Stress up-regulates oxidative burst in juvenile Chinook salmon leukocytes

When fish perceive stressful scenarios, their hypothalamus-pituitary-interrenal axis is activated resulting in the release of corticotropin releasing hormone, adrenocorticotropic hormone (ACTH), and finally cortisol. The physiologic stress response of fish has most often been linked to the reduced performance of the immune system, with a few exceptions where the immune system is activated. In this report, we tested the hypothesis that oxidative burst activity levels in juvenile Chinook salmon (Oncorhynchus tshawytscha) are altered when the fish is presented with a stressor. Fish were subjected to a stressor for 3 h and then allowed to recover for 20 h following the stressor. Plasma and spleens were collected from euthanized fish before the stressor, at the end of a 3 h stressor, and 23 h after the start of the experiment. Plasma was held at -80 °C until cortisol radioimmunoassay analysis was performed to confirm stress. Spleens were held in Dulbecco's Modified Eagle Medium overnight and analyzed the day following collection. Oxidative burst activity was measured in splenic leukocytes after being stimulated with phorbol 12-myristate 13-acetate. We found a significant increase in activated oxidative burst from fish subjected to the stressor as compared to unstressed fish. Speculation is given to ACTH being the leukocyte priming agent in this experiment rather than the cortisol itself.

Swimming Endurance in Juvenile Chinook Salmon Infected with Salmincola californiensis

Juvenile Chinook Salmon Oncorhynchus tshawytscha moving downstream through tributaries of the upper Willamette River basin can spend months in reservoirs created by dams. While residing in the reservoirs, they often obtain heavy infections of the freshwater parasitic copepod Salmincola californiensis. The physiologic effect these parasites have on salmonids is poorly understood. We developed a method to infect juvenile Chinook Salmon in a laboratory with the copepodid stage of S. californiensis. Infected and uninfected fish were subjected to a swimming challenge to ascertain swimming endurance. Severity of gill damage was assessed using a dissecting microscope. Juvenile Chinook Salmon naturally infected with S. californiensis in Cougar Reservoir, Oregon, were also challenged and compared with their lab-infected counterparts. Copepod infection greatly impaired the swimming ability of laboratory fish, and the naturally infected fish were entirely incapable of swimming at low velocity. Chinook Salmon collected in the wild were more heavily infected than the laboratory fish and had trouble surviving collection and transport to our laboratory. The intensity of infection and severity of gill damage were positively correlated with diminished swimming ability, suggesting that heavy infection with copepods impairs gas exchange and osmotic regulation, which likely results in diminished fitness and decreased survival of infected fish.

Growth and annual survival estimates to examine the ecology of larval lamprey and the implications of ageing error in fitting models

This study used existing western brook lamprey Lampetra richardsoni age information to fit three different growth models (i.e. von Bertalanffy, Gompertz and logistic) with and without error in age estimates. Among these growth models, there was greater support for the logistic and Gompertz models than the von Bertalanffy model, regardless of ageing error assumptions. The von Bertalanffy model, however, appeared to fit the data well enough to permit survival estimates; using length-based estimators, annual survival varied between 0·64 (95% credibility interval: 0·44-0·79) and 0·81 (0·79-0·83) depending on ageing and growth process error structure. These estimates are applicable to conservation and management of L. richardsoni and other western lampreys (e.g. Pacific lamprey Entosphenus tridentatus) and can potentially be used in the development of life-cycle models for these species. These results also suggest that estimators derived from von Bertalanffy growth models should be interpreted with caution if there is high uncertainty in age estimates.

Body morphology differs in wild juvenile Chinook salmon Oncorhynchus tshawytscha that express different migratory phenotypes in the Willamette River, Oregon, U.S.A

Body morphology of juvenile Chinook salmon Oncorhynchus tshawytscha in the upper Willamette River, Oregon, U.S.A., was analysed to determine if variation in body shape is correlated with migratory life-history tactics followed by juveniles. Body shape was compared between migrating juveniles that expressed different life-history tactics, i.e. autumn migrants and yearling smolts, and among parr sampled at three sites along a longitudinal river gradient. In the upper Willamette River, the expression of life-history tactics is associated with where juveniles rear in the basin with fish rearing in downstream locations generally completing ocean ward migrations earlier in life than fish rearing in upstream locations. The morphological differences that were apparent between autumn migrants and yearling smolts were similar to differences between parr rearing in downstream and upstream reaches, indicating that body morphology is correlated with life-history tactics. Autumn migrants and parr from downstream sampling sites had deeper bodies, shorter heads and deeper caudal peduncles compared with yearling smolts and parr from the upstream sampling site. This study did not distinguish between genetic and environmental effects on morphology; however, the results suggest that downstream movement of juveniles soon after emergence is associated with differentiation in morphology and with the expression of life-history variation.

Post-mortem sporulation of Ceratomyxa shasta (Myxozoa) after death in adult Chinook salmon

Ceratomyxa shasta (Myxozoa) is a common gastrointestinal pathogen of salmonid fishes in the Pacific Northwest of the United States. We have been investigating this parasite in adult Chinook salmon ( Oncorhynchus tshawytscha ) in the Willamette River, Oregon. In prior work, we observed differences in the pattern of development of C. shasta in adult salmon compared to juvenile salmon. Adult salmon consistently had large numbers of prespore stages in many of the fish that survived to spawn in the fall. However, myxospores were rarely observed, even though they were exposed and presumably infected for months before spawning. We evaluated the ability of C. shasta to sporulate following fish death because it is reported that myxosores are common in carcasses of Chinook salmon. We collected the intestine from 30 adult salmon immediately after artificial spawning and death (T0). A total of 23 fish were infected with C. shasta based on histology, but only a few myxospores were observed in 1 fish by histology. Intestines of these fish were examined at T0 and T7 (latter held at 17 C for 7 days) using quantified wet mount preparations. An increase in myxospore concentrations was seen in 39% of these fish, ranging between a 1.5- to a 14.5-fold increase. The most heavily infected fish exhibited a 4.6-fold increase from 27,841 to 129,352 myxospores/cm. This indicates, supported by various statistical analyses, that under certain conditions presporogonic forms are viable and continue to sporulate after death in adult salmon. Considering the life cycle of C. shasta and anadromous salmon, the parasite may have evolved 2, non-mutually exclusive developmental strategies. In young fish (parr and smolts), the parasite sporulates shortly after infection and is released into freshwater from either live or dead fish before their migration to seawater, where the alternate host is absent. The second strategy occurs in adult salmon, particularly spring Chinook salmon, which become infected upon their return to freshwater in the spring or early summer. For several months throughout the summer, only prespore stages are observed in most fish, even at the time of spawning. But once the fish dies, environmental conditions experienced by C. shasta change and viable presporogonic stages are induced to sporulate. As the post-spawned fish occur in the upper reaches of rivers, the myxospores would be released in a freshwater environment that would provide a reasonable opportunity for them to encounter their freshwater polychaete hosts, which reside downstream.

Identification of biomarkers indicative of barotrauma and recovery in black rockfish Sebastes melanops

A Sebastes-specific complementary DNA (cDNA) microarray was developed to identify potential biomarkers involved in the capture stress and recovery of Sebastes species if they are assisted in returning to their original depth of capture following barotrauma. Black rockfish Sebastes melanops were exposed to simulated decompression from 450 kPa (c. 35 m depth) (which resulted in barotrauma) and subsequent recompression. Sebastes melanops were sampled for liver tissue at days 3, 15 and 31 post-barotrauma. Potential candidate genes were identified from the microarray and then quantitative real-time PCR (qrt-PCR) was used to validate expression levels in biological replicates. Six potential biomarkers associated with the innate immune system were identified that were up-regulated in liver tissue at 3 days post-barotrauma: complement C1q-like protein 2, complement component C3, complement regulatory plasma protein, serum amyloid A-5, c-type lysozyme and hepcidin precursor type I. In addition, complement c1q was correlated to the presence of a ruptured swimbladder, providing further support that this gene may be a good biomarker of injury and recovery. Immune genes were no longer up-regulated at day 31 post-barotrauma, a good indication of recovery in S. melanops.

Recovery potential of black rockfish, Sebastes melanops Girard, recompressed following barotrauma

Overfished species of rockfish, Sebastes spp., from the Northeast Pacific experience high bycatch mortality because of 'barotrauma', a condition induced from the rapid change in pressure during capture. Field experiments show that it may be possible for rockfish to recover from barotrauma if quickly recompressed; however, no work has followed the physiological recovery of rockfish after recompression or determined whether it is possible for rockfish to survive barotrauma in the long term. Barotrauma was induced in adult black rockfish, Sebastes melanops Girard, from a simulated depth of 35 m, followed by recompression. Blood and selected tissues (eye, heart ventricle, head kidney, liver, rete mirabile and gonad) were sampled at days 3, 15 and 31 post-recompression to evaluate the tissue- and physiologic-level response during recovery. No mortality from barotrauma occurred during the experiments, and feeding resumed in 80% of both treatment and control fish. The primary injury in treatment fish was the presence of a ruptured swimbladder and/or a ruptured tunica externa (outer layer of swimbladder), which was slow to heal. Blood plasma was analysed for glucose, sodium, chloride, potassium, calcium, phosphorus, insulin-like growth factor-1 and cortisol. Plasma analyses indicated no strong effects because of barotrauma, suggesting overall handling stress outweighed any effect from barotrauma. Rockfish with ruptured swimbladders may face compromised competency in the wild; however, it appears the majority of black rockfish decompressed from 35 m have a high potential for recovery if recompressed immediately after capture. This research suggests recompression could be a valuable bycatch mortality reduction tool for rockfish in recreational fisheries.

Husbandry stress exacerbates mycobacterial infections in adult zebrafish, Danio rerio (Hamilton)

Mycobacteria are significant pathogens of laboratory zebrafish, Danio rerio (Hamilton). Stress is often implicated in clinical disease and morbidity associated with mycobacterial infections but has yet to be examined with zebrafish. The aim of this study was to examine the effects of husbandry stressors on zebrafish infected with mycobacteria. Adult zebrafish were exposed to Mycobacterium marinum or Mycobacterium chelonae, two species that have been associated with disease in zebrafish. Infected fish and controls were then subjected to chronic crowding and handling stressors and examined over an 8-week period. Whole-body cortisol was significantly elevated in stressed fish compared to non-stressed fish. Fish infected with M. marinum ATCC 927 and subjected to husbandry stressors had 14% cumulative mortality while no mortality occurred among infected fish not subjected to husbandry stressors. Stressed fish, infected with M. chelonae H1E2 from zebrafish, were 15-fold more likely to be infected than non-stressed fish at week 8 post-injection. Sub-acute, diffuse infections were more common among stressed fish infected with M. marinum or M. chelonae than non-stressed fish. This is the first study to demonstrate an effect of stress and elevated cortisol on the morbidity, prevalence, clinical disease and histological presentation associated with mycobacterial infections in zebrafish. Minimizing husbandry stress may be effective at reducing the severity of outbreaks of clinical mycobacteriosis in zebrafish facilities.

Using blood plasma for monitoring organochlorine contaminants in juvenile white sturgeon, Acipenser transmontanus, from the lower Columbia River

Organochlorine (OC) pesticide concentrations in blood plasma samples from 88 juvenile white sturgeon collected from the lower Columbia River were measured and compared to plasma sex steroid and OC tissue levels previously measured in corresponding fish. Significant squared correlation coefficients between summation operator DDT concentrations in sturgeon plasma and gonads and livers were 0.37 and 0.32, respectively. Significant negative correlations between plasma testosterone concentration and plasma Sigma DDT concentration in male fish (r(2)=0.26), plasma 17beta estradiol concentration and plasma Sigma DDT concentration in female fish (r(2)=0.38) and condition factor and plasma Sigma DDT concentration in all fish were found (r(2)=0.17). These results suggest that blood plasma may be a suitable nondestructive method for monitoring adult sturgeon population for persistent OC contaminants.

Chronic administration of fluoxetine alters locomotor behavior, but does not potentiate the locomotor stimulating effects of CRH in juvenile Chinook salmon (Oncorhynchus tshawytscha)

The present study investigated: 1) the behavioral effects of chronic administration of a serotonin uptake inhibitor (fluoxetine) in juvenile Chinook salmon, Oncorhynchus tshawytscha and, 2) whether chronic administration of fluoxetine alters the behavioral effects of corticotropin-releasing hormone (CRH). Chronic (20 day) treatment with fluoxetine decreased locomotor activity when compared to fish given long-term injections of saline. An intracerebroventricular (i.c.v.) injection of CRH had no effect on locomotor activity following a 20 day intraperitoneal treatment with either saline or fluoxetine. Chronic treatment with fluoxetine also increased the amount of time fish spent near the center of the tank. A similar increase was seen in fish given a chronic intraperitoneal (i.p.) series of saline followed by an acute i.c.v. injection of CRH. However, the effect was not additive when fish were given chronic i.p. injections of fluoxetine followed by an acute i.c.v. injection of CRH. These results provide evidence to support the hypothesis that the serotonergic system is involved in mediating locomotor activity and habitat choice in teleosts.

Mercury concentrations in gonad, liver, and muscle of white sturgeon Acipenser transmontanus in the lower Columbia River

This study determined the partitioning of total mercury in liver, gonad, and cheek muscle of white sturgeon (Acipenser transmonatus) in the lower Columbia River. The relationship between tissue mercury concentrations and various physiologic parameters was assessed. White sturgeon were captured in commercial fisheries in the estuary and Bonneville, The Dalles, and John Day Reservoirs. Condition factor (CF), relative weight (Wr), and gonadosomatic index (GSI) were determined for each fish (n = 57). Gonadal tissue was examined histologically to determine sex and stage of maturity. Liver (n = 49), gonad (n = 49), and cheek muscle (n = 57) were analyzed for total mercury using cold-vapor atomic fluorescence spectrophotometry. Tissue protein concentrations were measured by ultraviolet-visible spectroscopy. Plasma was analyzed for testosterone (T), 11-ketotestosterone (KT), and 17ss-estradiol (E2) using radioimmunoassay. Mean tissue mercury concentrations were higher in muscle compared with liver and gonad at all sampling locations, except Bonneville Reservoir where mean liver mercury content was the highest tissue concentration observed in the study. Significant negative correlations between plasma androgens (T and KT) and muscle mercury content and plasma E2 and liver mercury content were found. A significant positive linear relationship between white sturgeon age and liver mercury concentrations was evident. Significant negative correlations between CF and relative weight and gonad and liver mercury content were found. In addition, immature male sturgeon with increased gonad mercury content had decreased GSIs. These results suggest that mercury, in the form of methylmercury, may have an effect on the reproductive potential of white sturgeon.

Antibody-producing cells correlated to body weight in juvenile chinook salmon (Oncorhynchus tshawytscha) acclimated to optimal and elevated temperatures

The immune response of juvenile chinook salmon (Oncorhynchus tshawytscha) ranging in weight from approximately 10 to 55 g was compared when the fish were acclimated to either 13 or 21 degrees C. A haemolytic plaque assay was conducted to determine differences in the number of antibody-producing cells (APC) among fish of a similar age but different body weights. Regression analyses revealed significant increases in the number of APC with increasing body weight when fish were acclimated to either water temperature. These results emphasise the importance of standardising fish weight in immunological studies of salmonids before exploring the possible effects of acclimation temperatures.

Plasma androgen correlation, EROD induction, reduced condition factor, and the occurrence of organochlorine pollutants in reproductively immature white sturgeon (Acipenser transmontanus) from the Columbia River, USA

White sturgeon (Acipenser transmontanus) support an active fishery in the Columbia River, but there is poor reproductive success within the impounded sections. The poor reproductive success has been attributed to hydroelectric development; however, water pollution could be a significant factor. White sturgeon plasma, liver, and gonad samples were collected from four Columbia River locations and a California aquaculture facility. Total length and weight of the fish were measured, and plasma samples were analyzed for testosterone (T), 11-ketotestosterone (KT), 17 beta-estradiol (E2), and vitellogenin. Liver samples were analyzed for chlorinated pesticides and polychlorinated biphenyls, ethoxyresorufin-O-deethylase (EROD) activity and histopathology. Gonads were examined histologically to assess sexual maturity and characterize any lesions. Significant differences by location existed for p,p'-DDE, EROD activity, and condition factor. Plasma T was negatively correlated with p,p'-DDE in males and females, and plasma KT was negatively correlated in males. These data indicate that pollutants could be adversely affecting white sturgeon in the Columbia River basin.

Effects of stress on the reproductive performance of rainbow trout (Oncorhynchus mykiss)

We investigated the effects of stress over the final stages of sexual maturation on the reproductive performance of female rainbow trout, Oncorhynchus mykiss. Stress was administered over the period of early vitellogenesis (1.5 mo), late vitellogenesis-final maturation (1.5 mo), or during both periods (3 mo). Each stress treatment and control was triplicated, with eight females in each replicate (n = 24 fish per treatment). The eggs and progeny of each female were kept separate, and observations were made for 4 mo after transfer to rearing tanks. Fish that experienced stress during final maturation and those that were under stress during the whole experiment ovulated on average 2 wk earlier than the control group. In contrast, fish stressed during the period of early vitellogenesis ovulated at the same time as controls. Absolute fecundity and fertilization were not significantly affected in any treatment group, but significant differences in relative fecundity were found. Stress applied early in vitellogenesis resulted in smaller eggs and swim-up fry. No significant differences were found in juvenile weight 8 wk after hatching. Furthermore, we found no differences in survival of the progeny or resistance to the fish pathogen Vibrio anguillarum. Thus, mild acute stresses applied to rainbow trout females may affect certain reproductive performance parameters such as timing of ovulation and relative fecundity; however, the progeny of such stressed females perform as well as controls with regard to juvenile growth and disease resistance.

Physiological levels of testosterone kill salmonid leukocytes in vitro

Adult spring chinook salmon (Oncorhynchus tshawytscha) elaborate high plasma concentrations of testosterone during sexual maturation, and these levels of testosterone have been shown to reduce the salmonid immune response in vitro. Our search for the mechanism of testosterone's immunosuppressive action has led to the characterization of an androgen receptor in salmonid leukocytes. In the present study we examined the specific effects that testosterone had on salmonid leukocytes. Direct counts of viable leukocytes after incubation with and without physiological levels of testosterone demonstrate a significant loss of leukocytes in cultures exposed to testosterone. At least 5 days of contact with testosterone was required to produce significant immunosuppression and addition of a "conditioned media" (supernatant from proliferating lymphocytes not exposed to testosterone) did not reverse the immunosuppressive effects of testosterone. These data lead us to conclude that testosterone may exert its immunosuppressive effects by direct action on salmonid leukocytes, through the androgen receptor described, and that this action leads to the death of a significant number of these leukocytes.

Endogenous steroid metabolism is indicated by fluctuations of endogenous steroid and steroid glucuronide levels in early development of the steelhead trout (Oncorhynchus mykiss)

Concentrations of endogenous steroids and their glucuronide conjugates fluctuated during early development in steelhead trout Oncorhynchus mykiss. Whole body content of sex steroids and steroid glucuronides of both bisexual and gynogenetic (all female) steelhead trout were quantified by radioimmunoassay. Concentrations of 17beta-estradiol (E2) and cortisol increased 2-4 days before hatch. Two days after hatch, 11-ketotestosterone (11KT) increased in concentrations in both gynogenetic and bisexual populations, and 11KT glucuronide concentrations increased in the gynogenetic population. Testosterone (T) and E2 concentrations were at their lowest at 39 days postfertilization (dpf) for T and 39 and 61 dpf for E2. Changes in levels of steroid glucuronides were not consistently parallel to free steroids through time. T-, E2-, and 17alpha, 20beta dihydroxyprogesterone glucuronides declined slower than their free forms. Based on fluctuating concentrations of all steroid glucuronides, both populations of fish demonstrated an ability to form glucuronide conjugates of all steroids at the embryonic stage. The changes in levels of both free steroids and their glucuronides during early development of the trout indicate that steroid metabolism is active during development. This study also implicates steroid metabolism as an integral part of embryonic and postembryonic development.

Brain-pituitary-gonadal axis during early development and sexual differentiation in the rainbow trout, Oncorhynchus mykiss

Profiles of testosterone, 11-ketotestosterone, androstenedione, and estradiol were determined by RIA, and immunocytochemical techniques were employed to identify gonadotropin (GTH) I and II and gonadotropin releasing hormone (GnRH) in monosex and mixed sex populations of rainbow trout from 1 to 126 days postfertilization (dpf). Steroid levels were relatively high at 1 dpf and declined until 25 dpf. At 30 and 48 dpf (hatching) steroid levels increased slightly before they fell by 78 dpf and remained relatively constant thereafter. Trends toward differences in steroid content between males and females became evident around the time gonadal differentiation was histologically discernible (78 and 90 dpf). GTH I was present in the proximal pars distalis at all dates (48-126 dpf), whereas GTH II was not detectable. GnRH was found at all dates (48-126 dpf) and was distributed in several areas of the brain including the nucleus preopticus periventricularis, nucleus lateralis tuberis, and the pituitary in the region where GTH I was found. No differences were seen between males and females in the timing of appearance, localization, or intensity of staining of these peptide hormones. Given that the brain-pituitary-gonadal axis seems to be intact during the process of sexual differentiation and the fluctuations of steroid levels during this process, sex steroids may play the driving role for sexual differentiation of rainbow trout.

In vivo steroid metabolism in embryonic and newly hatched steelhead trout (Oncorhynchus mykiss)

Radioactive pregnenolone (P5), testosterone (T), or 17-beta-estradiol (E2) was microinjected into steelhead trout, Oncorhynchus mykiss, embryos and newly hatched yolk-sac fry (alevins) to detect in vivo metabolism. We also assayed the water used to incubate animals for 10 hr after microinjection to detect possible metabolite excretion. High pressure liquid chromatography and thin layer chromatography were used to separate and tentatively identify steroid metabolites. Metabolites of P5 were androstenedione (AN), E2, T, and glucuronides of E2 and T in embryos and AN, E2, progesterone, 17-alpha, 20-beta-dihydroxyprogesterone, and P5 glucuronide in alevins. E2 and its glucuronide were synthesized from precursor T in the embryos and alevins; however, the amounts of E2 and E2 glucuronide synthesized in the embryos were 10 and 3 magnitudes greater than those detected in alevins. Testosterone glucuronide was synthesized in similar amounts in both stages of development. Embryos did not synthesize free metabolites from E2 precursor, but E2 glucuronide was detected from E2 precursor. Estradiol in alevins was metabolized into unidentified free and glucuronide-conjugated steroids. Three unknown metabolites synthesized from P5 precursors and seven unknown substances produced in animals injected with testosterone or estradiol precursors were detected. Free metabolites were detected in the incubation water that held the animals (embryos and alevins) for 10 hr after microinjection with T or E2. Glucuronide metabolites were not excreted by embryos into the incubation water 10 hr after microinjections with any of the steroid precursors; however, alevins excreted glucuronides into the incubation water when supplied with precursor T. These results imply that endogenous steroid metabolism of maternally contributed steroids is active during early development.

Characterization of a glucocorticoid receptor in the brains of chinook salmon, Oncorhynchus tshawytscha

The effects of chronic stress on the brains of salmon may have important implications in light of the extremely high glucocorticoid levels experienced by migrating and spawning adults. The identification and the characterization of glucocorticoid receptors in salmon brains are the first steps in elucidating the effects of stress and high glucocorticoid levels on the brain. We have identified high-affinity, low capacity glucocorticoid receptors in chinook salmon (Oncorhynchus tshawytscha) brain cytosol and report the binding characteristics for the synthetic glucocorticoid triamcinolone acetonide (TA) and the naturally occurring salmonid glucocorticoid, cortisol. The binding characteristics for TA (Kd = 0.85 +/- 0.13 nM, B(max) = 22.4 +/- 2.97 fmol/mg protein, n = 7) and cortisol (Kd = 4.54 +/- 0.06 nM, B(max) = 25.40 +/- 2.20 fmol/mg protein, n = 2) demonstrated high-affinity, low capacity, and specificity for glucocorticoids. In competitive binding assays, TA, cortisol, and dexamethasone (a synthetic glucocorticoid) displaced [3H]TA most effectively, followed by RU38486. Corticosterone and RU28362 were weaker competitors. Cortisone was not a strong competitor nor were the sex steroids. Specific DNA binding was detected in DNA-cellulose chromatography assays. Receptors in nuclear extracts were not detected. These binding characteristics are consistent with published data on glucocorticoid receptors in other salmonid tissues.

Characterization of an androgen receptor in salmonid lymphocytes: possible link to androgen-induced immunosuppression

Cytosol of rainbow trout (Oncorhynchus mykiss) leukocytes demonstrated specific and saturable binding of [3H]testosterone (Kd = 0.99 +/- 0.17 nM and Bmax = 30.4 +/- 4.9 fmol/mg protein; based on a total of 6 determinations from three different cytosolic pools). Specific binding of [3H]testosterone was high in leukocytes and other tissues with known androgen binding affinity (plasma, skin, and liver) and low in other tissues (heart, muscle, and red blood cells). Specific binding of [3H]testosterone was displaced by testosterone and dihydrotestosterone. Androstenedione displaced 50% of specifically bound [3H]testosterone between 10- and 100-fold excess, while 17-alpha-methltestosterone, 11-ketotestosterone, and progesterone displaced 50% of specifically bound [3H]testosterone between 100- and 500-fold excess. Cortisol, 17 beta-estradiol, 17 alpha,20 beta-dihydroxyprogesterone, the synthetic androgen mibolerone, and the synthetic estrogen ethenylestradiol did not displace [3H]testosterone binding, even at 500-fold excess. Treatment of cytosol with proteolytic enzyme significantly reduced the specific binding of [3H]testosterone. HPLC analysis determined that [3H]testosterone was not metabolized during assay incubation with cytosol. These data strongly suggest that androgen receptors exists in salmonid leukocytes and support the hypothesis that these receptors play a role in androgen induced immunosuppression.

Binding characteristics of an androgen receptor in the ovaries of coho salmon, Oncorhynchus kisutch

Androgen receptors were identified in the cytosol from ovaries of juvenile coho salmon, Oncorhynchus kisutch. The binding for the synthetic androgen mibolerone was specific and saturable (Kd = 0.32 +/- 0.02 nM; Bmax = 15.31 +/- 4.31 fmol/mg protein). Bound [3H]mibolerone was much higher in ovarian cytosol than in cytosolic extracts from heart, liver, and muscle. [3H]mibolerone specific binding was 50% lower in the plasma than in the ovarian cytosolic extracts. [3H]mibolerone binding was displaced most effectively by those 17 alpha-methylated synthetic androgens (mibolerone, methyltestosterone, methylandrostanolone) that can induce functional masculinization in fish. The naturally occurring androgens 11-ketotestosterone and 5 alpha-dihydrotestosterone both displaced [3H]mibolerone binding, but they were 10- to 100-fold less effective than the 17 alpha-methylated androgens. Testosterone, 11 beta-hydroxyandrostenedione, estradiol, progesterone, and 17 alpha,20 beta-dihydroxyprogesterone were not potent competitors. [3H]mibolerone specific binding was reduced after preincubation with trypsin. About 25% of the binding in the cytosolic extract had DNA binding affinity under experimental conditions. The characteristics of this androgen binding site are consistent with a model of receptor-mediated steroid-induced sex inversion.

In vitro steroid secretion during early development of mono-sex rainbow trout: sex differences, onset of pituitary control, and effects of dietary steroid treatment

Sex differentiation in many teleost species can be controlled by treatment with steroids. To investigate the development of steroidogenesis during both natural and controlled sex differentiation, the production of androstenedione, testosterone, and estradiol were determined in tissues from populations of all-female and all-male rainbow trout (Oncorhynchus mykiss). At various times from hatching through gonadal sex differentiation, explants of steroidogenic tissues were incubated in vitro alone or in the presence of partly purified salmon gonadotropin and the resulting media were assayed for steroids. Androstenedione and testosterone were produced at higher levels in media from testes than from ovaries within 2 weeks of the onset of feeding (before any dramatic gonadal differentiation). Gonadal estradiol secretion was nondetectable until about 1 month after the onset of feeding when females produced more than males. Gonadotropin stimulated gonadal steroid production only after differentiation, but stimulated anterior kidney (interrenal) production of androstenedione much earlier in development. Dietary treatment of rainbow trout with either estradiol or 17 alpha-methyltestosterone (MT) inhibited in vitro gonadal steroid production and this effect persisted in MT-fed fish even after withdrawal of dietary steroids.

Seasonal changes in cortisol sensitivity and glucocorticoid receptor affinity and number in leukocytes of coho salmon

To determine if there were organ-specific changes in immune responses or immune-endocrine interaction, we monitored in vitro immune response, cortisol sensitivity and number and affinity of glucocorticoid receptors (GR) in leukocytes from freshwater-adapted juvenile coho salmon (Oncorhynchus kisutch) during the physiological changes that prepare them to enter the marine environment. During this period, absolute immune response declined, but splenic leukocytes generated more antibody-producing cells than did cells from anterior kidney. Splenic leukocytes were initially more sensitive to the suppressive effects of cortisol and had fewer GR than leukocytes from the anterior kidney. Leukocytes from the anterior kidney were initially insensitive to cortisol but developed sensitivity at about the same time as the dissociation constant and number of GR increased. In vitro incubation of anterior kidney leukocytes in cortisol altered GR variables when experiments were conducted during March through September but not during November through February. In some years, changes in GR or immune responses were correlated with plasma cortisol titers, but in other years there was no correlation. Thus, the exact relation between cortisol, GR and immune response in anadromous salmonids is unclear and other factors are involved.

Testosterone alters the immune response of chinook salmon, Oncorhynchus tshawytscha

We examined the effects of the high concentrations of sex steroid hormones detected in the plasma of sexually maturing chinook salmon (Oncorhynchus tshawytscha) on the functioning of the immune system. Leukocytes from the anterior kidney of juvenile spring chinook salmon were incubated in the presence of steroid, and their ability to form specific antibody-producing cells (plaque-forming cells) was used as a measure of immunocompetence. Testosterone and cortisol, but not 17 beta-estradiol or aldosterone, were found to significantly reduce the plaque-forming response in vitro. Testosterone and cortisol administered together had a significantly greater effect than did either administered alone. The plaque-forming response was significantly affected by season. The magnitude of the steroid-induced immunosuppression was reduced during winter and increased again in spring.

Evidence for ultra-short-loop feedback in ACTH-induced interrenal steroidogenesis in coho salmon: acute self-suppression of cortisol secretion in vitro

Interrenal tissues from coho salmon (Oncorhynchus kisutch) were incubated in a defined medium under blood-gas atmosphere at 17 degrees. Rates of cortisol secretion by tissues incubated in media containing 50 mU/ml porcine-ACTH were initially much greater than those of resting tissues in hormone-free media, but after 3 to 6 hr returned to resting rates. The time course of cortisol accumulation in ACTH-containing media was the same when tissues were incubated in different volumes; the final concentrations of cortisol in these incubations were similar to each other and resembled peak in vivo concentrations in juvenile coho subjected to acute stress. Cortisol secretion rates of tissues sequentially transferred to fresh ACTH-containing media every 6 hr did not return to resting levels but remained elevated for at least 24 hr. Cortisol secretion in response to ACTH was attenuated or completely abolished in tissues incubated in media containing exogenous cortisol; this effect was reversible and dose-dependent. Our results suggest that in coho salmon, cortisol may exert ultra-short-loop negative feedback directly at the level of the interrenal gland to effect self-suppression.

Stress and cortisol treatment changed affinity and number of glucocorticoid receptors in leukocytes and gill of coho salmon

To determine whether glucocorticoid receptors were altered during observed changes in immune function after stress or cortisol treatment, we conducted a series of experiments in which juvenile coho salmon (Oncorhynchus kisutch) were acutely or chronically stressed or fed a single meal containing cortisol. We then determined glucocorticoid receptor (GR) binding in cell-free gill preparations and whole leukocytes from spleen and anterior kidney using the synthetic hormone triamcinolone acetonide as radioligand. The affinities of GR were consistently lower in all tissues from chronically stressed fish than in tissues from controls; however, numbers of GR increased in whole leukocytes and decreased in gill. Acute stress had no affect on GR in gill and did not affect the affinity of GR in anterior kidney leukocytes, but did increase the number of GR in those leukocytes. Acute stress reduced affinity and increased numbers of GR in splenic leukocytes, suggesting heterogeneity of response to stress. Feeding cortisol to fish resulted in changes in GR from gill similar to those caused by chronic stress. Incubating leukocytes in cortisol for 3 or 24 h prior to assaying GR resulted in increased number and decreased affinity of GR in anterior kidney leukocytes, but had no effect on cells from spleen.

Cortisol effects on plasma electrolytes and thyroid hormones during smoltification in coho salmon Oncorhynchus kisutch

Plasma concentrations of Na+, K+, triiodothyronine (T3), and thyroxine (T4) and muscle water content were measured in yearling coho salmon. Oncorhynchus kisutch, after injection of cortisol in April, May, and June in fresh water (FW) and during acclimation to seawater (SW). Cortisol (17-21 micrograms/g), injected intraperitoneally in a melted cocoa butter suspension, suppressed the rise of plasma Na+ during SW acclimation in April but not in May or June. Muscle water content increased during SW acclimation in cortisol-treated fish in April and June. These observations suggest a hypoosmoregulatory function for cortisol during SW acclimation. Cortisol treatment also induced elevations of plasma K+ in FW (April only) and SW (April and May only). Cortisol treatment increased plasma T3 during SW acclimation in June and T4 in FW in May. The results suggest that cortisol may modify osmoregulatory processes and thyroid hormone activity during smoltification and acclimation to SW in yearling coho salmon.

Plasma prolactin and cortisol concentrations of stressed coho salmon, Oncorhynchus kisutch, in fresh water or salt water

Juvenile coho salmon, Oncorhynchus kisutch, adapted to fresh water or seawater were either acutely handled or continuously stressed by severe confinement. Chronic stress, independent of external salinity, caused a gradual increase in the concentration of circulating prolactin that persisted for 1 to 5 days but lagged behind the cortisol response which peaked much more rapidly and remained elevated. Acutely stressed fish showed a rapid, more transient increase in plasma cortisol titer with no apparent effect on prolactin. Confinement appeared to be more stressful to fish in salt water than to those in fresh water, as judged by their sodium regulatory ability, hormone profiles, and mortality. Stress always elevated plasma prolactin concentrations, regardless of medium or developmental stage.

Sex steroid profiles of coho salmon (Oncorhynchus kisutch) during early development and sexual differentiation

Sex steroids were measured by radioimmunoassay in whole-body extracts of coho salmon, Oncorhynchus kisutch, during early development and sexual differentiation. Profiles were developed for fish from the time of fertilization until 87 days postfertilization (dpf) for six steroids: testosterone (T), 11-ketotestosterone (KT), androstenedione (A), progesterone (P4), 17 alpha-hydroxy-20 beta-dihydroprogesterone (DHP), and 17 beta-estradiol (E2). Ovarian fluid was also examined for steroid content. Steroid profiles of unfertilized eggs essentially paralleled those of ovarian fluid. In one experiment, steroids in developing embryos declined precipitously after fertilization until 30 dpf; at hatching, all steroids increased slightly and then declined during yolk sac absorption. Results from a second experiment basically supported those of the first except that only testosterone increased at the time of hatching. Bimodality was evident in the data on steroid levels for fish collected between 42 and 56 dpf and again after 87 dpf. The hormone levels generally decreased or remained constant after the onset of exogenous feeding. Histological analyses during the first experiment showed the presence of undifferentiated gonads between hatching and 70 dpf, but by 77 dpf ovarian development was evident. In the second experiment, in which fish were more frequently sampled for histological analysis, undifferentiated gonads were present from hatching to 59 dpf. Development of oogonia was observed between 66 and 73 dpf and by 75 dpf ovarian development could be easily discerned. The sex of fish sampled at 101 dpf was determined by examining gonadal morphology, and steroid levels of those fish were determined. A sexual dimorphism was apparent in levels of T, KT, and A, but not of DHP or E2. The dynamics of steroid content of developing coho salmon at hatch, coupled with their bimodal distributions during yolk sac absorption, may suggest a role of sex steroids in the process of sexual differentiation apparent later in development. Changes in whole-body steroid levels at hatch may also be indicative of the onset of sexual differentiation even though no signs of gondal differentiation were histologically discernible at that time.

Glucocorticoid receptors in leukocytes and gill of juvenile coho salmon (Oncorhynchus kisutch)

We demonstrated that cytosol from the gill of coho salmon (Oncorhynchus kisutch) had saturable, high affinity, low capacity binding to radiolabeled [3H]cortisol (Kd = 2.24 +/- 0.28 nM, mean +/- 1 SE; Nmax = 41.4 +/- 7.4 fmol/mg protein) and radiolabeled [3H]triamcinolone acetonide (TA; Kd = 0.38 +/- 0.03 nM, Nmax = 37.8 +/- 4.9 fmol/mg protein). Similarly, TA bound to cytosolic fractions of leukocytes harvested from spleen (Kd = 0.32 +/- 0.03 nM, Nmax = 8.3 +/- 2.0 fmol/mg protein) and anterior kidney (Kd = 0.37 +/- 0.03 nM, Nmax = 30.2 +/- 5.2 fmol/mg protein) and to whole leukocytes from spleen (Kd = 0.30 +/- 0.04 nM, Nmax = 445 +/- 57 sites/cell) and anterior kidney (Kd = 0.40 +/- 0.04 nM, Nmax = 1198 +/- 180 sites/cell). The competition hierarchies of steroid competitors were the same for both ligands and all tissues (TA greater than cortisol greater than 17 alpha-hydroxyprogesterone greater than cortisone greater than aldosterone greater than testosterone). The differences in ligand binding in leukocytes from spleen and anterior kidney are consistent with previously reported organ-dependent sensitivity of leukocytes to cortisol.

Regulation of the interrenal of fishes: non-classical control mechanisms

The regulation of the interrenal of teleostean fishes is reviewed from the perspective of non-classical control mechanisms and new evidence is presented suggesting gonadotropic control of the interrenal. Cortisol secretion by the interrenal, in addition to regulation by ACTH, appears to be mediated by other hormones. Physiologically relevant, direct control of interrenal function by hydromineral factors is unclear.In vitro experiments with interrenals of coho salmon (Oncorhynchus kisutch) indicate that salmon gonadotropin is extremely corticotropic and both ACTH and gonadotropin stimulate the secretion of large quantities of androstenedione from the interrenal.

Stress alters immune function and disease resistance in chinook salmon (Oncorhynchus tshawytscha)

We examined the effects of acute stress on the immune system and disease resistance of juvenile chinook salmon (Oncorhynchus tshawytscha) in laboratory and clinical trials. Immune function, as measured by the ability of lymphocytes from the anterior kidney to generate specific antibody-producing cells (APC) in vitro, was depressed 4 h after stress, when plasma cortisol levels were highest. At the same time, resistance to the fish pathogen, Vibrio anguillarum, was also depressed. Compared with controls, plasma cortisol and APC of stressed fish were unchanged after 24 h, and disease resistance was enhanced as evidenced by higher survival rate and longer mean time to death of mortalities. After 7 days, even though numbers of APC were depressed, plasma cortisol concentration and disease resistance did not differ from controls. This pattern was generally the same, independent of the type of stress applied: i.e. being held out of water in a dipnet for 30 s, manipulation during hatchery operations for 4 h, or transportation for 9 h. These and earlier findings suggest that similar endocrine-immune interactions operate in the mammalian and salmonid systems during acute stress.

Spontaneous and ACTH-induced interrenal steroidogenesis in juvenile coho salmon (Oncorhynchus kisutch): effects of monovalent ions and osmolality in vitro

We determined the in vitro effects of changes in extracellular monovalent ion levels and osmotic pressure on the spontaneous and adrenocorticotropin (ACTH)-stimulated interrenal activity of coho salmon (Oncorhynchus kisutch). We used a perifusion system of incubation and monitored interrenal activity by measuring the effluent cortisol content with a radioimmunoassay. An increase in the medium osmolality with mannitol, from 206 to 290 or 353 mosmol, caused an increase in the spontaneous release of cortisol only slightly (compared with the much greater increase induced by porcine-ACTH). A similar minor increase was observed when NaCl was elevated from 130 to 180 mM. On the other hand, the spontaneous release of cortisol was not affected by increasing the KCl level from 3.2 to 9.6 mM, but was clearly increased when KCl was raised from 3.2 mM to a supraphysiological level of 27.2 mM. Ionic or osmolality changes, within the physiological range observed in coho salmon plasma, did not affect the characteristics of interrenal secretion of cortisol in response to porcine-ACTH. If our results with interrenal cells in vitro are representative of the basic functioning of the cells in vivo, then one would have to conclude that changes in concentrations of plasma monovalent ions or in osmotic pressure may not play a significant physiological role in the regulation of interrenal steroidogenesis or corticosteroid release in coho salmon.

Interrenal secretion of corticosteroids and plasma cortisol and cortisone concentrations after acute stress and during seawater acclimation in juvenile coho salmon (Oncorhynchus kisutch)

We determined the major corticosteroids secreted by interrenal tissue and those present in plasma of juvenile coho salmon (Oncorhynchus kisutch). Incubation medium of interrenal tissue, unstimulated or stimulated with exogenous ACTH in vitro, and plasma of resting or acutely stressed salmon were extracted and qualitatively analyzed for steroid composition using high-performance liquid chromatography. Concentrations of plasma cortisone and cortisol following an acute handling stress or exposure to seawater were quantitatively measured by radioimmunoassay (RIA). Cortisol was the only corticosteroid detected by HPLC in media after incubation of interrenal tissue in the absence or presence of ACTH in vitro. However, both cortisone and cortisol were detected by HPLC in plasma sampled 1 hr after fish were acutely stressed by handling. Stress and seawater acclimation produced marked elevations in plasma levels of both steroids as determined by RIA and also resulted in long-lasting changes in the plasma cortisone:cortisol ratios. In resting fish, cortisone concentrations were similar or higher than cortisol levels. We concluded that cortisol is the primary steroid secreted by the interrenal tissue of coho salmon, and that plasma cortisone arises primarily from the peripheral conversion of cortisol to cortisone. The relatively high levels of cortisone in resting fish and its increase following stress and seawater acclimation suggest the possibility of a biologically significant role for this hormone.

Adenylate cyclase activators and inhibitors, cyclic nucleotide analogs, and phosphatidylinositol: effects on interrenal function of coho salmon (Oncorhynchus kisutch) in vitro

The role of cyclic AMP (cAMP) as mediator of ACTH action on interrenal steroidogenesis was evaluated in juvenile coho salmon (Oncorhynchus kisutch). Head kidneys (containing the interrenal cells) were incubated in the absence or presence of putative adenylate cyclase activators (forskolin and cholera toxin), ACTH combined with putative adenylate cyclase inhibitors (hydrolysis-resistant ATP analogs), dibutyryl cyclic (dbc) AMP, dbcGMP, or phosphatidylinositol. The cortisol content of the incubation medium was subsequently determined by radioimmunoassay. Forskolin markedly stimulated cortisol secretion by interrenal cells. Adenylate cyclase inhibitors depressed the steroidogenic response to ACTH. Dibutyryl cAMP, but not dbcGMP, enhanced steroid secretion. Thus, cAMP seems to be an important "second messenger" for ACTH action on salmon interrenal cells. In contrast to findings in mammalian adrenocortical cells, exogenous phosphatidylinositol and cholera toxin failed to stimulate corticosteroid secretion in salmon interrenal cells. However, it was unclear whether these negative findings were an artifact resulting from the use of kidney tissue fragments instead of isolated interrenal cells.

Profiles of plasma sex steroids and gonadotropin in coho salmon, Oncorhynchus kisutch, during final maturation

The plasma concentrations of estradiol-17 alpha (E2), 17 mu-hydroxy-20 beta-dihydroprogesterone (DHP), progesterone, 11-ketotestosterone (11-KT), testosterone (T), and gonadotropin (GtH) were measured in adult coho salmon (Oncorhynchus kisutch) concomitantly with advancing maturation of the gonads during the spawning season. Concentrations of E2 were higher in females with eggs showing central (premigrating) germinal vesicles, migrating germinal vesicles, or peripheral germinal vesicles than in fish in which the eggs had undergone germinal vesicle breakdown or ovulation. Conversely, plasma levels of DHP were low at central, migrating, and peripheral germinal vesicle stages and then increased dramatically at germinal vesicle breakdown and ovulation. Levels of 11-KT, T, and GtH generally increased with advancing maturity of the eggs. When time rather than degree of maturation was used as a variable, plasma E2 was high in females first returning from the ocean but then dropped off precipitously in fish sampled at the end of the run. Plasma DHP followed the opposite pattern, being low in fish sampled early in the run and high in fish sampled late in the run. No easily discernible patterns emerged from the profiles of the other steroids and GtH, although some significant variation in the concentrations occurred. Even though sampling was initiated at the time when the salmon first returned from the ocean, most of the hormones were not variable in males when viewed according to sampling date or predominant cell type in the testes, which indicates that the males were extremely close to final maturity during the sampling period. Plasma levels of DHP were higher in males that were producing milt than in any other group of males. Concentrations of 11-KT were higher than those of T in all males, but no patterns in the levels of either steroid emerged.

Sexual dimorphism of plasma sex steroid levels in juvenile coho salmon, Oncorhynchus kisutch, during smoltification

Concentrations of plasma sex steroids, cortisol, and thyroxine were measured by radioimmunoassay in hatchery coho salmon (Oncorhynchus kisutch during winter and early spring. Mean plasma 11-ketotestosterone (11-KT) and estradiol levels fell into two distinct categories: 11-KT was 181-373% higher in males than in females, and estradiol was 109-143% higher in females than in males. No changes in plasma levels of estradiol in fish of both sexes, or in levels of 11-KT in males, were evident during spring when plasma thyroxine and cortisol were markedly elevated, indicating that the fish were undergoing smoltification. Although plasma 11-KT in females appeared to be lower in late April than in February, it showed no correlation with plasma thyroxine or cortisol in these individuals. Our finding of sexual dimorphism in 17 alpha-20 beta-dihydroxy-4-pregnen-3-one was inconsistent between stocks of fish and among sampling dates, thus making interpretation of the results difficult. However, no relationship between this steroid and plasma thyroxine or cortisol was observed. Therefore, plasma levels of sex steroids do not seem to be related to the changes in plasma thyroxine or cortisol observed during smoltification of coho salmon.

Changes in plasma cortisol during stress and smoltification in coho salmon, Oncorhynchus kisutch

The cortisol stress response in juvenile coho salmon, Oncorhynchus kisutch, considered as the difference between resting and peak poststress cortisol levels, increased from 80 ng/ml in March to 166 ng/ml in July, the period when smoltification normally occurs. Resting plasma cortisol levels also continually increased from 4 ng/ml in March to a maximum of 39 ng/ml in May, but then declined again to 3 ng/ml in July. The results indicate that there is an increased interrenal responsiveness to stress during the period of smoltification in coho salmon.

Clearance of plasma corticosteroids during smoltification of coho salmon, Oncorhynchus kisutch

The plasma clearance rate (PCR) of radioactivity after a single intracardial injection of 3H-cortisol was elevated during the spring in yearling coho salmon, Oncorhynchus kisutch. Graphical analysis suggested a seasonal correlation between PCR and gill Na/K-ATPase activity. An explanation for this correlation is suggested. The major metabolite of 3H-cortisol in plasma was 3H-cortisone. It appeared rapidly following injection of the original radiotracer.

Cortisol and its effects on plasma thyroid hormone and electrolyte concentrations in fresh water and during seawater acclimation in yearling coho salmon, Oncorhynchus kisutch

Plasma cortisol, thyroxine (T4), and triiodothyronine (T3) concentrations increased during seawater (SW) acclimation in yearling coho salmon, Oncorhynchus kisutch. Maximal concentrations of cortisol (220 ng/ml) occurred within 1.5 hr after the ambient water was changed from fresh water (FW) to SW; after 21 days in SW, cortisol levels were still slightly elevated (23 ng/ml) compared to those in FW fish (8 ng/ml). Plasma T4 concentrations peaked (14 ng/ml) at 12 and 72 hr after exposure to SW, and they were higher than those in FW fish (4 ng/ml) at all samples times. Maximal concentrations of T3 (8 ng/ml) occurred within 12 hr after exposure to SW, followed by a return to FW control levels (4 ng/ml) within 24 hr. Chronic treatment with cortisol significantly lowered plasma T3 concentrations in FW and during SW exposure, but it had no significant effect on T4 concentrations. Cortisol treatment lowered gill Na-K-ATPase activity in FW fish, but it did not affect plasma osmolarity, Na, K, Ca, or Mg in fish in FW or during SW acclimation.

Clearance of corticosteroids in yearling coho salmon, Oncorhynchus kisutch, in fresh water and seawater and after stress

The metabolic clearance rate of corticosteroids determined after a single injection of [3H]cortisol was higher in fish acclimated to seawater (SW) than in those acclimated to fresh water (FW). Uptake and retention of corticosteroids in liver, gill filaments, and gall bladder bile were greater in SW than in FW fish. The stress of long-term (5 days), but not short-term (12 hr), continuous confinement apparently increased the clearance rate of corticosteroids in both FW and SW fish. Chronic, but not acute, administration of exogenous cortisol, yielding cortisol titers close to the physiological range, appeared to increase the clearance rate of corticosteroids in FW fish.

Possible Adaptive Significance of Certain Enzyme Polymorphisms in Steelhead Trout (Salmo gairdneri)

In experimentally reared winter steelhead trout (Salmo gairdneri) fry, mean weights, lengths, and condition factors of three isozyme phenotypes of lactate dehydrogenase (LDH) enzyme differed significantly. Time of emergence from the gravel was unrelated to LDH phenotype. Relative mortality of the phenotypes between eyed-egg stage and emergence was unaffected by different subgravel conditions of temperature and dissolved oxygen. Differential tolerance to acute challenges of high temperature and low dissolved oxygen was observed between phenotypes of isocitrate dehydrogenase (IDH) enzyme and LDH in juvenile trout. Parental effects may have biased the result for LDH. Differences between IDH phenotypes may be related to intrinsic properties of variant isozymes. Key words: isozymes, lactate dehydrogenase, isocitrate dehydrogenase, temperature, dissolved oxygen, adaptive significance, Salmo gairdneri