Female Sockeye Salmon Research Articles

Abnormalities in the ovaries of sockeye salmon spawners in the Kamchatka River in 2019

Based on histological analysis of the ovaries of sockeye salmon spawners in the Kamchatka River, numerous abnormalities leading to resorption are shown in the structure of vitellogenic oocytes. Fertility of mature female sockeye salmon decreases due to the resorption of the vitellogenic oocytes. Presence of ovaries with abnormal oocytes implies oocyte resorption in fish being a phenomenon associated with mechanism of natural regulation of final fertility. Such mechanism is required in view of changing fish body length and weight in the sockeye salmon of the Kamchatka River. Numerous abnormalities in the ovaries indicate of a decreasing reproductive capacity of spawners and commercial quality of the eggs.

Exceptionally high mortality of adult female salmon: a large-scale pattern and a conservation concern

In recent decades, the relative proportion of female sockeye salmon (Oncorhynchus nerka) on spawning grounds of several British Columbia populations has declined. Coincident with the decline has been large changes to oceanic, estuarine, and river migration environments. Over the past 30 years, numerous telemetry tracking and laboratory studies have examined mortality of adult Fraser River sockeye salmon during ocean and freshwater migrations. We reviewed 19 studies that provided 40 situations where male and female mortality could be directly compared. Female mortality averaged 2.1 times greater than that of males and up to eightfold higher. High female mortality was also evident in migrating coho salmon (Oncorhynchus kisutch) and Chinook salmon (Oncorhynchus tshawytscha) and for sockeye salmon in other systems. Female mortality was highest when migration conditions were challenging (e.g., high or turbulent flows, high temperatures, confinement, or handling) and towards end of river migration. We review mechanisms for differential mortality, including energy exhaustion, cardiac performance, physiological stress, and immune factors. Female-specific mortality will become even more pronounced in coming years as ocean and riverine conditions continue to change.

Sex-specific differences in physiological recovery and short-term behaviour following fisheries capture in adult sockeye salmon (Oncorhynchus nerka)

Numerous laboratory and field studies have found that female Pacific salmon have higher mortality than males during their once-in-a-lifetime upriver spawning migration. However, the proximate cause(s) of this increased mortality are poorly understood. This study exposed sockeye salmon (Oncorhynchus nerka) to a mild capture and tagging stressor and evaluated physiological recovery and movement behaviour at 1 and 4 h postrelease. Female sockeye salmon did not expend more anaerobic energy in response to the stressor but did have higher plasma lactate levels 4 h after the stressor, indicating that females took longer to physiologically recover compared with males. In addition, female salmon had lower plasma glucose but higher plasma cortisol, plasma K+, and cardiac lactate levels compared with males. Male and female salmon had markedly different postrelease behaviours within the first hour of release; males were more likely to hold position within the staging area. Two potential mechanisms leading to increased mortality in female salmon were identified in this study: (a) prolonged recovery duration (possibly mediated by elevated plasma cortisol levels) and (b) insufficient oxygen delivery to the heart.

Physiological condition and migratory experience affect fitness‐related outcomes in adult female sockeye salmon

AbstractRelating fish physiology, behaviour and experience to fitness‐related outcomes at the individual scale is ecologically significant, but presents difficulties for free‐ranging fishes in natural systems. Physiological state (e.g. level of stress or maturity) and experience (e.g. habitat use or exposure to stressors) may alter the probability of survival or reproduction. This study examined the relative influence of physiology and migratory experience on survival, migration duration, reproductive longevity, and egg retention in adult female sockeye salmon (Oncorhynchus nerka) from a Fraser River population. One hundred and thirty‐five females were plasma sampled and tagged with radio transmitters and archival temperature loggers. Fish were tracked 55 km through two natal lakes to spawning grounds, following passage of a hydroelectric dam. For 39 females, we assessed the proportion of time within an optimal temperature (ToptAS) window (13.4–19.5°C), which provides ≥90% of maximum aerobic scope. Females with lower plasma glucose concentrations were more likely to reach spawning grounds. Early migrants spent longer in natal lakes. More time in the ToptAS window was associated with greater reproductive longevity and lower probability of egg retention. Later arriving females had reduced longevity on spawning grounds, as did females that retained eggs. Exposure to higher dam discharge was associated with reduced reproductive longevity and greater probability of egg retention, but not lower survival, indicating a delayed effect of dam passage. Our results underscore the complexity of factors governing fitness‐related outcomes for salmonids, particularly the importance of female experience in the days and weeks prior to spawning.

Does maternal captivity of wild, migratory sockeye salmon influence offspring performance?

The environment females experience during sexual maturation can have cascading effects on offspring. For free-swimming populations of wild fish, ecological factors including predation and competition are known to alter offspring phenotype. For wild-caught fish targeted for stock enhancement, logistical factors, such as the widespread practice of holding fish in captivity/confinement, also have the potential to modify offspring phenotype. Understanding how maternal captivity affects offspring, in comparison to offspring reared from fish that have matured in the wild, is an important but relatively understudied aspect of fish culture. We examined egg and offspring traits for wild-caught female sockeye salmon reared in captivity during the final stages of sexual maturation or captured on spawning grounds following in-river maturation. Compared to females that matured in-river, captive females had smaller eggs and offspring in poorer body condition. These same offspring did however swim for longer durations. These results suggest that maternal captivity prior to spawning elicits intergenerational phenotypic change. Whether captivity-induced maternal effects are maladaptive/reduce offspring fitness will be dependent upon the environment in which offspring are being released into (e.g. captive or wild). Intergenerational effects of short-term captivity of wild fish may be a contributing mechanism mediating the success of fisheries supplementation programmes.

Mother knows best, even when stressed? Effects of maternal exposure to a stressor on offspring performance at different life stages in a wild semelparous fish

The environment mothers are exposed to has resonating effects on offspring performance. In iteroparous species, maternal exposure to stressors generally results in offspring ill-equipped for survival. Still, opportunities for future fecundity can offset low quality offspring. Little is known, however, as to how intergenerational effects of stress manifest in semelparous species with only a single breeding episode. Such mothers would suffer a total loss of fitness if offspring cannot survive past multiple life stages. We evaluated whether chronic exposure of female sockeye salmon (Oncorhynchus nerka) to a chase stressor impaired offspring performance traits. Egg size and early offspring survival were not influenced by maternal exposure to the repeated acute stressor. Later in development, fry reared from stressed mothers swam for shorter periods of time but possessed a superior capacity to re-initiate bouts of burst swimming. In contrast to iteroparous species, the mechanisms driving the observed effects do not appear to be related to cortisol, as egg hormone concentrations did not vary between stressed and undisturbed mothers. Sockeye salmon appear to possess buffering strategies that protect offspring from deleterious effects of maternal stress that would otherwise compromise progeny during highly vulnerable stages of development. Whether stressed sockeye salmon mothers endow offspring with traits that are matched or mismatched for survival in the unpredictable environment they encountered is discussed. This study highlights the importance of examining intergenerational effects among species-specific reproductive strategies, and across offspring life history to fully determine the scope of impact of maternal stress.

Alterations to dam‐spill discharge influence sex‐specific activity, behaviour and passage success of migrating adult sockeye salmon

ABSTRACTDelays in the freshwater spawning migrations of anadromous fishes at upstream barriers are well documented, but underlying mechanisms causing slowed migrations are seldom known. Using acoustic accelerometer transmitters and generalized linear models, we investigated how alterations in flow at a diversion dam in British Columbia, Canada, affected the activity, behaviour and passage success of a Fraser River sockeye salmon population (Oncorhynchus nerka). Spilling excess water through the radial gate of the dam decreased the attraction efficiency of a vertical‐slot fishway by 90% and increased delay below the dam by 2 h, which had adverse effects on passage. Relative to males, female sockeye salmon had significantly lower passage success (73% vs 94%), attraction efficiency (79% vs 100%) and passage efficiency (89% vs 94%) at a fishway, delayed longer in the dam tailrace (mean ± standard error (SE): 20·8 ± 4·1 h vs 14·6 ± 3·2 h), and resorted to anaerobic swimming efforts for a greater percentage of time (0–61·3% vs 0·7–2·7%). Given that the persistence of Pacific salmon populations relies on the spawning success of females, understanding how males and females vary in their response to modified flow regimes will improve the management of complex fish‐passage problems. Copyright © 2013 John Wiley & Sons, Ltd.

Nest site preference and intrasexual competition in female sockeye salmon, Oncorhynchus nerka

Many ecological circumstances present individuals with a conflict between the inherent benefits of a particular habitat and the costs incurred in acquiring or retaining use of the habitat in the face of competition. For example, the reproductive biology of female Pacific salmon (Oncorhynchus spp.) led us to hypothesize that female nest site choice should reflect a compromise between the benefits of obtaining a high quality nest site and the cost of competing for and defending it. To test this hypothesis we studied female sockeye salmon (O. nerka) spawning on beaches in Iliamna Lake, Alaska using a combination of snorkel surveys, tagging, behavior observations, and models. Females showed spatial preferences in nest site selection (for shallower water, where water circulation was higher), aggressive competition in preferred areas was higher, and there was evidence for costs associated with this increased competition. Over the course of the season, spawning activity shifted from shallower to deeper water, consistent with a tradeoff between benefits for embryo survival associated with shallow sites and the costs of competing for them. However, it was also consistent with date-specific optimal sites related to the probability of embryo mortality for eggs spawned in shallow water late in the season, due to annual cycles in lake level and temperature.

Selecting for the phenotypic optimum: size‐related trade‐offs between mortality risk and reproductive output in female sockeye salmon

Summary Selection drives evolutionary changes but is often difficult to quantify and tightly link to phenotypic trait distributions in wild populations. Sampling a single population of sockeye salmon (Oncorhynchus nerka) for over a decade, we calculated the expected spawning success for females resulting from: (i) increased fecundity with body length, (ii) the mortality cost of prolonged marine residency necessary to achieve large size, (iii) size‐selective natural mortality on the spawning grounds from biotic (bear and gull predation) and abiotic (stranding) processes and (iv) exploitation by a size‐selective commercial fishery. We quantified the size‐specific probability of different modes of death and the resultant potential for successful spawning, and then modelled the theoretical relationship between female length and fitness (spawning success) in the population. This optimal distribution closely matched the observed length distribution of the focal population, when removals by the size‐selective commercial fishery were included. We then used a likelihood‐based approach to compare competing model predictions to length distributions from other populations in the watershed with different levels of size‐selective freshwater mortality, as determined by the physical characteristics of the spawning grounds. This study provides a quantitative framework for assessing female spawning success in wild populations, as represented by the expected number of eggs deposited per spawning female. These results advance previous analyses of natural selection in that predictions for phenotypic distributions were generated and then compared to those observed in situ, rather than assuming the adaptive nature of observed distributions.

High river temperature reduces survival of sockeye salmon (Oncorhynchus nerka) approaching spawning grounds and exacerbates female mortality

Recent studies have shown that warm temperatures reduce survival of adult migrating sockeye salmon ( Oncorhynchus nerka ), but knowledge gaps exist on where high-temperature-related mortality occurs along the migration and whether females and males are differentially impacted by river temperature. In this study, we monitored 437 radio-tagged Fraser River sockeye salmon and used capture–mark–recapture modelling approaches to investigate whether river thermal conditions differentially influence (i) spatial patterns of survival along a 413-km stretch of migration and (ii) survival of the sexes. Regardless of water temperature, survival decreased in the river section containing the most hydraulically difficult passages of the migration. However, when water temperature was warm (19 °C), survival decreased even further in the final 186 km of the migration prior to reaching the spawning grounds, particularly in females. Female and male survival differed but only when they experienced warm river temperatures. Under such conditions, the overall freshwater migration survival of males was 1.6 times higher (0.79 ± 0.09 standard error, SE) than that of females (0.50 ± 0.11 SE). As maturing female sockeye salmon maintain higher levels of plasma cortisol compared with males, we suspect that females could be immuno-compromised and thus less resistant to pathogens whose rates of development are accelerated by warm temperatures.

Sex and Proximity to Reproductive Maturity Influence the Survival, Final Maturation, and Blood Physiology of Pacific Salmon When Exposed to High Temperature during a Simulated Migration

Some Pacific salmon populations have been experiencing increasingly warmer river temperatures during their once-in-a-lifetime spawning migration, which has been associated with en route and prespawn mortality. The mechanisms underlying such temperature-mediated mortality are poorly understood. Wild adult pink (Oncorhynchus gorbuscha) and sockeye (Oncorhynchus nerka) salmon were used in this study. The objectives were to investigate the effects of elevated water temperature on mortality, final maturation, and blood properties under controlled conditions that simulated a "cool" (13°C) and "warm" (19°C) freshwater spawning migration. After 10 d at 13°C, observed mortality was 50%-80% in all groups, which suggested that there was likely some mortality associated with handling and confinement. Observed mortality after 10 d at 19°C was higher, reaching ≥98% in male pink salmon and female pink and sockeye salmon. Thus, male sockeye salmon were the most thermally tolerant (54% observed mortality). Model selection supported the temperature- and sex-specific mortality patterns. The pink salmon were closer to reproductive maturation and farther along the senescence trajectory than sockeye salmon, which likely influenced their survival and physiological responses throughout the experiment. Females of both species held at 19°C had reduced plasma sex steroids compared with those held at 13°C, and female pink salmon were less likely to become fully mature at 19° than at 13°C. Male and female sockeye salmon held at 19°C had higher plasma chloride and osmolality than those held at 13°C, indicative of a thermally related stress response. These findings suggest that sex differences and proximity to reproductive maturity must be considered when predicting thermal tolerance and the magnitude of en route and prespawn mortality for Pacific salmon.

Egg retention in relation to arrival timing and reproductive longevity in female sockeye salmon (Oncorhynchus nerka)

Some female Pacific salmon ( Oncorhynchus spp.) arrive at spawning grounds but fail to complete spawning prior to death. One hypothesis regarding egg retention is that some individuals do not have sufficient time on spawning grounds for successful completion of spawning. We investigated this hypothesis by quantifying the relationships among arrival timing, reproductive longevity, and egg retention in female sockeye salmon ( Oncorhynchus nerka ) from Weaver Creek Spawning Channel (British Columbia, Canada) in 2006. 250 females were tagged over three sampling periods and followed until death. Earlier-arriving females lived longer than later-arriving females (p < 0.001), but patterns of egg retention were not different across sampling dates (p > 0.40). Complete spawners tended to establish a redd sooner after arrival than incomplete spawners (p = 0.001); there was no relationship between spawning completion and reproductive maturity or fork length (p > 0.30). Consistent with the time limitation hypothesis, females retained a lower proportion of eggs with increasing reproductive longevity. Several long-lived females (>7 days) failed to spawn completely before death, indicating that time limitation was not a factor for spawning success in all females. Further research examining the role of individual-specific behavioural physiology on egg retention in sockeye salmon is needed.

Sex differences in circulatory oxygen transport parameters of sockeye salmon (Oncorhynchus nerka) on the spawning ground

Upon reaching sexual maturity, several species of male salmonids possess a relative ventricular mass (rM(V)) that may be up to 90% larger than females. This can increase maximum cardiac stroke volume and power output, which may be beneficial to increasing the oxygen transport capacity of male salmonids during the spawning period. It may be further hypothesized, therefore, that other variables within the circulatory oxygen transport cascade, such as blood oxygen-carrying capacity and heart rate, are similarly enhanced in reproductively mature male salmonids. To test this idea, the present study measured a range of circulatory oxygen transport variables in wild male and female sockeye salmon (Oncorhynchus nerka) during their spawning period, following a 150 km migration from the ocean. The rM(V) of male fish was 13% greater than females. Conversely, the haemoglobin concentration ([Hb]) of female fish was 19% higher than males, indicative of a greater blood oxygen-carrying capacity (138 vs. 116 ml O2 l(-1), respectively). Surgically implanted physiological data loggers revealed a similar range in heart rate for both sexes on the spawning ground (20-80 beats min(-1) at 10 degrees C), with a tendency for male fish to spend a greater percentage of time (64%) than females (49%) at heart rates above 50 beats min(-1). Male fish on average consumed significantly more oxygen than females during a 13-h respirometry period. However, routine oxygen consumption rates (.)MO2 ranged between 1.5 and 8.5 mg min(-1) kg(-1) for both sexes, which implies that males did not inherently possess markedly higher routine aerobic energy demands, and suggests that the higher [Hb] of female fish may compensate for the smaller rM(V). These findings reject the hypothesis that all aspects of the circulatory oxygen transport cascade are inherently superior in male sockeye salmon. Instead, it is suggested that any differences in (.)MO2 between sexually mature male and female sockeye salmon can likely be attributed to activity levels.

Dynamic in-lake spawning migrations by female sockeye salmon

Abstract – Precise homing by salmon to natal habitats is considered the primary mechanism in the evolution of population-specific traits, yet few studies have focused on this final phase of their spawning migration. We radio tagged 157 female sockeye salmon (Oncorhynchus nerka) as they entered Lake Clark, Alaska, and tracked them every 1–10 days to their spawning locations. Contrary to past research, no specific shoreline migration pattern was observed (e.g., clockwise) nor did fish enter a tributary unless they spawned in that tributary. Tributary spawning fish migrated faster (mean = 4.7 km·day−1, SD = 2.7, vs. 1.6 km·day−1, SD = 2.1) and more directly (mean linearity = 0.8, SD = 0.2, vs. 0.4, SD = 0.2) than Lake Clark beach spawning fish. Although radio-tagged salmon migrated to within 5 km of their final spawning location in an average of 21.2 days (SD = 13.2), some fish migrated five times the distance necessary and over 50 days to reach their spawning destination. These results demonstrate the dynamic nature of this final phase of migration and support studies indicating a higher degree of homing precision by tributary spawning fish.

Dynamic in‐lake spawning migrations by female sockeye salmon

Abstract – Precise homing by salmon to natal habitats is considered the primary mechanism in the evolution of population‐specific traits, yet few studies have focused on this final phase of their spawning migration. We radio tagged 157 female sockeye salmon (Oncorhynchus nerka) as they entered Lake Clark, Alaska, and tracked them every 1–10 days to their spawning locations. Contrary to past research, no specific shoreline migration pattern was observed (e.g., clockwise) nor did fish enter a tributary unless they spawned in that tributary. Tributary spawning fish migrated faster (mean = 4.7 km·day−1, SD = 2.7, vs. 1.6 km·day−1, SD = 2.1) and more directly (mean linearity = 0.8, SD = 0.2, vs. 0.4, SD = 0.2) than Lake Clark beach spawning fish. Although radio‐tagged salmon migrated to within 5 km of their final spawning location in an average of 21.2 days (SD = 13.2), some fish migrated five times the distance necessary and over 50 days to reach their spawning destination. These results demonstrate the dynamic nature of this final phase of migration and support studies indicating a higher degree of homing precision by tributary spawning fish.

Tooth size and skin thickness in mature sockeye salmon: evidence for habitat constraints and variable investment between the sexes

Abstract – Pacific salmon develop many sexually dimorphic features at maturity, and the extent of development varies among populations. In this study, we compared a suite of traits including body length, body depth, jaw length, tooth size and skin mass in male and female sockeye salmon breeding in beach and creek habitats. Both tooth size and skin mass varied positively with body length. Within each of the breeding habitats, males had longer teeth than females, and within each sex, beach spawners had longer teeth than creek spawners. Males also had heavier skin than females in each habitat but, unlike the case with tooth size, creek spawners showed a much stronger relationship between skin mass and body length than did beach spawners. Tooth length was positively related to jaw length and skin mass among individuals within a given sex and habitat. Taken together, these results suggested that variation in tooth size parallels variation in other sexually dimorphic traits. Males and beach spawners tend to exhibit large trait values relative to females and creek spawners, and ‘well‐endowed’ individuals displayed high values of all traits rather than a trade‐off as might occur if investment in one trait compromised investment in others. However, the finding that creek fish tended to have thicker skin for a given body length than did beach fish suggested that factors other than merely defense against conspecifics during battle, such as abrasion and desiccation resistance in small streams, may influence the evolution of skin mass in mature sockeye salmon.

Using Time-Lapsed Video to Estimate Survey Life for Area-under-the-Curve Methods of Escapement Estimation

Accurate estimation of spawner populations for Pacific salmon Oncorhynchus spp. is critically important in stock assessment for fisheries management and science. Survey life (SL) is one essential component for area-under-the-curve (AUC) estimation of spawner populations. However, AUC spawner estimates often rely on a constant or borrowed estimate of SL because reliable estimates require extensive and costly annual field programs. Using a constant SL estimate can introduce serious bias when estimating spawner populations for sockeye salmon O. nerka as well as other salmon species. In this study, inexpensive video observations of redd residency time (RRT) of female sockeye salmon were used to approximate SL. Data from past studies indicate that RRT and SL are often close in duration for sockeye salmon, Chinook salmon O. tshawytscha, and coho salmon O. kisutch. In addition, SL estimates for male and female sockeye salmon are not significantly different. As a result, the RRT of female sockeye salmon, as observed by time-lapsed video recordings, can be used to provide an inexpensive annual estimate of sockeye salmon SL.

Egg burial depth by sockeye salmon (Oncorhynchus nerka): implications for survival of embryos and natural selection on female body size

We studied the relationship between female sockeye salmon (Oncorhynchus nerka) body size and egg burial depth in a small Alaskan stream to better understand the ways in which stream-bed scour or digging by other females might destroy embryos. Two different measurements of egg burial depth were taken: depth from the original stream-bed surface to the top of the egg pocket and depth from the disturbed substrate directly above the egg pocket to the top of the egg pocket. The former may represent the depth to which stream-bed scouring would have to go to reach the eggs, but the latter represents the depth to which a second female would have to dig to disturb the egg pocket. Larger females buried their eggs deeper, relative to the original substrate level, than smaller females. This suggests that streams with frequent scour events would select for larger females. However, mean depth from the disturbed substrate level was significantly shallower than mean depth from the original stream-bed level, suggesting that even the smallest females could dig deep enough to disturb the egg pockets of the largest females. Finally, the egg burial depth - fish size relationship that we observed was compared with published data on other salmonid species, revealing considerable variation but a clear positive relationship between female size and burial depth. Because embryonic survival is affected by scour and nest disturbance, and because changes in fish body size, density, and flow regime can affect the vulnerability of embryos to such mortality, we recommend further, standardized measurements of the relationship between egg burial depth and female body size.

Mortality of Sockeye Salmon Raised Under Light Backgrounds of Different Spectral Composition

Salmon from a 3 male×3 female sockeye salmon cross were raised under three different light regimes (UV: 300–850 nm for 8 hours day-1, visible: 400–850 nm for 8 hours day-1, and a control in which eggs were kept in darkness until hatching, followed by increasing amounts of light as the embryos approached swim-up) up to the parr interval (105 days from fertilization). The dose rates (W m-2), and total intensities per day (J m-2), for each treatment were less than 0.46 and 0.28 times of those measured in the shaded part of a stream, respectively. Egg mortality was higher for the UV and visible treatments compared to the control, and the number of alevins alive after day 83 was significantly different between treatments: highest for the control and lowest for the UV treatment. Eggs and hatched fish from female F2 suffered higher mortality than those from the other two females raising the possibility that sensitivity to UV-B in sockeye salmon may be either a maternally inherited trait or due to physiological or physical differences between egg groups. Alevins and parr from the control treatment transferred to the UV treatment suffered little mortality compared to animals raised in the UV and visible treatments from fertilization. These results, in conjunction with light measurements from streams, suggest that light and UV-B radiation could be a source of young salmon mortality in perturbed environments such as those resulting after forestry intervention.

Effects of acute stress on plasma cortisol, sex steroid hormone and glucose levels in male and female sockeye salmon during the breeding season

In the sockeye salmon ( Onchorynchus nerka), as well as a number of migratory salmonid species, the plasma level of cortisol, the main stress hormone in fish, increases during the spawning period, and then fish die after the spawning. The response of cultured individuals of this species to an artificial acute stress during the spawning period was investigated by measuring levels of cortisol, sex steroid hormones and glucose in blood plasma. In addition, physiological changes in hormone and glucose levels were studied in wild individuals caught in the lake at different spawning stages of the natural condition. The fish cultured and matured in the research pond were confined in a small net and their blood was collected at 3, 5, 15 or 30 min after the start of confinement. Wild sockeye salmon were caught when they gathered in the mouth of their maternal river and their blood was collected just before and after ovulation and also after the artificial removal of eggs and sperm in pre-ovulatory individuals. Acute stress increased cortisol and glucose levels in males. However, in females, the initial cortisol level in the plasma was higher than that in males and did not change significantly with the confinement time. Testosterone levels in both sexes, 11-ketotestosterone levels in males and plasma 17α-progesterone levels in females decreased with the confinement time. In the natural condition, cortisol levels were higher in females than in males and did not change until the last sampling. In males, the cortisol levels increased from the time of spawning until just before death. Glucose levels in males and females increased whereas cortisol levels in females did not change. It is concluded that during the breeding season, male sockeye salmon respond to confinement stress with elevated cortisol levels and decreased sex steroid levels, while female sockeye salmon show only the latter response. It is postulated that female sockeye salmon are not refractory to stress in the breeding period, because their sex steroid levels decrease in response to stress. In females, cortisol levels probably reach maximum levels before blood samples are taken.