Juvenile Chinook Salmon Oncorhynchus Tshawytscha Research Articles

Understanding how restoration reduces competition for habitat by combining theory, observation, and experiment.

Habitat selection theory enables inferences about species habitat choice across a range of observed population densities. However, it is relatively uncommon to use habitat selection theory in studies of habitat restoration efficacy to understand the effect of restoration on habitat competition. We combined observational density data and resource selection functions to analyze habitat correlations with both habitat selection theory and a mark-recapture experiment to show how habitat restoration can mitigate competition between species with similar habitat preferences. To restore degraded and channelized riverine habitat for juvenile Chinook salmon (Oncorhynchus tshawytscha) and steelhead trout (Oncorhynchus mykiss) engineered log jams (ELJs) have been installed to create pools to enhance growth and rearing. Application of habitat selection theory first showed that both species share a preference for ELJ-treated habitat over unrestored habitat. Linear models showed that steelhead are generalists with respect to depth in unrestored habitat, whereas both species' abundance varies along a depth gradient in ELJ-treated habitat. Selective versus opportunistic use of deep and shallow ELJ pools was density-dependent. We found a range of densities at which a "ghost of competition" exists, where Chinook are selective on deep ELJ-treated pools and steelhead are selective on shallow pools. A mark-recapture experiment confirmed that steelhead limit Chinook movement into unrestored habitat, but this competitive effect vanished in ELJ-treated habitat where selection occurred with respect to pool depth. The experiment, combined with theory, enabled (1) the identification of a mechanism allowing for shared preference of restored habitat and (2) the description of how restoration can mitigate competition.

Unique diet and Philonema sp. infections in reservoir‐rearing juvenile Chinook Salmon

AbstractObjectiveDams and reservoirs can alter juvenile growth and survival of migratory salmonids through several physical and biological mechanisms. Juvenile Chinook Salmon Oncorhynchus tshawytscha that are produced upstream of large hydropower dams may have associated passage mortality, but the reservoirs created by these dams can support rapid growth. Characterizing the biotic drivers of growth and mortality in reservoirs may aid in understanding the cumulative effects of river impoundments on migratory salmonid populations. The purpose of this study was to understand how reservoirs facilitate rapid growth in juvenile Chinook Salmon.MethodsWe analyzed stomach contents to determine diet composition throughout the summer and fall. We also recorded prevalence of the parasitic nematode Philonema sp. in the coeloms of fish.ResultWe found that juvenile Chinook Salmon frequently consumed young‐of‐year centrarchids, which likely contributed to rapid growth. Piscivory was highest from July through October and decreased with surface temperature from November through December. Correspondingly, zooplankton and arthropod consumption increased in November and December. Prevalence of visible Philonema sp. infections in the coelom was high (34.6%), negatively associated with time, and nonlinearly associated with fork length.ConclusionThese findings reveal unique diet patterns and suggest potential parasite‐associated mortality in reservoir‐rearing Chinook Salmon, but more detailed studies across a longer time scale are needed to robustly assess the population‐level effects of this parasite.

The Roles of Diet and Habitat Use in Pesticide Bioaccumulation by Juvenile Chinook Salmon: Insights from Stable Isotopes and Fatty Acid Biomarkers.

Stable isotopes (SI) and fatty acid (FA) biomarkers can provide insights regarding trophic pathways and habitats associated with contaminant bioaccumulation. We assessed relationships between SI and FA biomarkers and published data on concentrations of two pesticides [dichlorodiphenyltrichloroethane and degradation products (DDX) and bifenthrin] in juvenile Chinook Salmon (Oncorhynchus tshawytscha) from the Sacramento River and Yolo Bypass floodplain in Northern California near Sacramento. We alsoconducted SI and FA analyses of zooplankton and macroinvertebrates to determine whether particular trophic pathways and habitats were associated with elevated pesticide concentrations in fish. Relationships between DDX and both sulfur (δ34S) and carbon (δ13C) SI ratios in salmon indicated that diet is a major exposure route for DDX, particularly for individuals with a benthic detrital energy base. Greater use of a benthic detrital energy base likely accounted for the higher frequency of salmon with DDX concentrations > 60ng/g dw in the Yolo Bypass compared to the Sacramento River. Chironomid larvae and zooplankton were implicated as prey items likely responsible for trophic transfer of DDX to salmon. Sulfur SI ratios enabled identification of hatchery-origin fish that had likely spent insufficient time in the wild to substantially bioaccumulate DDX. Bifenthrin concentration was unrelated to SI or FA biomarkers in salmon, potentially due to aqueous uptake, biotransformation and elimination of the pesticide, or indistinct biomarker compositions among invertebrates with low and high bifenthrin concentrations. One FA [docosahexaenoic acid (DHA)] and DDX were negatively correlated in salmon, potentially due to a greater uptake of DDX from invertebrates with low DHA or effects of DDX on FA metabolism. Trophic biomarkers may be useful indicators of DDX accumulation and effects in juvenile Chinook Salmon in the Sacramento River Delta.

Estimating predation rates of stocked juvenile Chinook Salmon using novel acoustic predation transmitters

AbstractObjectiveAcoustic telemetry is a tool commonly used to better understand the fates of fish following a stocking event. Until recently, predation on stocked fish could only be inferred by observing and identifying abrupt changes in tag detection histories. Recently, a novel predation transmitter has provided better insight into how predation affects poststocking survival. The main objective of this study was to use these transmitters to estimate predation rates on stocked juvenile Chinook Salmon Oncorhynchus tshawytscha in a landlocked system (Lake Oahe, South Dakota). Our secondary objectives were to determine how quickly predation was occurring, the time of day of predation events, and if there were differences in size between predated and nonpredated fish.MethodsIn 2021 and 2022, we surgically implanted Innovasea V5D 180‐kHz transmitters in 110 juvenile Chinook Salmon. These tagged Chinook Salmon were stocked at two locations on Lake Oahe along with a large cohort of untagged individuals. The tagged Chinook Salmon were passively tracked using an array of Innovasea VR2W 180‐kHz acoustic receivers.ResultAcross the two study years, we observed a mean predation rate (±SE) of 32.9 ± 6% on the tagged Chinook Salmon. Predation on the tagged Chinook Salmon occurred relatively quickly, with a mean time to predation of 86.7 ± 10.8 h. Additionally, 70% of the observed predation occurred during the daytime. Interestingly, results indicated that total length did not differ between fish that were preyed upon or and those that were, but there was a difference in total weight between the two groups.ConclusionOur results provide insight into recruitment bottlenecks within this population and offer information to other researchers regarding the advantages and limitations of this novel technology. Understanding how to mitigate predation could help to guide future stocking strategies to ensure a better return on investment for costly stocking programs.

Survival implications of diversion entrainment for out‐migrating juvenile Chinook Salmon and steelhead

AbstractObjectiveEfforts to ameliorate the negative effects of diversion dams on aquatic species of concern are important in rivers where water withdrawal supports agricultural economies, and they are likely to become increasingly important with impending climate change. A multi‐year study was conducted to evaluate the survival consequences of diversion dam passage for juvenile Chinook Salmon Oncorhynchus tshawytscha and steelhead O. mykiss in the highly managed Yakima River, Washington. Canal entrainment and passage were evaluated at four diversion dams in the lower Yakima River where seaward‐migrating juvenile salmon and steelhead either pass dams via spill bays or enter canals with downstream fish‐screening facilities designed to collect entrained fish and return them to the main‐stem river.MethodsWe used acoustic telemetry to monitor the downstream migration behavior of 4403 juvenile Chinook Salmon and steelhead during 2018–2021.ResultPercent entrainment into canals was substantial (6–59%) at three of the four diversion dams studied, and entrainment probability was positively associated with the proportion of streamflow diverted into canals. Survival probability estimates for groups of tagged fish that were entrained into canals were lower than survival probability estimates for tagged fish that passed through spill bays on the dams. Absolute differences in survival probabilities between routes ranged from 0.099 to 0.369, demonstrating that canal entrainment reduced the survival of outmigrating juvenile Chinook Salmon and steelhead. We also found that entrainment resulted in migration delays, which could further affect survival because fish are increasingly exposed to predation and decreased water quality as water temperature increases throughout the migration season.ConclusionCanal entrainment resulted in increased mortality and travel time through the study area, highlighting the need to limit the entrainment of juvenile salmon and steelhead at diversion dams in rivers where salmon recovery is important.

Size‐selective predation effects on juvenile Chinook salmon cohort survival off Central California evaluated with an individual‐based model

AbstractVariation in the recruitment of salmon is often found to be correlated with marine climate indices, but mechanisms behind environment–recruitment relationships remain unclear and correlations often break down over time. We used an ecosystem modeling approach to explore bottom‐up and top‐down mechanisms linking a variable environment to salmon recruitment variations. Our ecosystem model incorporates a regional ocean circulation submodel for hydrodynamics, a nutrient‐phytoplankton‐zooplankton submodel for producing planktonic prey fields, and an individual‐based model (IBM) representing juvenile Chinook salmon (Oncorhynchus tshawytscha), combined with observations of foraging distributions and diet of a seabird predator. The salmon IBM consists of modules, including a juvenile salmon growth module based on temperature and salmon–prey availability, a behavior‐based movement module, and a juvenile salmon predation mortality module based on juvenile salmon size distribution and predator–prey interaction probability. Seabird–salmon interactions depend on spatial overlap and juvenile salmon size, whereby salmon that grow past the size range of the prey distribution of the predator will escape predation. We used a 21‐year historical simulation to explore interannual variability in juvenile Chinook salmon growth and predation‐mediated survival under a range of ocean conditions for sized‐based mortality scenarios. We based a series of increasingly complex predation scenarios on seabird observational data to explore variability in predation mortality on juvenile Chinook salmon. We initially included information about the predator spatial distribution, then added population size, and finally the predator's diet percentage made up of juvenile salmon. Model agreement improves with added predator complexity, especially during periods when predator abundance is high. Overall, our model found that when the fraction of juvenile salmon in seabird diet increased relative to alternate prey (e.g., Northern anchovy Engraulis mordax, and juvenile rockfish Sebastes spp.), there was a concomitant decrease in salmon cohort survival during their first year at sea.

FIRST REPORT IN CANADA OF CERATONOVA GASTEROSTEA (CNIDARIA: MYXOZOA) IN THREESPINE STICKLEBACK GASTEROSTEUS ACULEATUS FROM THE FRASER RIVER, BRITISH COLUMBIA.

A survey of threespine stickleback (Gasterosteus aculeatus L.) and juvenile Chinook salmon (Oncorhynchus tshawytscha (Walbaum)) from the lower Fraser River, British Columbia, Canada and adjacent marine waters was conducted for the presence of myxozoan parasites of the genus Ceratonova Atkinson, Foott, and Bartholomew, 2014 (Bivalvulida, Ceratomyxidae). Quantitative polymerase chain reaction (PCR; small subunit ribosomal RNA gene) combined with conventional PCR and sequencing (ribosomal internal transcribed spacer region 1) detected Ceratonova gasterostea Atkinson, Foott, and Bartholomew, 2014 in 89 of 269 (33.1%) sticklebacks from the Fraser River and in 1 of 51 (2%) from Howe Sound. In contrast, Ceratonova shasta (Atkinson, Foott, and Bartholomew, 2014) was detected in 2 of 26 (7.7%) Chinook salmon from the Fraser River. Reciprocal infections were not detected. Light microscopic examination of the stickleback intestine revealed the presence of an intense infection with large numbers of pseudoplasmodia which interacted with enterocytes through pseudopodia-like projections. Myxospores with characteristic elongate valve cells were visible within mature pseudoplasmodia and free in the intestinal lumen. This is the first report of C. gasterostea in Canada.

Ceratonova shasta infection in lower Feather River Chinook juveniles and trends in water-borne spore stages

We performed a five-year (2015–2020) survey of juvenile natural Chinook Salmon (Oncorhynchus tshawytscha), adult salmon carcasses, and river water from the lower Feather River to determine infection prevalence, distribution, and spore quantity of the myxozoan parasite Ceratonova shasta. Average prevalence of infection in juvenile salmon collected from the high flow channel ranged from 45–58% depending on assay method. Initial infection of fry and detection of actinospore stage in river water began in late January or early February. Overt disease occurred in March and was lethal. Infection of the gill was detected weeks ahead of intestinal infection. Water-borne spore measurement and fish infection demonstrate an infectious zone beginning at the outlet of the Thermalito Afterbay. This zone is expanding downriver past the confluence of the Yuba River. Adult carcasses produce billions of myxospores annually that move downriver over the winter. C. shasta infection is one of several factors (predation, limited rearing habitat, elevated water temperature, water withdrawal, etc.) limiting natural Chinook Salmon recruitment in the Feather River.

A correlation between seasonally changing photoperiod, whole body lipid, and condition factor in juvenile spring Chinook salmon (Oncorhynchus tshawytscha).

The regulation of lipid stores is a central process for the physiology and ecology of fishes. Seasonal variation in lipid stores has been directly linked to survival of fishes across periods of food deprivation. We assessed whether a seasonally changing photoperiod was correlated to seasonal changes in energetic status to help better understand these important processes. Groups of first feeding Chinook salmon fry were introduced to a seasonal photoperiod cycle, but the point of entrance into the seasonal cycle varied from near the winter solstice (December), to either side of the spring equinox (February & May). Temperature and feeding rate were similar for all treatments. Subsequently, condition factor and whole body lipid content were assessed through a seasonal progression. Throughout most of the experiment, length and weight did not differ between the different photoperiod treatments, however whole body lipid and Fulton's condition factor did. Furthermore, changes in both whole body lipid and Fulton's condition factor in all treatment groups followed a similar seasonal pattern that was inversely related to day length (highest K and lipid levels found during days with the least light). These results suggest that regardless of age or size, there is a correlation between seasonal changes in photoperiod and changes in body composition in juvenile Chinook salmonids.

The influence of temperature on performance, biological indices, composition, and nutrient retention of juvenile Chinook salmon (Oncorhynchus tshawytscha) reared in freshwater

This study investigated the effects of different temperatures on the performance, composition, and nutrient retention of Chinook salmon reared in freshwater. Individuals (187.6 ± 27.1 g) were distributed into twelve tanks of 8000 L each (155 to 157 fish per tank) at a temperature of 14 °C. The tanks were transitioned from 14 °C (hatchery temperature) to 8, 12, 16, and 20 °C over seven days. Three fish assessments were performed, the first (initial) when the fish were distributed in the tanks, a second (interim) between days 9 and 16 at the start of the experiment, and a third (final) after 41 to 49 days at the target temperature. At the end of the trial, performance parameters, proximate composition, amino acid, and fatty acid composition, and nutrient retention were evaluated. Better growth performance was observed in fish at 16 °C and 20 °C compared to the lower temperatures. Fish at higher temperatures had higher levels of saturated fatty acids (SFA), while at lower temperatures fish had higher levels of n-3 and n-6 polyunsaturated fatty acids (PUFA), especially eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA). A polynomial relationship between nutrient retention and temperature showed that fish from all treatments retained more lipids than proteins, with higher retention of MUFA compared to the other fatty acid classes. Additionally, DHA retention was approximately three times higher than EPA retention. The results showed that 16 to 20 °C was the optimum temperature range for Chinook salmon, and the performance differences were mainly modulated by lipid retention/catabolism.

Estimating Juvenile Salmon Estuarine Carrying Capacities to Support Restoration Planning and Evaluation

Estimating juvenile salmon habitat carrying capacities is a critical need for restoration planning. We assimilated more than 4500 unique estimates of published juvenile densities (e.g., fish/m2) in estuarine and floodplain habitats. These density data were categorized by species and life stage, habitat type, seasonal period, and geographic region to develop frequency statistics (e.g., 25th and 75th percentiles, or quartiles). These frequency statistics were then used in a habitat expansion approach to estimate carrying capacities based on habitat extent. We demonstrate the habitat expansion approach by applying the quantiles of observed juvenile Chinook salmon (Oncorhynchus tshawytscha) and coho salmon (O. kisutch) densities (fish/ha) to spatial data describing current, historical or potential, and predicted (based on seal level rise) habitat extents for 16 coastal Oregon estuaries to estimate carrying capacities. Current carrying capacities based on 75th percentile springtime (Apr–Jun) densities ranged from 2902 to 33,817 fish/delta for Chinook salmon and 2507 to 20,206 fish/delta for coho salmon. Historic carrying capacities during the peak rearing period (spring) ranged from 3869 to 71,844 fish/delta for Chinook salmon and 3201 to 38,337 fish/delta for coho salmon, representing a 3 to 72% loss in Chinook salmon capacity and 2 to 67% loss in coho salmon capacity. Estimated carrying capacities were predicted to decline by 2 to 54% with 1.4 m of sea level rise in systems that are projected to lose vegetated tidal wetland habitat, while a 1 to 320% increase in capacity was predicted for systems that are predicted to increase in area with sea level rise. Finally, we demonstrate how the carrying capacity estimates can be used to estimate changes in juvenile Chinook and coho salmon capacity following restoration, which can be used to both design and evaluate restoration projects.

Gut microbiome composition associates with corticosteroid treatment, morbidity, and senescence in Chinook salmon (Oncorhynchus tshawytscha)

Pacific salmon experience prolonged elevation in corticosteroid hormones during important life history events including migration, reproduction, and senescence. These periods of elevated corticosteroids correspond with changes to immunity and energy metabolism; therefore, fish may be particularly vulnerable to mortality at these times. Recent studies found that stress-induced cortisol release associated with microbial community shifts in salmonids, raising the question of how longer-term corticosteroid dynamics that accompany life history transitions affect salmonid microbiomes. In this work, we experimentally evaluated the relationships between gut microbiome composition, chronically elevated corticosteroids, and mortality in juvenile Chinook salmon (Oncorhynchus tshawytscha). We found that treatment with slow-release implants of the corticosteroids cortisol or dexamethasone resulted in changes to the gut microbiome. Morbidity was also associated with microbiome composition, suggesting that the gut microbiome reflects individual differences in susceptibility to opportunistic pathogens. Additionally, we analyzed a small number of samples from adult fish at various stages of senescence. Results from these samples suggest that microbiome composition associated with gut integrity, and that the microbial communities of corticosteroid treated juveniles shift in composition toward those of senescent adults. Overall, findings from this work indicate that the gut microbiome correlates with mortality risk during periods of chronic corticosteroid elevation.

Differential heat shock protein responses in two species of Pacific salmon and their utility in identifying heat stress

Abstract Rapid and accelerating warming of salmon habitat has the potential to lower productivity of Pacific salmon (Oncorhynchus species) populations. Heat stress biomarkers can indicate where warming is most likely affecting fish populations; however, we often lack clear classifications that separate individuals with and without heat stress needed to make these tools operational. We conducted a heat exposure experiment with trials lasting 12 or 36 h using juvenile Chinook salmon (Oncorhynchus tshawytscha) and coho salmon (Oncorhynchus kisutch) to validate heat stress biomarkers in white muscle. Following habituation to 13°C, individuals were exposed to water temperatures that increased to 15°C, 17°C, 19°C, 21°C or 23°C. Heat shock protein 70 abundance (HSP70 measured by ELISA) and transcription of 13 genes (mRNA measured by qPCR) including three heat shock protein genes (hsp70, hsp90, hsp27) were measured. A distinct heat stress response was apparent by 21°C in juvenile Chinook salmon and 23°C in juvenile coho salmon using HSP70. A threshold for heat stress classification in Chinook salmon of > 2 ng HSP70 mg.1 total protein identified heat stress in 100% of 21 and 23°C treated individuals compared to 4% in cooler treatments. For coho salmon, > 3 ng HSP70 mg.1 total protein identified heat stress in 100% of 23°C treated individuals compared to 4% in cooler treatments. Transcription from a panel of genes separated individuals between cooler and stressful temperature experiences (≥21°C for Chinook salmon and ≥23°C for coho salmon) with ~ 85% correct classification. Our findings indicate that juvenile Chinook salmon were more temperature-sensitive than juvenile coho salmon and support the use of a HSP70 threshold sampled from muscle for assessing heat stress in individual wild Pacific salmon with an option for non-lethal biopsies for spawning adults.

Effect of temperature and dietary pesticide exposure on neuroendocrine and olfactory responses in juvenile Chinook salmon (Oncorhynchus tshawytscha)

Projected water temperature increases based on predicted climate change scenarios and concomitant pesticide exposure raises concern about the responses of aquatic organisms. To better understand the effect of pesticide mixtures and influence of water temperature to fish, juvenile Chinook salmon (Oncorhynchus tshawytscha) were dietarily exposed to a mixture of legacy and current use pesticides (p,p’-DDE, bifenthrin, chlorpyrifos, esfenvalerate, and fipronil) at concentrations detected from field-collected prey items in the Sacramento-San Joaquin Delta, California (Delta) and exposed under current and predicted future water temperature scenarios, 11, 14, or 17 °C, for 14 days. The expression of a subset of genes (deiodinase 2-dio2, gonadotropin releasing hormone 2-gnrh2, and catechol-o-methyltransferase-comt) involved in neuroendocrine, dopaminergic, and olfactory function previously shown to be altered by individual pesticide exposures germane to this study were determined and olfactory function assessed using a Y-maze behavioral assay. When total body burdens of pesticides were measured, a significant decrease in dio2 expression was observed in Chinook salmon exposed at 14 °C compared to fish kept at 11 °C. Increases in gnrh2 expression were also observed in fish exposed to 14 °C. Similarly, increases in comt expression was noted at 14 and 17 °C. Additionally, altered expression of all transcripts was observed, showing interactions between temperature and individual pesticide concentrations. Chinook salmon spent significantly more time actively avoiding the odorant arm at baseline conditions of 11 °C in the Y-maze. At higher temperatures, Chinook spent significantly more time not making a choice between the odorant or clean arm following exposure to the low pesticide mixture, relative to 11 °C. These results suggest that dietary exposure to pesticide mixtures can potentially induce neuroendocrine effects and behavior. Impaired olfactory responses exhibited by Chinook salmon could have implications for predator avoidance in the wild under increased temperature scenarios and impact populations in the future.

Behavioral cues enable native fishes to exit a California floodplain while leaving non‐native fishes behind

AbstractFloodplains are highly productive environments that provide critical rearing habitat and increased growth for diverse native fishes, including juvenile Chinook salmon (Oncorhynchus tshawytscha). Yet, stranding during the flood recession is a potential negative outcome in restored ecosystems where environmental cues are not always present as they were under historical conditions. Outmigration cues of native and non‐native fishes from a restored floodplain were evaluated along the Cosumnes River, CA, USA. This river is the only remaining major unregulated river in the Sierra Nevada that flows into the Central Valley. It remains unclear how native and non‐native fishes utilize spatiotemporally heterogeneous habitats in the Cosumnes River; however, a better understanding of these dynamics could yield insight into how degraded river ecosystems in the region could be rehabilitated to benefit native fishes. In 2018 and 2019, daily fyke net surveys of fish assemblages were conducted within the floodplain and in outmigration corridors, along with the collection of environmental data to identify possible cues. Bayesian modeling showed that temperature increases, along with the rate of floodplain drawdown and the average flow over a rolling seven‐day period, were important triggers for cuing native fishes to exit the floodplain. We conclude that the numerous benefits to the aquatic food web and growth of native fishes justify the risk of stranding that floodplain restoration poses, particularly when supplemented with outmigration cues.

In hot water? Assessing the link between fundamental thermal physiology and predation of juvenile Chinook salmon

AbstractIn light of ongoing environmental change, understanding the complex impact of interacting stressors on species, communities, and ecosystems is an important challenge. Many studies to date examine the effects of potential stressors on a single species of concern. Yet these effects often resonate throughout a community and may produce changes in ecosystem dynamics that are equally critical to species resilience. The aim of this study was to develop a mechanistic understanding of how a rapidly changing stressor, water temperature, will alter trophic interactions among ectothermic fish species. In our region, California's Sacramento–San Joaquin River Delta system, it has been speculated that the decreased survivorship of juvenile Chinook salmon (Oncorhynchus tshawytscha) in warming waters may be caused partly by increased predation. Temperature influences metabolic rate functions and the amount of energy available for fitness‐relevant parameters (i.e., swim performance and escape response). Consequently, we hypothesized that these patterns of predation emerge due to a physiological advantage of predators over prey at warmer temperatures. To explore this, our first objective was to determine the fundamental thermal physiology of juvenile Chinook salmon and their potential predators in the Delta. Three physiological performance traits were measured for each species across a spectrum of temperatures: aerobic scope, burst speed, and the ability to burst repeatedly. For our second objective, we assessed whether the effect of temperature on these performance traits predicted the outcome of predation trials conducted across the same temperature spectrum. We found that temperature effects were species or population specific. Additionally, absolute burst swimming ability and the relative burst performance between predator and prey were stronger indicators of trophic dynamics than aerobic scope. Our analyses also confirmed that a major predator in the Delta, specifically largemouth bass (Micropterus salmoides), is more thermally adapted to higher temperatures and will likely consume salmon with an increasing frequency as waters warm. Thus, we show that an improved understanding of how fundamental thermal physiology impacts predation can provide ecosystem managers with better tools to predictively model predation upon juvenile salmon based on prevailing and future water temperatures.

Territories within groups: the dynamic competition of drift-feeding juvenile Chinook salmon in 3-dimensional space

Salmonid populations are often regulated by territorial competition among juveniles for food and space. In the canonical view, salmonid territories are spaced horizontally across the river bottom in a two-dimensional mosaic. However, some juveniles instead feed in tight, three-dimensional (3-D) social groups. To investigate whether territoriality is possible within such groups, we applied a new concept—the momentary home range—to quantify the size, exclusivity, and temporal dynamics of 3-D space use by juvenile Chinook salmon ( Oncorhynchus tshawytscha) in the Chena River, AK. Individual strategies spanned a broad continuum of exclusivity and stationarity. However, some of the largest, most dominant fish in each group aggressively defended stationary, exclusive feeding spaces and thus were unambiguously territorial. Transient floaters entered and left the group quickly. A majority of fish were not aggressive but nevertheless occupied exclusive, stationary spaces that probably function as territories with regard to resource distribution and population regulation. The presence of territoriality within social groups, in a 3-D configuration, expands the known domain of this important behavior.

Interpopulation variation in thermal physiology among seasonal runs of Chinook salmon

Conservation of species facing environmental change requires an understanding of interpopulation physiological variation. However, physiological data are often scarce and therefore pooled across populations and species, erasing potentially important variability between populations. Interpopulation variation in thermal physiology has been observed within the Salmonidae family, although it has not been associated with seasonally distinct migratory phenotypes (i.e., seasonal runs). To resolve whether thermal physiology is associated with life-history strategy, we acclimated four Sacramento River juvenile Chinook salmon ( Oncorhynchus tshawytscha) populations (Coleman fall-run, Feather River fall-run, Feather River spring-run, and Sacramento River winter-run) exhibiting different seasonal migratory phenotypes (fall-, spring-, and winter-run), at 11, 16, and 20 °C and assessed variation in growth rate, critical thermal maxima, and temperature-dependent metabolic traits. We identified population differences in the physiological parameters measured and found compelling evidence that the critically endangered and endemic Sacramento River winter-run Chinook population exhibits thermal physiology associated with its early-migration life-history strategy. Acclimation to warm temperatures limited the growth and metabolic capacity of winter-run Chinook salmon, highlighting the risk of future environmental warming to this endemic population.

Comparative Susceptibilities of Selected California Chinook Salmon and Steelhead Populations to Isolates of L Genogroup Infectious Hematopoietic Necrosis Virus (IHNV).

Simple SummaryChinook salmon in California conservation hatcheries have suffered disease outbreaks and significant mortality due to the fish virus infectious hematopoietic necrosis virus (IHNV) since at least the 1940s. Although steelhead trout in California are also occasionally infected with the same virus they do not typically experience disease. In this study the susceptibility of California Chinook salmon and steelhead trout was defined in controlled experiments that exposed groups of juvenile fish to California IHNV strains (L genogroup). The results confirmed high mortality among Chinook salmon but very low mortality of steelhead trout, despite identical virus exposures. Tests of varying conditions found increasing mortality of Chinook salmon with increasing virus dose, but reduced mortality at higher temperatures or with increased age or size of fish. Among Chinook salmon that survived virus exposure the persistence of virus was detected in one fish 8 months after virus exposure. These findings demonstrate that L genogroup IHNV in California has host specificity for Chinook salmon, and provide an understanding of factors that contribute to disease epidemics in California hatchery Chinook salmon.Salmonid species demonstrate varied susceptibility to the viral pathogen infectious hematopoietic necrosis virus (IHNV). In California conservation hatcheries, juvenile Chinook salmon (Oncorhynchus tshawytscha) have experienced disease outbreaks due to L genogroup IHNV since the 1940s, while indigenous steelhead (anadromous O. mykiss) appear relatively resistant. To characterize factors contributing to the losses of California salmonid fish due to IHNV, three populations of Chinook salmon and two populations of steelhead native to California watersheds were compared in controlled waterborne challenges with California L genogroup IHNV isolates at viral doses of 104–106 pfu mL−1. Chinook salmon fry were moderately to highly susceptible (CPM = 47–87%) when exposed to subgroup LI and LII IHNV. Susceptibility to mortality decreased with increasing age and also with a higher temperature. Mortality for steelhead fry exposed to two IHNV isolates was low (CPM = 1.3–33%). There was little intraspecies variation in susceptibility among populations of Chinook salmon and no differences in virulence between viruses strains. Viral persistence was demonstrated by the isolation of low levels of infectious IHNV from the skin of two juvenile Chinook salmon at 215 d post exposure. The persistence of the virus among Chinook salmon used for stocking into Lake Oroville may be an explanation for the severe epidemics of IHN at the Feather River hatchery in 1998–2002.

Stress history affects heat tolerance in an aquatic ectotherm (Chinook salmon, Oncorhynchus tshawytscha)

The stress history of an ectotherm may be a pivotal predictor of how they cope with rapid spikes in environmental temperature. An understanding of how stressors in habitats and commercial operations affect ectotherm heat tolerance is urgently required so that management actions can be informed by thermal physiology. We hypothesised that brief exposure to mild stress would heighten tolerance to subsequent heat stress, indicative of a cross-tolerance interaction, whereas exposure to severe stress would reduce heat tolerance, reflecting a cross-susceptibility interaction. To test this hypothesis, we assessed how three acute stressors (salinity shock [10 or 33 ppt for 2 h]), air exposure (1 or 5 min) and crowding [95.6 kg m−3 for 2 h]), commonly experienced by fish, affected the heat tolerance (measured as critical thermal maximum, CTMAX) in juvenile Chinook salmon (Oncorhynchus tshawytscha). Fish were exposed to one of the three stressors and left for 24 h of recovery prior to measuring CTMAX. Heat tolerance was improved by ∼0.6 °C in fish exposed to salinity shock (10 ppt) and air exposure (5 min) compared to unstressed controls, demonstrating cross-tolerance. However, the development of cross-tolerance was non-linear with stressor severity, and crowding stress had no effect on CTMAX. Together these results show that some forms of stress can heighten acute heat tolerance in ectotherms, but the development of cross-tolerance is highly specific to both stressor type and stressor severity.