Assessment of spawning site fidelity in interior Fraser River Coho salmon Oncorhynchus kisutch using otolith microchemistry, in British Columbia, Canada

Coho Salmon Oncorhynchus kisutch show fidelity to natal spawning watersheds. Fine-scale homing, however, within rivers is not well understood. Interior Fraser Coho (IFC) salmon eggs were incubated at known spawning locations in the Coldwater River, two main stem sites and one-off channel pond site, providing otolith reference data for comparison to otolith signatures for returning adults using laser ablation inductively coupled plasma mass spectrometry. Elemental ratios for Ba:Ca and Sr:Ca in otoliths of juvenile O. kisutch differed significantly among the spawning locations examined. Juvenile otolith data were used to conduct a linear discriminant analysis to assess fine-scale homing in adults. Juvenile data were all assigned to the location where they had been incubated, producing a robust data set used to compare adult otoliths and define natal locations based on elemental signatures in otoliths of adult spawners. Homing and straying were apparent at the reach level; 57.1% of adults returned to their natal spawning locations, while 42.9% strayed to other spawning sites within the Coldwater River. Straying to novel incubation sites at the reach scale demonstrated plasticity in homing within a watershed.

Spatial and environmental effects on Coho Salmon life history trait variation

Adult size, egg size, fecundity, and mass of gonads are affected by trade-offs between reproductive investment and environmental conditions shaping the evolution of life history traits among populations for widely distributed species. Coho salmon Oncorhynchus kisutch have a large geographic distribution, and different environmental conditions are experienced by populations throughout their range. We examined the effect of environmental variables on female size, egg size, fecundity, and reproductive investment of populations of Coho Salmon from across British Columbia using an information theoretic approach. Female size increased with latitude and decreased with migration distance from the ocean to spawning locations. Egg size was lowest for intermediate intragravel temperature during incubation, decreased with migration distance, but increased in rivers below lakes. Fecundity increased with latitude, warmer temperature during the spawning period, and river size, but decreased in rivers below lakes compared with rivers with tributary sources. Relative gonad size increased with latitude and decreased with migration distance. Latitude of spawning grounds, migratory distance, and temperatures experienced by a population, but also hydrologic features-river size and headwater source-are influential in shaping patterns of reproductive investment, particularly egg size. Although, relative gonad size varied with latitude and migration distance, how gonadal mass was partitioned gives insight into the trade-off between egg size and fecundity. The lack of an effect of latitude on egg size suggests that local optima for egg size related to intragravel temperature may drive the variation in fecundity observed among years.

Behavioural changes during the parr-smolt transformation in coho salmon Oncorhynchus kisutch: is it better to be cool?

Behavioural changes that occur during the parr-smolt transformation were investigated in juvenile coho salmon Oncorhynchus kisutch. Fish from two populations were examined from the Fraser River catchment in British Columbia, Canada; a short and a long-distance migrating population. Fish showed a significant decrease in condition factor and significant increase in gill Na⁺ K⁺ -ATPase activity during the spring indicating that they became competent smolts, but no difference between populations. Temperature preference trials were conducted using a shuttlebox system throughout the spring. Mean temperature preference did not differ between the two populations, but preferred temperature decreased with development from 16.5 ± 0.3°C for parr to 15.5 ± 0.4°C for smolts. Mean swimming velocity was also greater in smolts than parr, but there was no difference between the two populations. The preference for warmer water temperature observed for parr in early spring may be advantageous for stimulating smolt development. Preference for slightly cooler temperatures observed for smolts would sustain elevated seawater tolerance during the smolt window by a short time and may ensure successful transition to the marine environment.

Is the duration of the smolt window related to migration distance in coho salmon Oncorhynchus kisutch?

Physiological changes during the parr-smolt transformation were investigated in short distance (Chilliwack River) and long-distance (Salmon River) migrating coho salmon Oncorhynchus kisutch populations in British Columbia, Canada. Biochemical and molecular indicators were used to monitor smolt development for fish reared at 10 °C throughout the spring. Fish grew well and developed the physical appearance of competent smolts. Both populations exhibited increases in gill Na⁺ -K⁺ -ATPase activity (NKA; an important indicator of seawater tolerance) at the same date and the duration of the increase in enzyme activity did not differ between populations. Gill messenger (m)RNA copies for two isoforms of the NKA α subunit, α1a and α1b, showed significant changes and the pattern was similar between populations. Growth hormone receptor and prolactin receptor mRNA from the gill showed modest changes associated with smolting in the spring for both populations, suggesting that these may not be useful indicators of smolt development in hatchery-reared O. kisutch. Consequently, the duration of the smolt window was not based on the region of origin in the present study.

Temporal and spatial differences in smolting among Oncorhynchus nerka populations throughout fresh and seawater migration

Physiological changes that occur in the spring are preparatory for salmonid smolts to successfully enter seawater, but variation is likely to exist within species with a wide geographic distribution. Whether differences in development of seawater tolerance exist among populations that differ in distance to the ocean, temporally during the spring, or as fish migrate downstream is not known. Juvenile sockeye salmon Oncorhynchus nerka from four regions in the Fraser River catchment, British Columbia, were intercepted to assess physiological differences among populations and at different times during migration to characterize the parr-smolt transformation. Pre-migratory fish had low levels of gill Na⁺ -K⁺ -ATPase (NKA) activity. High gill NKA activities were observed at the start of migration for some populations, but smolts leaving the lake did not consistently have higher gill NKA activity than non-migratory juvenile O. nerka sampled in their natal lakes. Gill NKA activity was highly variable at the start of migration with no relationship with distance from the ocean. Gill NKA activity changes with migration were also highly variable, but consistently smolts in the ocean had the highest enzyme activities. Internal and external factors may influence this variation, but the dynamic nature of smolting was not based on the region of origin, timing during migration or on the year of migration.

The effect of substrate rearing on growth, aerobic scope and physiology of larval white sturgeon Acipenser transmontanus

The effect of substratum on growth and metabolic rate was assessed in larval white sturgeon Acipenser transmontanus. Yolk-sac larvae (YSL) were reared in bare tanks or tanks with gravel as substratum from hatch until approximately 16 days post hatch (dph). The effect of an artificial substratum was also evaluated on growth alone. Substratum had a significant effect on mass, with larvae reared in gravel and artificial substrata being larger than those reared without substratum. Routine metabolic rates were significantly lower and relative aerobic scope (the difference between maximum and routine metabolic rate) was significantly higher for YSL and feeding larvae (FL) reared in gravel relative to those reared in bare tanks, particularly before fish started feeding exogenously. Furthermore, gravel-reared larvae had higher whole-body glycogen concentrations relative to bare-tank-reared larvae. Routine factorial scope (maximum metabolic rate divided by routine metabolic rate) was relatively low in all treatments (< 1·7) indicating a limited ability to elevate metabolic rate above routine early in development and mass exponents for metabolic rate exceeded 1. Taken together, these data indicate that YSL reared without substratum may divert more of their energy to non-growth related processes impairing growth. This finding underscores the importance of adequate rearing substratum for growth of A. transmontanus and may provide support for habitat restoration and alternative hatchery rearing methods associated with sturgeon conservation.

Is fishing selective for physiological and energetic characteristics in migratory adult sockeye salmon?

There is extensive evidence that fishing is often selective for specific phenotypic characteristics, and that selective harvest can thus result in genotypic change. To date, however, there are no studies that evaluate whether fishing is selective for certain physiological or energetic characteristics that may influence fish behaviour and thus vulnerability to capture. Here, adult sockeye salmon (Oncorhynchus nerka) were used as a model to test the null hypothesis that fishing is not selective for specific physiological or energetic traits. Fish were intercepted during their spawning migrations, implanted with a gastric radio transmitter, and biopsied (i.e., non-lethally sampled for blood, gill tissue and quantification of energetic status). In both 2003 and 2006, we tagged and biopsied 301 and 770 sockeye salmon, respectively, in the marine environment en route to their natal river system to spawn. In 2006 an additional 378 individuals were tagged and biopsied in freshwater. We found that 23 (7.6%) of the marine fish tagged in 2003, 78 (10.1%) of the marine fish tagged in 2006 and 57 (15.1%) of the freshwater fish tagged in 2006 were harvested by one of three fisheries sectors that operate in the coastal marine environment and the Fraser River (i.e. commercial, recreational or First Nations fisheries between the site of release and Hell's Gate in the Fraser River, approximately 250 km upriver and 465 km from the ocean tagging site). However, fisheries were not open continually or consistently in different locations and for different fisheries sectors necessitating a paired analytical approach. As such, for statistical analyses we paired individual fish that were harvested with another fish of the same genetic stock that was released on the same date and exhibited similar migration behaviour, except that they successfully evaded capture and reached natal spawning grounds. Using two-tailed Wilcoxon matched pairs signed-rank tests, we revealed that the physiological and energetic characteristics of harvested fish did not differ from those of the successful migrants despite evaluating a number of biochemical (e.g. plasma metabolites, cortisol, plasma ions, gill Na⁺/K⁺-ATPase) and energetic (e.g. gross somatic energy density) variables (P's all >0.10). However, for some analyses we suffered low statistical power and the study design had several shortcomings that could have made detection of differences difficult. We suggest that additional research explore the concept of fishing-induced selection for physiological characteristics because physiology is closely linked to three traits where fisheries-induced selection does occur (i.e. life-history, behaviour and morphology).

Freshwater movement patterns by juvenile Pacific salmon Oncorhynchus spp. before they migrate to the ocean: Oh the places you'll go!

Juvenile movement patterns for coho salmon Oncorhynchus kisutch and Chinook salmon Oncorhynchus tshawytscha from two large interior rivers of British Columbia, Canada, were examined. Otoliths from post-spawned fishes were collected on spawning grounds and elemental signatures were determined through transects from sectioned otoliths using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). Large variations in otolith elemental signatures were found during the freshwater life stage indicative of movement downstream to rivers and tributaries that differed in elemental signature. This study highlights that downstream movements occur before migration to the ocean during the parr-smolt transformation. Extensive downstream movements of parr appear to be a successful life-history strategy based on variations observed in the otolith elemental signatures of spawners. Movements downstream in parr and the remarkable homing ability of adults also suggest that imprinting to natal streams must occur prior to the parr-smolt transformation.

Differential physiological and endocrine responses of rainbow trout, Oncorhynchus mykiss, transferred from fresh water to ion-poor or salt water

To understand the physiological and molecular endocrine changes that occur in response to a salinity challenge, we transferred rainbow trout from fresh water to an ion-poor or 24‰ saltwater treatment for 14 days. An increase in gill Na(+), K(+)-ATPase (NKA) activity in salt water was associated with higher mRNA expression for the NKA α1b subunit. In contrast, there was little change in gill NKA activity following transfer to ion-poor water, but the mRNA expression of NKA α1a was significantly elevated. Endocrine signals were assessed by measuring plasma cortisol concentrations and by quantifying changes in mRNA extracted from the gill for glucocorticoid receptors 1 and 2 (GR1 and GR2), mineralocorticoid receptor (MR), growth hormone receptor (GHR1), and prolactin receptor (PrlR). Cortisol increased after transfer to ion-poor and salt water, but both GR and MR mRNA in the gill showed little change. PrlR mRNA was significantly higher when fish were transferred to the ion-poor water and GHR1 mRNA was elevated during the saltwater challenge. This study demonstrated an increase in gill PrlR mRNA that parallels the changes in gill NKA α1a when rainbow trout were transferred to a lower salinity level. Furthermore, the increase in gill GHR1 mRNA supports the importance of GH for seawater acclimation as there is a corresponding increase in the expression of gill NKA α1b, the saltwater isoform. GH and Prl, therefore, may differentially determine the function of cortisol in both fresh- and saltwater ionoregulation.

Genetic versus rearing-environment effects on phenotype: hatchery and natural rearing effects on hatchery- and wild-born coho salmon

With the current trends in climate and fisheries, well-designed mitigative strategies for conserving fish stocks may become increasingly necessary. The poor post-release survival of hatchery-reared Pacific salmon indicates that salmon enhancement programs require assessment. The objective of this study was to determine the relative roles that genotype and rearing environment play in the phenotypic expression of young salmon, including their survival, growth, physiology, swimming endurance, predator avoidance and migratory behaviour. Wild- and hatchery-born coho salmon adults (Oncorhynchus kisutch) returning to the Chehalis River in British Columbia, Canada, were crossed to create pure hatchery, pure wild, and hybrid offspring. A proportion of the progeny from each cross was reared in a traditional hatchery environment, whereas the remaining fry were reared naturally in a contained side channel. The resulting phenotypic differences between replicates, between rearing environments, and between cross types were compared. While there were few phenotypic differences noted between genetic groups reared in the same habitat, rearing environment played a significant role in smolt size, survival, swimming endurance, predator avoidance and migratory behaviour. The lack of any observed genetic differences between wild- and hatchery-born salmon may be due to the long-term mixing of these genotypes from hatchery introgression into wild populations, or conversely, due to strong selection in nature--capable of maintaining highly fit genotypes whether or not fish have experienced part of their life history under cultured conditions.

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.

Individual variation in migration speed of upriver-migrating sockeye salmon in the Fraser River in relation to their physiological and energetic status at marine approach

Little research has examined individual variation in migration speeds of Pacific salmon (Oncorhynchus spp.) in natural river systems or attempted to link migratory behavior with physiological and energetic status on a large spatial scale in the wild. As a model, we used three stocks of summer-run sockeye salmon (Oncorhynchus nerka) from the Fraser River watershed, British Columbia, to test the hypothesis that individual variation in migration speed is determined by a combination of environmental factors (i.e., water temperature), intrinsic biological differences (sex and population), and physiological and energetic condition. Before the freshwater portion of the migration, sockeye salmon (Quesnel, Chilcotin, and Nechako stock complexes) were captured in Johnstone Strait ( approximately 215 km from river entry), gastrically implanted with radio transmitters, and sampled for blood, gill tissue, and energetic status before release. Analyses focused solely on individuals that successfully reached natal subwatersheds. Migration speeds were assessed by an extensive radiotelemetry array. Individuals from the stock complex that migrated the longest distance (Nechako) traveled at speeds slower than those of other stock complexes. Females traveled slower than males. An elevated energetic status of fish in the ocean was negatively correlated with migration speed in most river segments. During the transition from the ocean to the river, migration speed was negatively correlated with mean maximum water temperature; however, for the majority of river segments, it was positively correlated with migration speed. Physiological status measured in the ocean did not explain among-individual variability in river migration speeds. Collectively, these findings suggest that there could be extensive variation in migration behavior among individuals, sexes, and populations and that physiological condition in the ocean explained little of this variation relative to in-river environmental conditions and energetic status. Interestingly, individual fish generally retained their rank in swimming speed across different segments, except when transiting a challenging canyon midway during the migration.

Are we missing a mineralocorticoid in teleost fish? Effects of cortisol, deoxycorticosterone and aldosterone on osmoregulation, gill Na+,K+ -ATPase activity and isoform mRNA levels in Atlantic salmon

It has long been held that cortisol, acting through a single receptor, carries out both glucocorticoid and mineralocorticoid actions in teleost fish. The recent finding that fish express a gene with high sequence similarity to the mammalian mineralocorticoid receptor (MR) suggests the possibility that a hormone other than cortisol carries out some mineralocorticoid functions in fish. To test for this possibility, we examined the effect of in vivo cortisol, 11-deoxycorticosterone (DOC) and aldosterone on salinity tolerance, gill Na(+),K(+)-ATPase (NKA) activity and mRNA levels of NKA alpha 1a and alpha 1b in Atlantic salmon. Cortisol treatment for 6-14 days resulted in increased, physiological levels of cortisol, increased gill NKA activity and improved salinity tolerance (lower plasma chloride after a 24h seawater challenge), whereas DOC and aldosterone had no effect on either NKA activity or salinity tolerance. NKA alpha 1a and alpha 1b mRNA levels, which increase in response to fresh water and seawater acclimation, respectively, were both upregulated by cortisol, whereas DOC and aldosterone were without effect. Cortisol, DOC and aldosterone had no effect on gill glucocorticoid receptor GR1, GR2 and MR mRNA levels, although there was some indication of possible upregulation of GR1 by cortisol (p=0.07). The putative GR blocker RU486 inhibited cortisol-induced increases in salinity tolerance, NKA activity and NKA alpha 1a and alpha 1b transcription, whereas the putative MR blocker spironolactone had no effect. The results provide support that cortisol, and not DOC or aldosterone, is involved in regulating the mineralocorticoid functions of ion uptake and salt secretion in teleost fish.

Dosage effects on heritability and maternal effects in diploid and triploid Chinook salmon (Oncorhynchus tshawytscha)

Induced triploidy (3N) in salmon results from a blockage of maternal meiosis II, and hence provides a unique opportunity to study dosage effects on phenotypic variance. Chinook salmon families were bred using a paternal half-sib breeding design (62 females and 31 males) and half of each resulting family was treated to induce triploidy. The paired families were used to test for dosage effects (resulting from triploidy) on (1) the distribution and magnitude of phenotypic variation, (2) narrow-sense heritability and (3) maternal effects in fitness-related traits (i.e., survival, size-at-age, relative growth rate and serum lysozyme activity). Quantitative genetic analyses were performed separately for diploid and triploid family groups. Triploidization resulted in significantly higher levels of phenotypic variance and substantial differences in patterns of variance distribution for growth and survival-related traits, although the patterns were reversed for lysozyme activity. Triploids exhibited higher narrow sense heritability values relative to diploid Chinook salmon. However, maternal effects estimates were generally lower in triploids than in diploids. Thus, the dosage effects resulting from adding an extra set of chromosomes to the Chinook salmon genome are primarily additive. Somewhat counterintuitively, however, the relative magnitude of the combined effects of dominance, epistasis and maternal effects is not affected by dosage. Our results indicate that inheritance of fitness-related quantitative traits is profoundly affected by dosage effects associated with induced triploidy, and that triploidization can result in unpredictable performance and fitness outcomes.

Mechanistic basis of individual mortality in Pacific salmon during spawning migrations

Reproductive-based migration is a challenging period for many animals, but particularly for Pacific salmonids, which must navigate from the high seas to freshwater natal streams. For the first time, we attempt to answer the question as to why some migratory adult Pacific salmon die en route to spawning grounds. Summer-run sockeye salmon (Oncorhynchus nerka) were used as a model, and the migration behavior of 301 fish was followed by intercepting them in the ocean about 215 km from the mouth of the Fraser River, British Columbia, Canada, and implanting a gastric radio transmitter. Before release, telemetered fish were also bio-sampled, which included drawing a blood sample, collecting a gill biopsy, and quantifying energetic status with a microwave energy meter. We tested the predictions that the fish that died prematurely would be characterized by low energy reserves, advanced reproductive development, elevated indicators of stress, and low osmoregulatory preparedness compared with fish that completed their river migration. Just over half (52.3%) of the sockeye tagged were subsequently detected in the Fraser River. Salmon that failed to enter the river had exhibited indicators of stress (e.g., elevated plasma lactate, glucose, and cortisol). Contrary to our prediction, fish that failed to enter the river tended to have higher gross somatic energy and be larger at the time of sampling in the ocean than fish that successfully entered the river. Of the fish that were detected in the river (i.e., 134 fish excluding fishery removals), 9.7% did not migrate beyond the lower reaches (approximately 250 km from ocean), and a further 14.2% reached the upper reaches but failed to reach natal sub-watersheds, whereas the remainder (76.1%) reached natal sub-watersheds. Of these, fish unsuccessful in the lower reaches tended to have a high plasma osmolality in the ocean, whereas fish failing in the upper reaches had lower levels of reproductive hormones in the ocean.

Ionoregulatory changes in different populations of maturing sockeye salmon Oncorhynchus nerka during ocean and river migration

We present the first data on changes in ionoregulatory physiology of maturing, migratory adult sockeye salmon Oncorhynchus nerka. Fraser River sockeye were intercepted in the ocean as far away as the Queen Charlotte Islands (approximately 850 km from the Fraser River) and during freshwater migration to the spawning grounds; for some populations this was a distance of over 700 km. Sockeye migrating in seawater toward the mouth of the Fraser River and upriver to spawning grounds showed a decline in gill Na+,K+-ATPase activity. As a result, gill Na+,K+-ATPase activity of fish arriving at the spawning grounds was significantly lower than values obtained from fish captured before entry into freshwater. Plasma osmolality and chloride levels also showed significant decreases from seawater values during the freshwater migration to spawning areas. Movement from seawater to freshwater increased mRNA expression of a freshwater-specific Na+,K+-ATPase isoform (alpha1a) while having no effect on the seawater-specific isoform (alpha1b). In addition, gill Na+,K+-ATPase activity generally increased in active spawners compared with unspawned fish on the spawning grounds and this was associated with a marked increase in Na+,K+-ATPase alpha1b mRNA. Increases in gill Na+,K+-ATPase activities observed in spawners suggests that the fish may be attempting to compensate for the osmotic perturbation associated with the decline in plasma chloride concentration and osmolality.

Routine and active metabolic rates of migrating adult wild sockeye salmon (Oncorhynchus nerka Walbaum) in seawater and freshwater

We present the first data on the differences in routine and active metabolic rates for sexually maturing migratory adult sockeye salmon (Oncorhynchus nerka) that were intercepted in the ocean and then held in either seawater or freshwater. Routine and active oxygen uptake rates (MO2) were significantly higher (27%-72%) in seawater than in freshwater at all swimming speeds except those approaching critical swimming speed. During a 45-min recovery period, the declining postexercise oxygen uptake remained 58%-73% higher in seawater than in freshwater. When fish performed a second swim test, active metabolic rates again remained 28%-81% higher for fish in seawater except at the critical swimming speed. Despite their differences in metabolic rates, fish in both seawater and freshwater could repeat the swim test and reach a similar maximum oxygen uptake and critical swimming speed as in the first swim test, even without restoring routine metabolic rate between swim tests. Thus, elevated MO2 related to either being in seawater as opposed to freshwater or not being fully recovered from previous exhaustive exercise did not present itself as a metabolic loading that limited either critical swimming performance or maximum MO2. The basis for the difference in metabolic rates of migratory sockeye salmon held in seawater and freshwater is uncertain, but it could include differences in states of nutrition, reproduction, and restlessness, as well as ionic differences. Regardless, this study elucidates some of the metabolic costs involved during the migration of adult salmon from seawater to freshwater, which may have applications for fisheries conservation and management models of energy use.

Census vs. effective population size in chinook salmon: large- and small-scale environmental perturbation effects

Population viability has often been assessed by census of reproducing adults. Recently this method has been called into question and estimation of the effective population size (Ne) proposed as a complementary method to determine population health. We examined genetic diversity in five populations of chinook salmon (Oncorhynchus tshawytscha) from the upper Fraser River watershed (British Columbia, Canada) at 11 microsatellite loci over 20 years using DNA extracted from archived scale samples. We tested for changes in genetic diversity, calculated the ratio of the number of alleles to the range in allele size to give the statistic M, calculated Ne from the temporal change in allele frequency, used the maximum likelihood method to calculate effective population size (NeM), calculated the harmonic mean of population size, and compared these statistics to annual census estimates. Over the last two decades population size has increased in all five populations of chinook examined; however, Ne calculated for each population was low (81-691) and decreasing over the time interval measured. Values of NeM were low, but substantially higher than Ne calculated using the temporal method. The calculated values for M were generally low (M < 0.70), indicating recent population reductions for all five populations. Large-scale historic barriers to migration and development activities do not appear to account for the low values of Ne; however, available spawning area is positively correlated with Ne. Both Ne and M estimates indicate that these populations are potentially susceptible to inbreeding effects and may lack the ability to respond adaptively to stochastic events. Our findings question the practice of relying exclusively on census estimates for interpreting population health and show the importance of determining genetic diversity within populations.

Heritability and Y-chromosome influence in the jack male life history of chinook salmon (Oncorhynchus tshawytscha)

Jacking in chinook salmon (Oncorhynchus tshawytscha) is an alternative reproductive strategy in which males sexually mature at least 1 year before other members of their year class. We characterize the genetic component of this reproductive strategy using two approaches; hormonal phenotypic sex manipulation, and a half-sib breeding experiment. We 'masculinized' chinook salmon larvae with testosterone, reared them to first maturation, identified jacks and immature males based on phenotype, and genotyped all fish as male ('XY') or female ('XX') using PCR-based Y-chromosome markers. The XY males had a much higher incidence of jacking than the XX males (30.8% vs 9.9%). There was no difference in body weight, gonad weight, and plasma concentrations of testosterone and 17beta-estradiol between the two jack genotypes, although XY jacks did have a higher gonadosomatic index (GSI) than XX jacks. In the second experiment, we bred chinook salmon in two modified half-sib mating designs, and scored the number of jacks and immature fish at first maturation. Heritability of jacking was estimated using two ANOVA models: dams nested within sires, and sires nested within dams with one-half of the half-sib families common to the two models. The sire component of the additive genetic variance yielded a high heritability estimate and was significantly higher than the dam component (h(2)(sire) = 0.62 +/- 0.21; h(2)(dam) = -0.14 +/- 0.12). Our experiments both indicated a strong sex-linked component (Y-chromosome) to jacking in chinook salmon, although evidence for at least some autosomal contribution was also observed.

Effects of an advanced temperature cycle on smolt development and endocrinology indicate that temperature is not a zeitgeber for smolting in Atlantic salmon

Atlantic salmon (Salmo salar) juveniles were reared under simulated conditions of normal photoperiod (LDN) or short days (LD 9:15) and ambient temperature (AMB: normal temperature increases in April) or an advanced temperature cycle (ADV: temperature increases in February). Under both photoperiod conditions, the timing of increased and peak levels of gill Na+,K+-ATPase activity were not altered by temperature,although the rate of increase was initially greater under ADV. ADV/LD 9:15 resulted in peak gill Na+,K+-ATPase activity that was half of that seen under normal photoperiod and temperature conditions. Plasma growth hormone (GH) levels increased threefold in late March under ADV/LDN,but not under ADV/LD 9:15, indicating that there is a photoperiod-dependent effect of temperature on levels of this hormone. Plasma insulin-like growth factor I (IGF-I) increased in spring in all groups, with increases occurring significantly earlier in the ADV/LDN group. In each photoperiod condition, the advanced temperature cycle resulted in large decreases in plasma thyroxine(T4) levels in March, which subsequently recovered, whereas plasma 3,5,3′-triiodo-L-thyronine (T3) levels were not substantially affected by either photoperiod or temperature. There was no consistent pattern of change in plasma cortisol levels. The results do not provide support for the role of temperature as a zeitgeber, but do indicate that temperature has a role in the timing of smolting by affecting the rate of development and interacting with the photoperiod.

The presence of high-affinity, low-capacity estradiol-17beta binding in rainbow trout scale indicates a possible endocrine route for the regulation of scale resorption

High-affinity, low-capacity estradiol-17beta (E(2)) binding is present in rainbow trout scale. The K(d) and B(max) of the scale E(2) binding are similar to those of the liver E(2) receptor (K(d) is 1.6 +/- 0.1 and 1.4 +/- 0.1 nM, and B(max) is 9.1 +/- 1.2 and 23. 1 +/- 2.2 fmol x mg protein(-1), for scale and liver, respectively), but different from those of the high-affinity, low-capacity E(2) binding in plasma (K(d) is 4.0 +/- 0.4 nM and B(max) is 625.4 +/- 63. 1 fmol x mg protein(-1)). The E(2) binding in scale was displaced by testosterone, but not by diethylstilbestrol. Hence, the ligand binding specificity is different from that of the previously characterized liver E(2) receptor, where E(2) is displaced by diethylstilbestrol, but not by testosterone. The putative scale E(2) receptor thus appears to bind both E(2) and testosterone, and it is proposed that the increased scale resorption observed during sexual maturation in both sexes of several salmonid species may be mediated by this receptor. No high-affinity, low-capacity E(2) binding could be detected in rainbow trout gill or skin.

Responsiveness of gill Na+/K+-ATPase to cortisol is related to gill corticosteroid receptor concentration in juvenile rainbow trout

A positive relationship between receptor concentration and tissue responsiveness is an often-assumed and rarely tested principle in endocrinology. In salmonids, seasonal changes in levels of plasma cortisol and gill corticosteroid receptors (CRs) during the spring indicate a potential role for this hormone in the parr-smolt transformation. It is not known whether these seasonal changes result in alterations in gill responsiveness to cortisol. The relationship between CR concentration and tissue responsiveness was, therefore, examined in the gills of juvenile rainbow trout (Oncorhynchus mykiss). Gill CR concentration (Bmax) and affinity (Kd) were assessed using a radioligand binding assay with the synthetic glucocorticoid triamcinolone acetonide. Gill responsiveness to cortisol was quantified by measuring in vitro Na+/K+-ATPase activity. Gill CR concentration was manipulated by stress or hormonal treatments. Repeated handling stresses resulted in a significant reduction in CR numbers. The decrease in CR Bmax corresponded to a reduction in gill responsiveness to cortisol. Triiodothyronine, but not growth hormone, treatment was found to increase CR Bmax significantly. The increase in CR numbers was correlated with a marked increase in gill responsiveness to cortisol. A significant positive linear relationship exists between the in vitro gill Na+/K+-ATPase activity response to cortisol and CR Bmax (r2=0.614, P&lt;0.001). We have demonstrated that binding sites for cortisol in the gills of rainbow trout have high affinity, high specificity and saturable binding and that the number of binding sites is correlated with the tissue response to cortisol.

Regulation of gill cytosolic corticosteroid receptors in juvenile Atlantic salmon: interaction effects of growth hormone with prolactin and triiodothyronine

The potential effects of growth hormone (GH), prolactin (Prl), and triiodothyronine (T3) on gill Na+,K+-ATPase activity and corticosteroid receptor (CR) concentration (Bmax) and dissociation constant (Kd) were examined in juvenile Atlantic salmon (Salmo salar). Compared to controls, fish injected with GH (ovine, 5.0 microgram g-1) had significantly greater gill Na+,K+-ATPase activity after 7 and 14 days. Gill CR Bmax and Kd were significantly elevated on day 7, but not day 14. T3 also significantly increased CR Bmax. The effect of GH on CR Bmax was also additive with T3 (5.0 microgram g-1) treatment. There was a synergistic effect on CR Bmax when purified coho salmon GH (csGH, 0.1 microgram g-1) was injected in combination with T3 (1.6 microgram g-1). Prl (ovine, 5.0 microgram g-1; purified coho salmon, 0.1 microgram g-1) did not significantly alter gill CR Bmax. Although Prl limited the increase in CR Bmax by GH, the effect was not signicant. T3 and Prl did not have an effect on Kd. GH significantly increased gill Na+,K+-ATPase activity, T3 administration did not have a significant effect, and Prl-treated fish had significantly lower gill Na+,K+-ATPase activity. The results indicate that T3 acts additively with GH, while Prl has no effect in regulating CR Bmax. An increase in cytosolic CR by GH and T3, but not Prl, may regulate gill responsiveness to cortisol and be an important mechanism in the endocrine control of physiological changes during the parr-smolt transformation.

Increases in gill cytosolic corticosteroid receptor abundance and saltwater tolerance in juvenile coho salmon (Oncorhynchus kisutch) treated with growth hormone and placental lactogen

Juvenile coho salmon (Oncorhynchus kisutch) were injected with one of two recombinant bovine hormones, growth hormone (bGH; 5.0 and 0.5 micrograms.g-1 body wt) or placental lactogen (bPL; 5.0, 0.5, and 0.05 micrograms.g-1 body wt) to determine the effect on growth, plasma cortisol concentration, cytosolic corticosteroid receptors (CR) in the gills, and the development of hypoosmoregulatory ability. One week following a single injection or six weekly injections of bGH or bPL, the fish were measured and sampled for CR concentration and Na+,K(+)-ATPase activity in the gills. Fish were also challenged with salt water (salinity 25%) for 24 hr to determine saltwater tolerance at the end of the 6-week treatment. Treatment with bPL and bGH significantly increased weight and length of the fish. The 0.05-micrograms bPL dose significantly elevated plasma cortisol concentration, whereas all other hormone treatments did not affect cortisol levels. bPL and bGH also significantly increased CR concentration and Na+,K(+)-ATPase activity in the gills. The perturbation in plasma sodium concentration was least in animals receiving the highest dose of bPL and the bGH-treated animals following transfer to seawater. An increase in cytosolic CR by bGH and bPL may increase responsiveness of the gills to cortisol and partially account for the increase in Na+,K(+)-ATPase activity and greater ability to regulate plasma sodium in seawater as exhibited by the experimental groups.

Downregulation of corticosteroid receptors in gills of coho salmon due to stress and cortisol treatment

The effect of stress and cortisol treatment on corticosteroid receptors (CRs) in the gills of coho salmon (Oncorhynchus kisutch) was examined. Plasma cortisol levels were elevated acutely by intraperitoneal injection of cortisol 21-hemisuccinate and chronically by implanting miniosmotic pumps filled with cortisol or by repeated daily handling stresses. CR concentration and affinity were measured by radioreceptor assay employing 3H-labeled triamcinolone acetonide as ligand. Acute administration of cortisol resulted in a reduction in CR numbers for 72 h with no change in affinity. Chronic cortisol treatment resulted in a decrease in CR concentration and affinity. The change in affinity occurred only while plasma cortisol levels remained elevated, but CR population remained significantly reduced for at least 10 days after cessation of hormone treatment. Repeated handling stresses resulted in a similar reduction in CR numbers but without an apparent change in affinity. The chronic or repeated elevation in plasma cortisol downregulates the sensitivity of the gills to cortisol by a persistent reduction in CR concentration, despite the return to nonstress levels of circulating cortisol.