Adult Sockeye Salmon Research Articles

Brown bear (Ursus arctos) foraging in a mosaic of spatially discrete and variable habitats over 25 years of shifting Pacific salmon densities

Many foraging models assume “perfect information” and “free movement” when describing predator foraging behavior, although this is rare in nature. Here, we quantified predation by brown bears ( Ursus arctos Linnaeus, 1758) on adult sockeye salmon ( Oncorhynchus nerka (Walbaum, 1792)) in a series of spatially proximate ponds that largely satisfied both assumptions. Salmon abundance varied among years, but pond area and depth were fixed, allowing us to examine interactions between prey abundance and habitat features. We applied versions of two models to 25 years of data on the number and proportion of salmon killed by bears, modifying these models to include habitat features and temporal variability. The functional response model with a year effect fit the data well, indicating bears could take almost all salmon in ponds when salmon were scarce, but bears were sated when salmon were abundant. The proportion of salmon killed by bears was similar across habitats after correcting for pond depth and area. Overall, bears foraged across all habitats but killed higher proportions of salmon in smaller and shallower habitats, consistent with ease of capture.

Using repeat injury assessments in adult sockeye salmon (Oncorhynchus nerka) to predict spawning success and describe severity of migration conditions

Pacific salmon (Oncorhynchus spp.) are semelparous and anadromous, and during their up-river migration to spawn can experience injuries and mortality from fisheries interactions and adverse environmental conditions. There is the potential to improve models used to predict those losses by including information on surficial fish condition, but for this information to be useful, we need to understand how injuries change over time, and how injury type influences this change. To do this, we used repeated individual assessments to examine whether injuries accrued during migration on Fraser River sockeye salmon (O. nerka), assessed in the last leg of their migration or on arrival at the spawning grounds, could be used to predict their final injury score at death after spawning. We found that injury scores increased over the spawning period, but this was not driven by any specific type of migration-related injury, and fish initially scored as ‘uninjured’ had the largest increase in injury score, indicating a ceiling effect to our scoring method. Females with higher migration-related injury scores were more likely to experience pre-spawn mortality. Initial injury score accrued during migration was positively correlated with final injury score after spawning but only when initially assessed 45 km from the spawning grounds, and females that spawned accrued more injuries than females that did not spawn. Thus our method was able to use pre-spawn injury scores to predict spawning success but was limited in its ability to use post-spawn injury scores to describe severity of migration conditions. This highlights the need to better understand injury and disease dynamics and further refine injury assessment and scoring methods at the individual level if information on surficial fish condition is to be used to either describe or predict migration severity or pre-spawn mortality at the population level.

Metabolic constraints and individual variation shape the trade-off between physiological recovery and antipredator responses in adult sockeye salmon.

Metabolic scope represents the aerobic energy budget available to an organism to perform non-maintenance activities (e.g., escape a predator, recover from a fisheries interaction, compete for a mate). Conflicting energetic requirements can give rise to ecologically relevant metabolic trade-offs when energy budgeting is constrained. The objective of this study was to investigate how aerobic energy is utilized when individual sockeye salmon (Oncorhynchus nerka) are exposed to multiple acute stressors. To indirectly assess metabolic changes in free swimming individuals, salmon were implanted with heart rate biologgers. Animals were then exercised to exhaustion or briefly handled as a control, and allowed to recover from this stressor for 48-h. During the first two hours of the recovery period, individual salmon were exposed to 90ml of conspecific alarm cues or water as a control. Heart rate was recorded throughout the recovery period. Recovery effort and time was higher in exercised fish, relative to control fish, whereas exposure to an alarm cue had no effect on either of these metrics. Individual routine heart rate was negatively correlated with recovery time and effort. Together, these findings suggest that metabolic energy allocation towards exercise recovery (i.e., an acute stressor; handling, chase, etc.) trumps antipredator responses in salmon, although individual variation may mediate this effect at the population level. This article is protected by copyright. All rights reserved.

Changes in infectious agent profiles and host gene expression during spawning migrations of adult sockeye salmon (Oncorhynchus nerka)

During spawning migrations, adult Pacific salmon ( Onco rhynchus spp.) must undergo a suite of morphological and physiological changes to prepare for spawning. Modern genomics approaches can be used to conduct nonlethal assessments of physiology and infectious agent presence in migrating salmon. We investigated gene expression changes and shifts in the prevalence and load of 18 infectious agents in the gill tissue of adult sockeye salmon ( Oncorhynchus nerka) during the final 50 km of their freshwater migration. We found 77% of fish successfully migrated to spawning grounds. Fish exhibited a decrease in thermal stress gene expression following lake migration, consistent with the hypothesized benefits of thermal refugia during spawning migrations. We also found evidence of cooler water selection (∼1.5 °C) during lake migration for fish with higher infection burden, suggesting behavioural changes for fish suffering higher infection burdens and potential benefits of thermal refuges for reducing the negative impacts of infection for migrating salmon. Collectively, our findings provide further evidence that fish behaviourally regulate exposure to stressful conditions such as high temperature that may undermine survival.

Thermal selection and delayed migration by adult sockeye salmon (Oncorhynchus nerka) following escape from simulated in-river fisheries capture

Two hypotheses were tested concerning the consequences to adult sockeye salmon (Oncorhynchus nerka) of escape from commonly used fishing gear (gillnet, seine net and tangle net). First, by experimentally exposing 214 fish to three commonly used fishing gear types (gillnets, tangle nets, or seine nets) and releasing to complete migration after PIT-tagging, we tested the hypothesis that migration success and behaviour are differentially impacted according to the type of fishing gear interaction and escape. Also, by fitting salmon with thermal loggers, we tested whether salmon behaviourally selected cooler lake water following an escape encounter. Migration success was unaffected after escape from the simulated gear types compared to control fish. Surviving sockeye salmon that had gillnet or seine net encounter, but not a tangle net encounter, on average took approximately 2 days longer to migrate to the spawning area (averaging 14.7 days and 14.9 days, respectively) compared to control fish (12.6 days). Furthermore, escaped fish migrated at cooler temperatures through a lake system (average of 14.0 °C) compared to control fish (15.3 °C), which would reduce their absolute cost of transport during this migration by reducing the standard metabolic rate by about 10%. Consequently, in addition to demonstrating that fish escapement from a fishing gear will alter their subsequent migration behaviour, we introduce the possibility that the associated increase in energy expenditure and migration delay can be partially compensated for by behavioural selection of cooler water, if it is available, to lower basic energy turnover in a fish that is entirely reliant on energy stores to fuel its spawning migration.

Host-pathogen-environment interactions predict survival outcomes of adult sockeye salmon (Oncorhynchus nerka) released from fisheries.

Incorporating host-pathogen(s)-environment axes into management and conservation planning is critical to preserving species in a warming climate. However, the role pathogens play in host stress resilience remains largely unexplored in wild animal populations. We experimentally characterized how independent and cumulative stressors (fisheries handling, high water temperature) and natural infections affected the health and longevity of released wild adult sockeye salmon (Oncorhynchus nerka) in British Columbia, Canada. Returning adults were collected before and after entering the Fraser River, yielding marine- and river-collected groups, respectively (N=185). Fish were exposed to a mild (seine) or severe (gill net) fishery treatment at collection, and then held in flow-through freshwater tanks for up to four weeks at historical (14°C) or projected migration temperatures (18°C). Using weekly nonlethal gill biopsies and high-throughput qPCR, we quantified loads of up to 46 pathogens with host stress and immune gene expression. Marine-collected fish had less severe infections than river-collected fish, a short migration distance (100km, 5-7days) that produced profound infection differences. At 14°C, river-collected fish survived 1-2weeks less than marine-collected fish. All fish held at 18°C died within 4weeks unless they experienced minimal handling. Gene expression correlated with infections in river-collected fish, while marine-collected fish were more stressor-responsive. Cumulative stressors were detrimental regardless of infections or collection location, probably due to extreme physiological disturbance. Because river-derived infections correlated with single stressor responses, river entry probably decreases stressor resilience of adult salmon by altering both physiology and pathogen burdens, which redirect host responses toward disease resistance.

Adult Sockeye Salmon Responses to Transplanting Upstream of an Impassable Dam

AbstractThe Yakama Nation manages a program for reintroducing Sockeye Salmon Oncorhynchus nerka upstream of Cle Elum Dam in the Yakima River basin. The program involves transplanting of a mix of adult Sockeye Salmon from two middle Columbia River donor stocks, Lake Wenatchee (WEN) and Osoyoos Lake (OSO), with the goal of establishing a self‐sustaining population in the basin. In 2017, a subset of the transplanted population was radio‐tagged and monitored to assess stock‐specific responses to transplanting, including downstream entrainment at Cle Elum Dam (“fallback”) and prespawn mortality. Tagged fish were active during the first 2 weeks after release. Some fish moved upstream into the Cle Elum River and then returned to the reservoir; other fish moved downstream and passed Cle Elum Dam. Movement within the study area diminished during August but increased in September when fish began migrating up the Cle Elum River for spawning. A greater proportion of WEN fish (76.3%) was detected entering the river compared to OSO fish (53.9%). The hazard ratio from a multistate analysis of fish behavior indicated that WEN fish were nine times more likely to enter the river than OSO fish. The WEN fish also initiated upstream movement 3 weeks earlier than OSO fish. We found that 27.0% of all tagged fish were lost to the spawning population through fallback (20.7%) and prespawn mortality (6.3%). Fallback and prespawn mortality occurred for a greater percentage of OSO fish (26.9% and 7.7%) than WEN fish (15.3% and 5.1%), but these differences were not statistically different. These results provide foundational information from the early phase of a reintroduction effort. Continued monitoring will be required as Sockeye Salmon evolve to conditions upstream of Cle Elum Dam, and results from these efforts will provide valuable insights for this reintroduction effort and others in river systems with similar characteristics.

Time Required for Brown Bears to Capture and Consume Pacific Salmon

Detection, pursuit, capture, and consumption of prey by predators may be time consuming or not, depending on predators and prey, and this time allocation affects other aspects of predator ecology. Pacific salmon contribute substantially to coastal brown bear (Ursus arctos) diets, with implications for predators, prey, and their ecosystem. Brown bears were observed capturing sockeye salmon (Oncorhynchus nerka) at Mikfik Creek, Alaska, to determine capture and consumption rates. Bears averaged 3:00 min (SD = 5:24) to capture and 2:26 min (SD = 1:33) to completely consume small adult sockeye salmon. These foraging activities thus occupied only a small fraction of their total time budget at this site.

Evaluating Dam Water Release Strategies for Migrating Adult Salmon Using Computational Fluid Dynamic Modeling and Biotelemetry

AbstractHydrodynamics in dam tailraces can influence the swimming behavior and survival of fish, and yet there have been few studies that have linked the movement patterns of fish to encountered flow patterns. In this study, we examined the flow field downstream of Seton Dam in British Columbia, Canada using acoustic Doppler current profilers (ADCPs), and computational fluid dynamic (CFD) modeling of two dam water release scenarios to understand their impacts on the swim speed and behavior of upriver‐migrating adult sockeye salmon (Oncorhynchus nerka). Forty‐five sockeye salmon were tagged with acoustic transmitters and tracked to understand their movement patterns, swim speed, residence time in the dam tailrace, and postdam passage survival. The water release scenarios produced contrasting tailrace hydrodynamics and altered the behavior of migrating sockeye salmon. Tagged fish avoided areas of high velocity (>2.4 m/s or four body lengths/s, twice their critical swim speed), high Reynolds shear stress (>21 Pa), and high turbulence kinetic energy (>0.12 m2 s−2) in the dam tailrace. Excessive use of anaerobic metabolism, together with high water shear stress adjacent to and downstream of the fishway entrance, were thought to be the main cause for low postdam passage survival of sockeye salmon. Additional dam water release strategies were assessed with the CFD model to target releases that improve the migration conditions for sockeye salmon. Our study highlights the importance of pairing hydraulic and ecological information to better understand and improve the migration conditions for wild fish.

Links between fluctuations in sockeye salmon abundance and riparian forest productivity identified by remote sensing

AbstractPacific salmon (Oncorhynchus spp.) carcasses can fertilize riparian forests with marine‐derived nutrients when populations make their annual return to natal streams to spawn; however, the strength of this cross‐system linkage likely varies substantially among years due to the interannual fluctuations in abundance that characterize most salmon populations. Here, we used a 36‐yr time series (1984–2019) of satellite imagery and salmon abundance estimates to assess spatiotemporal associations between forest greenness (a measure of plant productivity) and adult sockeye salmon (Oncorhynchus nerka) abundance in the lower Adams River, British Columbia, Canada. The Adams River sockeye population displays a quadrennial pattern of abundance, with a dominant cohort that spawns every four years in numbers that are typically two to three orders of magnitude larger than non‐dominant cohorts. We found that variation in forest greenness was consistently explained best by models including dominant cohort year, whereas models lacking an index of salmon abundance were the lowest‐ranked. Greenness of riparian vegetation increased by an average of 0.015 NDVI units (approximately 1%) in the summer after a dominant cohort return, and this effect on greenness persisted into the subsequent fall (11–13 months after spawning). The positive association between quadrennial pulses of salmon and riparian greenness occurred in plots both within 30 m of the stream and 95–125 m away from the stream, indicating that the spatial extent of fertilization may occur well beyond areas directly adjacent to the riverbank. These results suggest that forests respond to cyclical variation in salmon abundance and that overwinter storage of marine‐derived nutrients within catchments allows plants to capitalize on these nutrients in the following growing season. Continued advances in remote sensing technology will enhance our understanding of cross‐system resource linkages and can inform the ecosystem‐based management of Pacific salmon.

Resumption of Anadromy or Straying? Origins of Sockeye Salmon in the Elwha River

AbstractWhen barriers to migration are removed, anadromous fishes from other rivers may colonize newly accessible habitat or landlocked forms of the species may resume anadromy if conditions allow. For example, two large hydroelectric dams on the Elwha River, Washington, were removed between 2011 and 2014 to restore populations of Pacific salmon Oncorhynchus spp. after a century of isolation. Immediately prior to and following dam removal (2010–2017), upstream‐migrating adult Sockeye Salmon Oncorhynchus nerka were sampled, and DNA was extracted and compared with samples from the local Lake Sutherland population (above the lower dam’s former location) and a coastwide genetic baseline to determine the probable origin of each fish. Genetic analysis indicated that none of the 45 Sockeye Salmon sampled had originated from the local population or other populations nearby in the genetic baseline, and only a single fish originated from the abundant and proximate Fraser River populations. The strays reflected two different sources and processes: 22 were from the west coast of Vancouver Island (all in 2015, when these populations were very abundant and encountered conditions unfavorable for upstream migration in their natal system) and another 22 assigned to more northerly populations, mostly to riverine populations from the Stikine River. Test fishery samples collected in the Strait of Juan de Fuca contained very few fish with assignments similar to these northern rivers. It is not clear whether these individuals originated from the distant riverine populations or from a closer population not in the baseline that resembles them genetically. Taken together, the findings indicate that the genetic composition of any future population in the Elwha River may reflect signatures from diverse sources and subsequent selection in the new habitat.

The assessment of the adult escapement of the Ozernaya River sockeye salmon (Oncorhynchus nerka) with the hydroacustic method

Results of in situ testing of BioSonics DT-X and Didson hydroacustic systems set on migrating path to spawning grounds of adult sockeye salmon in the Ozernaya River are presented. Two million 374 thousand fishes was recognized and counted for the whole working period of the systems, including one million 276 thousand individuals registered by BioSonics DT-X echo-sounder and 1 million 98 thousand – by Didson imaging sonar. The data on the assessment of Pacific salmon escapement in the Ozernaya River allow to make recommendations on the use of the BioSonics DT-X and Didson hydroacustic systems as a tool for assessment of Pacific salmon escapement in rivers of Kamchatka.

Stable Isotopes Reveal Variation in Consumption of Pacific Salmon by Brown Bears, Despite Ready Access in Small Streams

AbstractBrown bears Ursus arctos consume a wide range of organisms, including ungulates and plants, but Pacific salmon Oncorhynchus spp. are especially important to their diet where their ranges overlap. Although some brown bears minimize antagonistic encounters with other brown bears or infanticide by avoiding streams where salmon spawn, studies generally assume that brown bears with ready access to salmon feed heavily on them. To test this assumption, and the hypothesis that male brown bears would feed more heavily on salmon than females (owing to their sexual size dimorphism), we collected hair samples from brown bears by using barbed wire placed on six small tributaries of Lake Aleknagik, Alaska, USA, where adult Sockeye Salmon Oncorhynchus nerka are readily accessible and frequently consumed by brown bears. Analysis of DNA distinguished among the different brown bears leaving the hair samples, some of which were sampled multiple times within and among years. We assessed the contribution of salmon to the diet of individual brown bears by using carbon and nitrogen stable isotope signatures. The 77 samples analyzed from 31 different bears over 4 y showed isotopic ratios consistent with reliance on salmon, but the wide range of isotopic signatures included values suggesting variable, and in one case considerable, use of terrestrial resources. Stable isotope signatures did not differ between male and female brown bears, nor did they differ between two sides of the lake, despite marked differences in Sockeye Salmon density. We collected the hair samples when salmon were present, so there was some uncertainty regarding whether they reflected feeding during the current or previous season. Notwithstanding this caveat, the results are consistent with the hypothesis that salmon were sufficiently available to provide food for the brown bears and that the considerable isotopic variation among brown bears with access to salmon reflected their age, status, and behavior.

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.

Trace element content in tissues of sockeye salmon from several waterbodies of the Kamchatka Peninsula

The results of the study of macro- (Ca, Mg) and micro- (Mn, Zn, Cu, Fe) elements content in the pectoral fin tissue of adult sockeye salmon Oncorhynchus nerka from the rivers Bolshaya and Kamchatka as well as from Azabachye Lake (Kamchatka Peninsula) are presented. The significant differences in the contents in samples from the three waterbodies were found. It was ascertained that the largest ash proportion (and the mass fraction of trace elements) in dry matter contained in the tissues of sockeye salmon from Kamchatka River, the smallest in the sample of Azabachye Lake. The Ca content was 98 % of the ash mass in all samples; Mg accounted for slightly less than 2 %. The proportion of the remaining elements varied within 0.1 %, and exactly in this range the differences in the microelement composition between the samples were revealed. In sockeye salmon tissues from the Kamchatka River, the content of heavy metals as well as Ca and Mg was higher than in other samples, and their percentage ratio considerably differed from the others. Obviously, the water bodies studied sufficiently distinguish in hydrochemical indices, and that is the cause of differences in bioaccumulation intensities of trace elements in fish tissues at the freshwater life stage.

Modified Commercial Fish Trap to Help Eliminate Salmonid Bycatch Mortality

AbstractThe utility of commercial salmonid Oncorhynchus spp. traps in the U.S. Pacific Northwest was recently revisited for the first time in decades to enable selective harvesting of hatchery‐origin salmonids while reducing mortality of Endangered Species Act (ESA)‐listed salmonids. Modifications to historical gear designs resulted in dramatic improvements in salmonid bycatch survival rates relative to conventional commercial gears in the lower Columbia River. Expanding upon this work, an experimental commercial fish trap was further modified to largely eliminate net contact, air exposure, handling, and crowding of fish. Studies were conducted from May to November 2019 in the lower Columbia River to estimate survival of bycatch and evaluate potential benefits from the modified passive capture design. Analyzed through two separate survival estimation techniques, the modified trap demonstrated no detectable effect on salmon release survival and a significant improvement over the previous prototype design. Estimated through a paired release–recapture methodology, the relative survival effect of catch and release compared to controls over a 400‐km migration was 1.017 ( = 0.032) for adult Sockeye Salmon O. nerka. For adult Coho Salmon O. kisutch that were held captive for a 48‐h postrelease period, estimated survival (S) was 1.000 (lower 95% confidence limit: S ≥ 0.978). These results suggest that trap modifications can be made to significantly reduce bycatch mortality of ESA‐listed salmonids and provide increased opportunity for harvest of hatchery‐origin salmonids.

The effect of temperature acclimation on the force-frequency relationship and adrenergic sensitivity of the ventricle of two populations of juvenile sockeye salmon.

We tested the hypothesis that cardiorespiratory differences known to exist among adult sockeye salmon populations also exist in the juveniles. To test this hypothesis, we compared cardiac contractility and adrenergic responsiveness of juvenile sockeye salmon from two geographically isolated populations that were reared from eggs under common garden conditions and at two acclimation temperatures (5°C and 14°C). However, we found no substantive differences in the force-frequency response (FFR) and the cardiac pumping capacity of juveniles from Weaver Creek and Chilko River populations, even when we considered wild-reared juveniles from one of the populations. An unexpected discovery for all fish groups at 5°C was a rather flat FFR during tonic β-adrenergic stimulation (βAR) stimulation. Curiously, while active tension nearly doubled with maximum βAR stimulation at low pacing frequencies for all fish groups, a negative FFR with maximum βAR stimulation meant that this inotropic benefit was lost at the highest pacing frequency (0.8Hz). Active tension with tonic βAR stimulation was similar at 14°C, but maximum pacing frequency doubled and all fish groups displayed a modest negative FFR. Maximum βAR stimulation again doubled active tension and this benefit was retained even at the highest pacing frequency (1.6Hz) at 14°C. Even though subtle population differences were apparent for the FFR and pumping capacity, their biological significance is unclear. What is clear, however, is that the cardiac pumping capacity of juvenile sockeye would benefit more from βAR stimulation swimming at 15°C than when swimming at 5°C.

Adult Sockeye Salmon Gastrically Tagged Near Spawning Grounds Exhibit Lower Survival Rates throughout the Spawning Period than Externally Tagged Conspecifics

AbstractTelemetry is a common tool for studying the behavior and fate of migrating adult Pacific salmon Oncorhynchus spp., yet few field studies have compared behavior and fate associated with different tagging techniques. In this study, adult Harrison River (British Columbia) Sockeye Salmon O. nerka were captured in their natal river near spawning areas, radio‐tagged by gastric insertion or external attachment in the dorsum, and released. Tagging occurred on 5 d spread over 3–8 weeks prior to spawning, thus encompassing fish in varying stages of maturity and freshwater residency. Tagged individuals were monitored over the spawning season by using fixed receiver stations and mobile tracking. The probability of fish moving upstream or downstream of the tagging site within 35 h of tagging was a function of tagging date but not tag type. Tag type significantly influenced fate, with almost twice as many externally tagged fish (41.6%; 42 of 101) surviving to reach spawning areas compared to gastrically tagged fish (22.4%; 21 of 94). The number of active externally tagged fish in the Harrison River system was consistently greater than the number of active gastrically tagged fish that received tags on the same date for four of the five tagging dates. External tag attachment may be a better approach than gastric insertion for studies that tag adult salmon near or on spawning areas.

Adult Sockeye Salmon Assessment in a Tidal, Turbid River: A Comparison of Sonar and Test Fishing Methods

AbstractThe feasibility of using sonar in Alaska's lower Kvichak River, a region with a 3.8 m tidal flux, to assess the abundance of adult Sockeye Salmon Oncorhynchus nerka was evaluated as a replacement for an in‐river gill‐net test fishery. Test fish indices are one tool that is used to manage commercial and subsistence fisheries in Bristol Bay. On this river, an observation tower project near the clear headwaters provides accurate abundance estimates for this species, but fishery managers require timelier estimates for in‐season daily management. The lower‐river test fish index is timelier, but it is less precise than tower estimates are. We had to determine whether to replace the test fish project with sonar based on how well each compared with tower estimates and an assessment of their cost and effort. Two dual‐frequency identification sonars (DIDSONs) were deployed along one bank of the lower river (one pointed toward shore and one away from shore) to assess salmon passage across all tidal stages in an attempt to produce an index with higher precision. Compared with the tower estimates, the test fish index showed a slightly stronger correlation in each of the three study years than the sonar index did, despite the stronger sampling power of the sonar. The test fish and sonar indices were strongly correlated. In the lower river, upriver salmon migration (85–98%) occurred primarily during ebb tides and downriver passage (97–100%) during flood tides, indicating that salmon do not use selective tidal stream transport to minimize their energy expenditures in this river. We were able to successfully use fixed‐location sonar in a tidal nearshore region to produce an index of fish abundance. Our recommendation was to continue the test fishing project rather than to replace it with sonar because of the similarity between their indices and the higher cost of the sonar project.

Increased prespawning mortality threatens an integrated natural- and hatchery-origin sockeye salmon population in the Lake Washington Basin

The life cycle of diadromous fishes such as salmonids involves natural mortality in a series of distinct life history stages, occurring sequentially in different habitats. Decades of research have emphasized mortality at the embryo, juvenile, and sub-adult stages but it is increasingly clear that some adults that survive and return to freshwater habitats die during the final homeward migration or after they reach the spawning grounds, prior to breeding. These are termed “en route” and “prespawning” mortality, respectively, and can threaten populations depleted by mortality at previous stages. In this study, we present evidence that the sockeye salmon, Oncorhynchus nerka, population that returns to the Lake Washington Basin, in Washington State, USA, is experiencing both forms of adult mortality. Counts of the salmon entering the basin on their return migration in June and July were compared to counts in the major spawning grounds in September through November for 1995–2018. The disparity has increased markedly in recent years. The counts on the spawning grounds have decreased as a proportion of the number entering the system with an average 49 % of sockeye unaccounted for, consistent with increased en route mortality. In addition, prespawning mortality rates have increased in salmon that reach the Cedar River, the main spawning tributary, both at a hatchery holding adult fish in 1995–2018, and in the naturally spawning populations when monitored in the last five years. Hatchery records indicated <10 % prespawning mortality for 1995–2010, increasing to an average 43 % for 2014 – 2018. Recent carcass surveys in the Cedar River documented that 33.6% (2014), 22.3% (2015), 30.3% (2016) and 50.0% (2018) of female sockeye died before completing spawning. These recent increases in prespawning mortality have been associated with warm water during entry to freshwater, but comparably warm water in past decades had no such effect. Steady warming of river temperatures around the median run completion date from < 8.0 °C to > 13.0 °C was correlated with increased prespawning mortality rates at the hatchery from 1995–2018. We conclude that warming conditions during migration and spawning, in concert with other factors such as infections with pathogens, are responsible for the increased prespawning mortality of adult sockeye salmon that are high enough to threaten the population’s viability.