp. 2) by implanting passive integrated transponder tags into subgroups of treated and control smolts and placing them in a stream raceway. Irrespective of treatment, nocturnal downstream movement was initiated in all groups on April 23, switching to diurnal movement in late May. Average swimming speed was estimated to be higher than current speed, indicating active migration. E2 and OP-100 fish migrated at lower frequency than control fish, suggesting a reduced migratory drive. The data suggests that waterborne exposure of salmon to xenoestrogens reduce both physiological and behavioural components of smoltification, even when exposure occurs several months prior to smolting.

L-Kynurenine, an amino acid identified as a sex pheromone in the urine of ovulated female masu salmon

Many animals employ sex pheromones to find mating partners during their reproductive seasons. However, most sex pheromones of vertebrates remain to be identified. Over the past 20 years, steroids and prostaglandins have been identified as sex pheromones in several fishes. These pheromones are broadly termed "hormonal pheromones" because they or their precursors act as hormones in these fishes. Hitherto, no other type of sex pheromone has been unambiguously identified in teleost fish. Here we report the identification of a "nonhormonal pheromone" in teleost fish. The urine of the reproductively mature female masu salmon (Oncorhynchus masou) contains a male-attracting pheromone. Bioassay-guided fractionation yielded an active compound that was identical to L-kynurenine in spectral and chromatographic properties. L-Kynurenine is a major metabolite of L-tryptophan in vertebrates. This pheromone elicits a male-specific behavior at even picomolar concentrations; its electrophysiological threshold is 10(-14) M. L-Kynurenine is a reasonable substance for female masu salmon to advertise their readiness for mating.

Effects of insulin-like growth factor I on GnRH-induced gonadotropin subunit gene expressions in masu salmon pituitary cells at different stages of sexual maturation

Effects of insulin-like growth factor I (IGF-I) and salmon gonadotropin-releasing hormone (sGnRH) on expression of gonadotropin (GTH) subunit genes were examined using primary pituitary cell cultures of masu salmon (Oncorhynchus masou). Fishes were assessed at three reproductive stages, i.e., in April (early maturation), in June (maturing), and in September (spawning). Amounts of GTH subunit mRNAs in pituitary cells were determined using real-time PCR after incubation with IGF-I and/or sGnRH. IGF-I alone had almost no effects on three GTH subunit mRNAs in both sexes, except for decrease in follicle-stimulating hormone (FSH) beta mRNA in males in June. sGnRH alone was effective in stimulation of FSHbeta and luteinizing hormone (LH) beta gene expression in males in April. Thereafter it had no significant effects on GTH subunit mRNAs, although in September it tended to increase FSHbeta and LHbeta mRNAs in females. Co-administered IGF-I counteracted the sGnRH-induced expression of FSHbeta and LHbeta genes in males in April, but not in females in September. These results suggest that IGF-I is involved in direct regulation of GTH subunit genes during sexual maturation. In particular, IGF-I differently modulates sGnRH-induced GTH subunit gene expression, depending on reproductive stages.

Effects of salmon GnRH and sex steroid hormones on expression of genes encoding growth hormone/prolactin/somatolactin family hormones and a pituitary-specific transcription factor in masu salmon pituitary cells in vitro

Expression of genes encoding growth hormone (GH), prolactin (PRL), and somatolactin (SL) in growing and maturing salmon was stimulated by gonadotropin-releasing hormone (GnRH) analog during particular periods of the life cycle. GnRH therefore appears to directly and/or indirectly regulate gene expression for GH, PRL, and SL in combination with the pituitary-gonadal axis, such as sex steroid hormones. Direct effects of salmon GnRH (sGnRH), estradiol-17beta (E2), testosterone, and 11-ketotestosterone (11KT) on the amounts of GH, PRL, and SL mRNAs were thus examined using primary pituitary cell cultures of masu salmon at the four reproductive stages. We also determined the amounts of mRNA encoding pituitary specific POU homeodomain transcription factor (Pit-1) by real-time polymerase chain reactions. The amounts of GH, PRL, and SL mRNAs in the control cells elevated with gonadal maturation, coincidently with those of Pit-1 mRNA. sGnRH at 1.0 nM elevated the amounts of all mRNAs examined in the pre-spawning females, whereas significant effects were not observed with 100 nM sGnRH at any reproductive stages. Sex steroid hormones had no significant effects before initiation of gonadal maturation and at the maturing stage. In the males, E2 tended to decrease the amounts of SL mRNA in the pre-spawning stage. In the females, E2 and 11KT increased the amounts of PRL and SL mRNAs in the pre-spawning stage, but halved those of PRL mRNA in the spawning stage. The amounts of Pit-1 mRNA changed coincidently with those of PRL and SL mRNAs at all examined stages. The effects of E2 alone were abolished by 100 nM sGnRH. The present results indicated that both sGnRH and steroid hormones directly modulate synthesis of Pit-1, and further expression of PRL and SL genes. sGnRH may indirectly regulate GH/PRL/SL family hormone genes through the pituitary-gonadal axis, particularly in the late stage of gametogenesis.

Elevation of gene expression for salmon gonadotropin-releasing hormone in discrete brain loci of prespawning chum salmon during upstream migration

Our previous studies suggested that salmon gonadotropin-releasing hormone (sGnRH) neurons regulate both final maturation and migratory behavior in homing salmonids. Activation of sGnRH neurons can occur during upstream migration. We therefore examined expression of genes encoding the precursors of sGnRH, sGnRH-I, and sGnRH-II, in discrete forebrain loci of prespawning chum salmon, Oncorhynchus keta. Fish were captured from 1997 through 1999 along their homing pathway: coastal areas, a midway of the river, 4 km downstream of the natal hatchery, and the hatchery. Amounts of sGnRH mRNAs in fresh frozen sections including the olfactory bulb (OB), terminal nerve (TN), ventral telencephalon (VT), nucleus preopticus parvocellularis anterioris (PPa), and nucleus preopticus magnocellularis (PM) were determined by quantitative real-time polymerase chain reactions. The amounts of sGnRH-II mRNA were higher than those of sGnRH-I mRNA, while they showed similar changes during upstream migration. In the OB and TN, the amounts of sGnRH mRNAs elevated from the coast to the natal hatchery. In the VT and PPa, they elevated along with the progress of final maturation. Such elevation was also observed in the rostroventral, middle, and dorsocaudal parts of the PM. The amounts of gonadotropin IIbeta and somatolactin mRNAs in the pituitary also increased consistently with the elevation of gene expression for sGnRH. These results, in combination with lines of previous evidence, indicate that sGnRH neurons are activated in almost all the forebrain loci during the last phases of spawning migration, resulting in coordination of final gonadal maturation and migratory behavior to the spawning ground.

17-Beta estradiol and 4-nonylphenol delay smolt development and downstream migration in Atlantic salmon, Salmo salar

The effect of 17-beta estradiol (E2) and 4-nonylphenol (4-NP) on smoltification and downstream migration of Atlantic salmon was studied in an integrated laboratory and field study. In a stock of hatchery-raised 1-year-old salmon, smoltification progressed from February until late May as judged by increased gill Na+, K+ -ATPase activity and 24 h sea water (SW)-tolerance. Starting late March, three groups of 150 fish were each given 6 serial injections over 20 days of 2 microg/g body weight E2, 120 microg/g 4-NP dissolved in peanut oil or peanut oil (4 microl/g) as control. After the last injection, all fish were individually tagged (Passive Integrated Transponder tags) and a non-lethal gill biopsy was taken. Two days later (8 April), 100 fish per group were transported to the field site and released into a small stream. Smolt migration was registered by measuring arrival time at a trap downstream of the release site. Serum vitellogenin levels increased several-fold in both male and female E2- and 4-NP-treated fish. Overall, E2- and 4-NP-treatment impaired smolting as judged by elevated condition factor, reduced gill Na+, K+ -ATPase activity and alpha-subunit Na+, K+ -ATPase mRNA level, reduced muscle water content and increased mortality following 24 h SW-challenge. After release, control fish initiated downstream migration immediately, with 50% of the total number of migrants appearing in the trap within 10 days. E2- and 4-NP-treated fish appeared in the trap with a delay in comparison to controls of 6 and 8 days, respectively. After the smolt run, no fish were registered by electro-fishing upstream of the trap. The total number of fish reaching the trap and thus post-release survival was in the order control (81%), E2 (53%), 4-NP (12%). Representatives from all treatment groups held under simulated natural conditions in the laboratory survived 100% through the migration period, suggesting that a combination of behavioural and in-stream factors (predation by herons) may contribute to the differential mortality. The study indicates that short-term exposure to natural and environmental estrogens may impair smolt development and survival and delay subsequent downstream migration in Atlantic salmon.

Synergistic effects of salmon gonadotropin-releasing hormone and estradiol-17beta on gonadotropin subunit gene expression and release in masu salmon pituitary cells in vitro

Effects of salmon gonadotropin-releasing hormone (sGnRH) and estradiol-17beta (E2) on gene expression and release of gonadotropins (GTHs) were examined in masu salmon (Oncorhynchus masou) using primary pituitary cell cultures at three reproductive stages, initiation of sexual maturation in May, pre-spawning in July, and spawning in September. Amounts of GTH subunit mRNAs were determined by real-time polymerase chain reaction, and levels of GTH released in the medium were determined by RIA. In control cells, the amounts of three GTH subunit mRNAs (alpha2, FSHbeta, and LHbeta) peaked in July prior to spawning. FSH release spontaneously increased with gonadal maturation and peaked in September, whereas LH release remained low until July and extensively increased in September. Addition of E2 to the culture extensively increased the amounts of LHbeta mRNA in May and July in both sexes. It also increased the alpha2 mRNA in July in the females. In contrast, sGnRH alone did not have any significant effects on the amounts of three GTH subunit mRNAs at all stages, except for the elevation of alpha2 and FSHbeta mRNAs in July in the females. Nevertheless, synergistic effects by sGnRH and E2 were evident for all three GTH subunit mRNAs. In May, sGnRH in combination with E2 synergistically increased the amounts of LHbeta mRNA in the males and alpha2 mRNA in the females. However, in July the combination suppressed the amounts of alpha2 and FSHbeta mRNAs in the females. sGnRH alone stimulated LH release at all stages in both sexes, and the release was synergistically enhanced by E2. Synergistic stimulation of FSH release was also observed in May and July in both sexes. These results indicate that a functional interaction of sGnRH with E2 is differently involved in synthesis and release of GTH. The synergistic interaction modulates GTH synthesis differentially, depending on subunit, stage, and gender, whereas it potentiates the activity of GnRH to release GTH in any situation.

Seasonal changes of responses to gonadotropin-releasing hormone analog in expression of growth hormone/prolactin/somatolactin genes in the pituitary of masu salmon

Gonadotropin-releasing hormone (GnRH) is considered to stimulate secretion of growth hormone (GH), prolactin (PRL), and somatolactin (SL) at particular stages of growth and sexual maturation in teleost fishes. We therefore examined seasonal variation in the pituitary levels of GH/PRL/SL mRNAs, and tried to clarify seasonal changes of responses to GnRH in expression of GH/PRL/SL genes, in the pituitaries of growing and maturing masu salmon (Oncorhynchus masou). Pituitary samples were monthly collected one week after implantation with GnRH analog (GnRHa). The levels of mRNAs encoding GH, PRL, and SL precursors in single pituitaries were determined by a real-time polymerase chain reaction method. The fork lengths and body weights of control and GnRHa-implanted fish of both sexes gradually increased and peaked out in September of 2-year-old (2+) when fish spawned. GnRHa implantation did not stimulate somatic growth, nor elevate gonadosomatic index (GSI) of 1+ and 2+ males, whereas it significantly increased GSI of 2+ females in late August to early September. The GnRHa-implanted 1+ males had higher levels of GH and PRL mRNAs in July, and SL mRNA from June to August than the control males. The levels of GH, PRL, and SL mRNAs in the control and GnRHa-implanted 1+ females, however, did not show any significant changes. Afterward, the PRL mRNA levels elevated in the control 2+ fish of both sexes in spring. GnRHa elevated the GH mRNA levels in both males and females in 2+ winter, and the PRL mRNA levels in females in early spring. Regardless of sex and GnRHa-implantation, the SL mRNA levels increased during sexual maturation. In growing and maturing masu salmon, expression of genes encoding GH, PRL, and SL in the pituitary is thus sensitive to GnRH in particular seasons probably in relation to physiological roles of the hormones.

Sex steroids regulate intermediary metabolism in Oreochromis mossambicus

The effect of long-term administration of testosterone, progesterone, and a synthetic estrogen, diethylstilbestrol (DES), on intermediary metabolism was studied in a freshwater fish Oreochromis mossambicus. The present study reveals that testosterone, progesterone, and Des specifically control key enzymes involved in carbohydrate, protein and lipid metabolism in the liver of O. mossambicus implying a general influence of sex steroids on intermediary metabolism. The activities of malic enzyme (ME), glucose-6-phosphate dehydrogenase (G6PDH), isocitrate dehydrogenase (ICDH), glucose 6 phosphatase (G-6-Pase), aspartate aminotransferase (AST), and alanine aminotransferase (ALT) are either stimulated or inhibited following the administration of sex steroids. The long-term in vivo i.p. injection of sex steroids intensely reveals that testosterone and progesterone are hyperglycemic, DES is hypoglycemic, testosterone and DES lipogenic, and progesterone antilipogenic (lipolytic) in the present study. It is also established that amino acid catabolism, mostly that of alanine, may be a major source of substrate for gluconeogenesis. A genomic mode of action is proposed for sex steroids for long term treatment, as their action is sensitive to transcription and translation inhibitors.

Maturational changes in brain contents of salmon GnRH in rainbow trout as measured by a newly developed time-resolved fluoroimmunoassay

A newly developed time-resolved fluoroimmunoassay (TR-FIA) for salmon gonadotropin-releasing hormone (sGnRH) was applied to investigate changes in sGnRH content in discrete brain areas at three different gonadal stages in the rainbow trout, Oncorhynchus mykiss. The sensitivity (6.8 pg/well), specificity, intraassay coefficients of variation (<7.4%), and interassay coefficients of variation (<10.3%) of the assay system were almost the same as those for the radioimmunoassay. Displacement curves of serially diluted brain extracts of nine teleost fish (freshwater fish and seawater fish) including rainbow trout paralleled that of the sGnRH standard, indicating that the sGnRH TR-FIA is widely applicable to the measurement of the brain sGnRH contents of various fishes. The sGnRH content in female hypothalamus decreased during final gonad maturation, whereas the sGnRH levels in pituitary were highest at the time of spermiating in males or ovulating in females, decreasing significantly thereafter. In contrast, there were no changes in the sGnRH contents of olfactory bulbs, telencephalon, optic tectum + thalamus, and cerebellum + medulla oblongata during final maturation, except for olfactory bulbs of males. Changes in sGnRH contents in the hypothalamus and the pituitary indicate that sGnRH is involved in final maturation (ovulation or spermiation) in the rainbow trout.

Prepubertal increases in the levels of two salmon gonadotropin-releasing hormone mRNAs in the ventral telencephalon and preoptic area of masu salmon

Ontogenic changes in the expression levels of two salmon gonadotropin-releasing hormone genes (sGnRH-I and -II) were examined in the forebrain region including the ventral telencephalon and preoptic area of masu salmon by competitive reverse transcription-polymerase chain reaction (RT-PCR). Two genes showed similar expression patterns throughout the lifetime in both sexes, although the levels of sGnRH-II mRNA were about 20 times higher than those of sGnRH-I mRNA. In males, the levels of sGnRH mRNAs increased at the beginning of the second year and reached their maximum in the autumn. The levels decreased gradually until the autumn of the third year when fish sexually matured. In females, the levels reached their maximum in the first autumn and fluctuated considerably along with the seasons in the third year. These results suggest that, in the salmon brain, sGnRH genes are activated long before the sexual maturation under sexually different control mechanisms.

The effects of testosterone on upstream migratory behavior in masu salmon, Oncorhynchus masou

Effects of testosterone (T) on upstream migratory behavior in masu salmon, Oncorhynchus masou, were studied by use of artificial raceways. In Experiment 1, yearling precocious males castrated in August were implanted with a capsule of medical silicone tube containing 500 microg of T in September. Their upstream migration was recorded in an artificial raceway with castrated and sham-operated precocious males implanted with a capsule containing vehicle for 2 months. In Experiment 2, upstream migratory behavior of yearling immature parr implanted with a capsule containing T 500 microg or vehicle was observed from September through November. In Experiment 3, upstream migratory behavior of castrated, castrated + T 50 microg, castrated + T 500 microg, and sham-operated precocious males was observed from September through October. In Experiment 4, upstream migratory behavior of the control, T 50 microg-, T 500 microg-, and T 1000 microg-treated immature parr was observed from September through October. In each experiment, plasma and pituitary samples were taken from the fish that moved upstream and remained to measure levels of T, gonadotropin (GTH) II, thyroxine (T(4)), and triiodothyronine (T(3)) by radioimmunoassay. Administration of T caused increases in plasma T levels within the physiological range. In Experiment 1, the frequency of migration upstream was 0, 19.2, and 35.7% in the castrated, castrated + T 500 microg, and sham-operated groups, respectively. The frequency was higher in the castrated + T 500 microg and sham-operated groups than in the castrated group. In Experiment 2, the frequency was higher in T 500 microg-treated parr (22%) than in the control (2.6%). In Experiments 1 and 2, pituitary contents of GTH II in the T-treated groups and precocious males were higher than those in the castrated precocious males and immature parr. In Experiment 3, castrated + T 50 microg, castrated + T 500 microg, and sham-operated fish showed upstream migratory behavior, whereas castrated fish without T did not. In Experiment 4, the frequency was 5.7, 22.9, 17.1, and 28.6% in the control, T 50 microg-, T 500 microg-, and T 1000 microg-treated groups, respectively. In each experiment, plasma levels of T(4) in migrants were lower than those in nonmigrants, whereas plasma levels of T(3) did not show such changes. From these results, it is inferred that T is a factor influencing upstream migration in masu salmon.