Temperature can alter contaminant toxicity in aquatic ectotherms, yet thermal context is rarely incorporated explicitly in pesticide risk assessments. Early life stages of cold-water fishes are particularly sensitive to such interactions, as temperature governs metabolic rate, developmental timing, and physiological tolerance to chemical stress. Here, we examined how bifenthrin interacts with water temperature to exert sublethal effects on larval Green (Acipenser medirostris) and White Sturgeon (Acipenser transmontanus). Larvae (3 day-post-hatch) were exposed for 96 h to 0, 10, 100, or 500 ng/L bifenthrin (nominal concentrations) at 12, 15, or 18°C, followed by a 28-day recovery period; exposure concentrations were analytically verified and presented along with nominal treatment categories. Acute mortality was low (<1% on average), whereas deformity and post-exposure survival showed endpoint- and species-specific temperature dependence. Green Sturgeon showed reduced survival and higher deformity at cooler temperatures, with mean deformity reaching 13.10% ± 2.48% across treatments, whereas White Sturgeon exhibited reduced survival at 18°C but low deformity overall (0.29% ± 0.16%). Temperature also modulated bifenthrin effects on post-exposure survival and growth in a species-specific manner, including enhanced survival in Green Sturgeon at the higher nominal exposure treatments under 12 and 15°C, but weaker and inconsistent responses in White Sturgeon. Elevated temperatures promoted growth in both species, although bifenthrin effects varied with temperature and species. High bifenthrin exposure (nominally 500 ng/L) was associated with behavioral changes in both species, and locomotor reduction was most evident in White Sturgeon, where total distance moved decreased by 123 ± 32 cm relative to controls across temperatures. Acetylcholinesterase activity increased in several warmer and higher-exposure contexts, but this response varied by species, temperature, and exposure concentration. Overall, these results indicate that thermal context can modify bifenthrin responses in early life-stage sturgeon, supporting the incorporation of temperature as a complementary factor in ecological risk assessment under current and future climate conditions.

Aquatic ectotherms are hypothesized to be vulnerable to warming and deoxygenation associated with environmental change because temperature and oxygen (O 2 ) supply can restrict aerobic scope (AS) in captivity. However, evidence of a direct association between AS and fitness in the wild is lacking, inspiring debate about the circumstances under which AS is the primary driver of population fluctuations. Using respirometry data, telemetry studies, long-term population monitoring, and in situ predator-prey experiments, we related AS to two Chinook salmon ( Oncorhynchus tshawytscha ) population bottlenecks in the wild, juvenile rearing and migration. We found that AS, which we quantified using the metabolic index (ɸ), was associated with success probability for these bottlenecks only under a relatively narrow window of viable environmental conditions, depending on intraspecific metabolic trait diversity and hydrologic conditions. Opportunities for potentially high-impact temperature- and O 2 -specific conservation and management actions using existing hydraulic engineering infrastructure could therefore exist when AS is between critical (ɸ crit ) and stable (ɸ stable ) values. Outside of this ecological threshold, changes in AS did not yield appreciable fitness benefits because successful rearing and migration were either exceptionally improbable (i.e., AS<ɸ crit ), or seemingly independent of AS (i.e., AS>ɸ stable ). In addition, AS impairments likely increased susceptibility to predation, and this may have been involved in the putative association between AS and fitness in the wild.

Delta smelt Hypomesus transpacificus is a Critically Endangered fish native to the San Francisco Estuary. Respirometry methods quantify O2 consumption (MO2) to estimate metabolic activity, providing insights on organismal energetic budgets critical for devising conservation strategies. Due to its widespread adoption and utility, respirometry protocols have been established to foster quality control. However, delta smelt respirometry attempts over the past 12 yr have been largely unsuccessful, and high mortality rates during tests were only circumvented with very short measurement duration. This prevented the measurement of routine metabolic rate (RMR) and critical O2 tension (Pcrit) and obstructed delta smelt physiological research and conservation. Here, we detailed the anecdotal, behavioral, and empirical evidence that culminated in successful delta smelt RMR and Pcrit measurements at 10, 12, 15, 17 and 19 degrees C. We discovered the delta smelt is physostomous and requires frequent air bladder refills to maintain buoyancy and found the inclusion of an air pocket within their respirometry chamber greatly improved measurements. Daytime respirometry measurements yielded greater survival compared to overnight measurements; time of day did not cause significant RMR differences. Moreover, fasting did not significantly influence MO2. Delta smelt RMR values at 10 and 12 degrees C were significantly lower than fish acclimated to 15, 17, and 19 degrees C. Delta smelt Pcrit showed a similar pattern, but only the 19 degrees C treatment showed significantly higher Pcrit level. Altogether, this methodology allows for acquisition of critical knowledge needed for delta smelt conservation research, reducing conservation aquaculture operating costs, and improving survival during transport and field supplementation.

The efficacy of population supplementation (i.e., stocking) may be limited by physiological stress incurred upon transported and released fishes. In an experimental supplementation program for the critically endangered delta smelt (Hypomesus transpacificus), acclimatization enclosures have been developed to facilitate stress recovery. However, the magnitude and duration of the stress response and the optimal time of release from enclosures remain unknown. To address these knowledge gaps, we quantified whole-body cortisol, glucose, lactate, and gill ionocyte morphology at 10 time points across 72 h following transport to enclosures at high and low densities. Cortisol and glucose peaked within 1 h following transport into enclosures then declined significantly by 27 h. Lactate was moderately affected, and transport density had no effect on cortisol, glucose, or lactate. Gill ionocyte surface area significantly increased in response to shifts in water chemistry throughout transport, and we observed three distinct types of microvilli on the surface of gill ionocytes, including two never previously reported. This work highlights the importance of ensuring stress recovery during the supplementation of delta smelt and other species.

The presence of chemical contaminants in freshwater systems poses a threat to many aquatic organisms, and understanding the extent and nature of this threat can facilitate conservation management actions. Sturgeon are considered threatened worldwide and they differ in many important ways from other fishes. Two sturgeon species, green sturgeon (Acipenser medirostris) and white sturgeon (A. transmontanus), are found in California and utilize anthropogenically impacted freshwater habitats of the Central Valley. This study evaluated the behavioural effects in endogenously feeding larvae (3–7 days post hatch) of both sturgeon species following an acute exposure (96 hours) to the pyrethroid pesticide bifenthrin at aqueous concentrations ranging from 10 to 2000 ng/l, with selected levels based on previous environmental monitoring. Sturgeon had high survival at all concentrations tested (~95%), yet at higher concentrations (>1000 ng/l) they displayed altered behavioural patterns, including reduced activity, increased meander of the movement path and reduced thigmotaxis. While these higher concentrations of bifenthrin have been observed within water samples from the sturgeon habitats of California, they appear uncommon. The present study suggests that sturgeon larvae are not highly sensitive to acute aqueous exposure under environmentally relevant concentrations of bifenthrin (1–10 ng/l), yet these aqueous concentrations do have behavioural effects that may be of concern for the conservation of these declining species. Additionally, impacts to these species may also occur through exposure to sediment-bound bifenthrin or dietary bioaccumulation, and more work needs to be done to understand the implications of these exposure routes.

Sturgeon are threatened by anthropogenic changes to river systems, including entrainment or impingement at water diversions (i.e. the unwanted passage of fish through a water intake or physical contact with a barrier screen, likely caused by high intake velocities). Though there are no universally accepted protocols to determine water diversion risk, previous studies on sturgeon suggest that laboratory evaluations of swimming performance are an effective way to describe susceptibility to entrainment or impingement. The swimming performance of juvenile Green Sturgeon (~5 cm fork length), Acipenser medirostris, was quantified for fish acclimated to 13 and 18°C for 2 weeks using fixed water velocity endurance tests. Water velocities ranged from 25 to 55 cm s-1, and time-to-fatigue was measured at 5 cm s-1 increments. Green Sturgeon were quicker to exhaust at the lower acclimation temperature (13°C) compared to fish acclimated to 18°C, for example at 40 cm s-1 13°C acclimated fish impinged ~7.7 times faster than 18°C acclimated fish and ~41.3 times quicker at water velocities of 45 cm s-1. Whole-body cortisol grouped by time-to-fatigue (i.e. sustained swimming: time-to-fatigue >200 min, prolonged swimming: time-to-fatigue between 5 and 200 min, rapid swimming: time-to-fatigue <5 min, and non-swimming: control fish) was highest following the swimming experiment for fish utilizing prolonged swimming strategies regardless of temperature exposure. Furthermore, whole body lactate was elevated in fish utilizing prolonged and rapid swimming strategies compared to sustained and control non-swimming fish. Taken together, when swimming to exhaustion, these results suggest that Green Sturgeon were upregulating stress markers and relying on anaerobic metabolism, although both the above trends were driven by 18°C acclimated fish. The time-to-fatigue data suggest that the risk of entrainment was reduced to zero at water speeds ≤ 29.4 cm s-1 for 18°C and ≤ 22.6 cm s-1 for 13°C acclimated fish.

As fish populations face compounding pressures under climate change, highly modified rivers are receiving increasing research and conservation attention as important sites for restoration. Across the North Pacific Ocean, Chinook salmon (Oncorhynchus tshawytscha) have experienced unprecedented declines and extirpations because of habitat loss and fragmentation and climate variability. Here, we studied a rare example of a novel salmon population developing in Putah Creek, California, a dam-controlled stream flowing through an area of intensive agriculture where salmon were unlikely to occur prior to the rehabilitation of a more natural flow regime. We used otoliths from adult Chinook salmon carcasses recovered from Putah Creek to determine river or hatchery of origin for five spawning year classes. Our results provide evidence of successful salmon reproduction, outmigration survival, and natal homing of individuals back to Putah Creek in recent years. Although hatchery-origin fish that strayed to Putah Creek to spawn dominated returns each year, this study documents life-cycle completion required for the potential development of a self-sustaining salmon population. This study demonstrates that targeted restoration and flow rehabilitation efforts can generate viable new salmon spawning habitat in dam-controlled river systems, which could help mitigate habitat lost to dams and developments. Further, successful anadromous life-cycle completion can occur in new spawning habitat within the first several generations, which will help inform efforts to restore or reintroduce salmon in other altered river systems.

Sprint swimming performance in fishes is relatively understudied despite its critical role in predation attempts, prey evasion, spawning events, and overcoming hydraulic challenges. Sprint swimming is characterized by fast acceleration, over a short distance and of limited duration. The bulk of sprint performance research uses analysis of high-speed recordings of fish behavior. While behavioral video analysis has improved, it is still expensive in both processing time and computational resources, limiting the ability to develop reaction norms for sprint performance which necessitate large sample sizes. Here we present a laser-gated sprint performance chamber (SPC) that improves upon past designs by introducing an adjustable number of lasers (≤ 25) that facilitates greater resolution on sprint performance. Use of customized arrangements can facilitate measurement of novel performance metrics of interest to a range of key questions (e.g., fatigue rate, residual anaerobic capacity, and sprint stamina). Using this chamber we quantified the sprint velocity, residual anaerobic capacity, sprint stamina, and fatigue rate of rainbow trout (Oncorhynchus mykiss), a widely distributed and studied species. We directly compared the results measured by our device to high-speed camera data collected simultaneously and found the velocity estimates from the sprint chamber to be highly accurate (R2 = 0.97). We also compared the sprint performance of individual rainbow trout with their individual UCRIT, a commonly measured metric of aerobic swimming performance. We found little correlation between the two traits, indicating that fish capable of rapid sprint swimming are not necessarily fast sustained swimmers. Finally, we defined and quantified three novel traits of sprint swimming performance: relative anaerobic scope, sprint stamina (the number of sprint events that can be elicited prior to performance decline), and fatigue rate (the rate of decline associated with repeated sprinting). The SPC is an adjustable platform for quantifying understudied elements of fish swimming physiology, improving design of fish passage infrastructure, and facilitating discoveries in how sprint performance changes with environmental conditions.

Coastal estuaries globally, including the San Francisco Estuary (SFE), are experiencing significant degradation, often resulting in fisheries collapses. The SFE has undergone profound modifications due to population growth, industrialization, urbanization and increasing water exports for human use. These changes have significantly altered the aquatic ecosystem, favouring invasive species and becoming less hospitable to native species such as the longfin smelt (Spirinchus thaleichthys). With longfin smelt abundance declining to <1% of historical numbers, there is a pressing need for laboratory-based experiments aimed at investigating the effects of varying environmental conditions on their stress response and physiology. This study explored the impact of temperature (11 and 14 degrees C) and turbidity maintained with algae (1, 4 and 11 nephelometric turbidity units (NTU)) on the physiological condition of juvenile longfin smelt. Fish were sampled after 2 and 4 weeks in experimental conditions and analysed for whole-body cortisol, glucose, lactate and protein. Condition factor was calculated using length and weight measurements. Critical thermal maximum trials were conducted to assess how prior rearing conditions affected upper thermal tolerance. Cortisol levels were significantly higher in fish held in low-turbidity conditions, whilst glucose levels were significantly greater at lower temperatures and higher turbidities. Protein-to-mass ratios were significantly greater in higher turbidity conditions, with a significant interaction between temperature and turbidity further influencing these ratios. Moreover, 14 degrees C led to diminished condition factors but increased upper thermal tolerances (26.3 +/- 0.05 vs 24.6 +/- 0.18) compared to longfin smelt at 11 degrees C, highlighting a potential trade-off between the induction of defense mechanisms and subsequent reductions in energy and growth. Data suggest that cooler temperatures (11 degrees C) and elevated turbidities (11 NTU) can benefit juvenile longfin smelt by reducing stress and enhancing growth and energy. These findings hold significant implications for informing and optimizing future endeavours in the culturing and conservation of this species. Lay Summary Juvenile longfin smelt displayed increased stress at lower turbidity levels and reduced energy at both elevated temperatures andlower turbidity. Elevated temperatures increased upper thermal tolerance and reduced condition factor. Cooler temperatures and higher turbidities improved physiological condition and should be considered for future rearing and supplementation efforts.

Respirometry quantifies O2 consumption to estimate metabolic activity across stressor(s), and the resulting data is central for devising conservation strategies for fishes. Due to its rapid adoption and widespread utility, protocols have been established to foster quality control and increase consistency. However, the application of established respirometry protocols on the Delta Smelt (Hypomesus transpacificus) have been largely unsuccessful, with reports over the past 12 years documenting high mortality rates that can only be circumvented with very short measurement duration (~4 h). This prevents the measurement of standard metabolic rate (SMR) and critical O2 tension (Pcrit), and ultimately obstructed physiological research, management actions and conservation tools development. In this study, we detailed the anecdotal, behavioral and empirical evidence that culminated in successful measurements of Delta Smelt SMR and Pcrit at 10, 12, 15, 17 and 19 degrees C. We discovered the Delta Smelt is physotomous and requires frequent refilling of its air bladder to maintain buoyancy. Inspired by air-breathing fish respirometry, the inclusion of an air pocket within their chamber led to considerably more robust respirometry measurements compared to past attempts. The influence of fasting and time of day on O2 consumption were also examined. Altogether, this methodology paves the way for acquiring critical knowledge needed to inform Delta Smelt research and conservation (e.g. the development of metabolic index). The lessons learned may also have implications for other aspects of Delta Smelt conservation such as decreasing conservation aquaculture operating costs and improving survival during fish transport and supplementation. ### Competing Interest Statement The authors have declared no competing interest.

Lay Summary: We reared green sturgeon at two temperatures and provided two rations. We then assessed the metabolic performance of fish from these four treatments across a range of temperatures (11 to 31#x00B0;C). The impacts of temperature and ration create a conservation conflict between green sturgeon and the endangered winter-run Chinook salmon. Green sturgeon (Acipenser medirostris) are an anadromous threatened species of sturgeon found along the Pacific coast of North America. The southern distinct population segment only spawns in the Sacramento River and is exposed to water temperatures kept artificially cold for the conservation and management of winter-run Chinook salmon (Oncorhynchus tshawytscha). Past research has demonstrated costs of cold-water rearing including reduced growth rates, condition and survivorship of juvenile green sturgeon. Our research investigates how the stressors of water temperature and food limitation influence the metabolic performance of green sturgeon. We reared green sturgeon at two acclimation temperatures (13 and 19 degrees C) and two ration amounts (100% and 40% of optimal feed). We then measured the routine and maximum metabolic rates (RMR and MMR, respectively) of sturgeon acclimated to these rearing conditions across a range of acute temperature exposures (11 to 31 degrees C). Among both temperature acclimation treatments (13 or 19 degrees C), we found that feed restriction reduced RMR across a range of acute temperatures. The influence of feed restriction on RMR and MMR interacted with acclimation temperature. Fish reared at 13 degrees C preserved their MMR and aerobic scope (AS) despite feed restriction, while fish fed reduced rations and acclimated to 19 degrees C showed reduced MMR and AS capacity primarily at temperatures below 16 degrees C. The sympatry of threatened green sturgeon with endangered salmonids produces a conservation conflict, such that cold-water releases for the conservation of at-risk salmonids may constrain the metabolic performance of juvenile green sturgeon. Understanding the impacts of environmental conditions (e.g. temperature, dissolved oxygen) on ecological interactions of green sturgeon will be necessary to determine the influence of salmonid-focused management.

Southern Distinct Population Segment (sDPS) green sturgeon spawn solely in one stretch of the Sacramento River in California. Management of this spawning habitat is complicated by cold water temperature requirements for the conservation of winter-run Chinook salmon. This study assessed whether low incubation and rearing temperatures resulted in carryover effects across embryo to early juvenile life stages on scaling relationships in growth and metabolism in northern DPS green sturgeon used as a proxy for sDPS green sturgeon. Fish were incubated and reared at 11 °C and 15 °C, with a subset experiencing a reciprocal temperature transfer post-hatch, to assess recovery from cold incubation or to simulate a cold-water dam release which would chill rearing larvae. Growth and metabolic rate of embryos and larvae were measured to 118 days post hatch. Reciprocal temperature transfers revealed a greater effect of low temperature exposure during larval rearing rather than during egg incubation. While 11 °C eggs hatched at a smaller length, log-transformed length–weight relationships showed that these differences in developmental trajectory dissipated as individuals achieved juvenile morphology. However, considerable size-at-age differences persisted between rearing temperatures, with 15 °C fish requiring 60 days post-hatch to achieve 1 g in mass, whereas 11 °C fish required 120 days to achieve 1 g, resulting in fish of the same age at the completion of the experiment with a ca. 37-fold difference in weight. Consequently, our study suggests that cold rearing temperatures have far more consequential downstream effects than cold embryo incubation temperatures. Growth delays from 11 °C rearing temperatures would greatly increase the period of vulnerability to predation in larval green sturgeon. The scaling relationship between log-transformed whole-body metabolism and mass exhibited a steeper slope and thus an increased oxygen requirement with size in 11 °C reared fish, potentially indicating an energetically unsustainable situation. Understanding how cold temperatures affect green sturgeon ontogeny is necessary to refine our larval recruitment estimations for this threatened species.

The Longfin Smelt (LFS, Spirinchus thaleichthys) population within the San Francisco Estuary, California, has experienced a substantial reduction, diminishing to <1% of their historical abundance. This decline has culminated in their classification as a threatened species under the purview of the California Endangered Species Act. Understanding their physiology and stress response in relation to varying environmental conditions, such as temperature and turbidity, is crucial for LFS culturing, management, and conservation. In this study, we assessed juvenile LFS (age range during exposure: 181 to 228 days post hatch, dph) performance as measured by growth and gene expression following four weeks at two temperatures (11 degrees C and 14 degrees C) and three turbidity levels (1, 4, and 11 nephelometric turbidity units (NTU)). At the end of the 4-week exposure period, we conducted assessments encompassing fork length, wet weight, condition factor, and examined alterations in the transcription of 12 genes. The selection of these genes aimed at determining responses associated with osmoregulation, growth, metabolism, and general stress, as all of which are potentially influenced by temperature and/or turbidity. Weight and condition factor was significantly higher at lower temperature, whereas turbidity had no effect on growth, condition factor, and transcriptomic stress-response. Instead, the lower expression levels of Catalase, Citrate Synthase and Growth Factor Receptor Bound Protein 10 at 14 degrees C were indicative of metabolic and growth-related changes governed by temperature. This suggests that rearing of LFS at 11 degrees C and low turbidity (<11 NTU) is suitable for the juvenile stage, whereas growth as well as metabolic capacity is limited at slightly warmer temperatures.

There is growing interest to integrate conservation initiatives into agricultural practices using a reconciliation ecology framework. In California's Central Valley, one approach to improving crucial nursery habitat for threatened and endangered fish species is by re-creating floodplain habitats through the inundation of agricultural fields during the non-growing season. We conducted a series of field experiments in agricultural floodplains (winter-flooded rice fields that historically were natural floodplains) to examine whether different field preparation methods enhanced growth and survival of floodplain-dependent fish species. Approximately 8000 juvenile fall-run Chinook Salmon (Oncorhynchus tshawytscha) were reared for at least 28 days on eight, 0.2 hectares (ha) experimental fields. Each experimental field represented one of four treatments (two replicates per treatment): addition of large wood, addition of in-field canals, addition of both large wood and in-field canals, and control. Controls fields received no additional field preparations beyond standard post-growing practices: that is chopping rice into stubble, baling, and removing excess rice straw, and discing the field with a single pass by plowing rice straw into the ground to promote decomposition. We found no significant difference in water temperature, fish growth or fish survival among habitat treatments. Across treatments, survival ranged from 50.1% to 78.3% and averaged 65.75% (+/- 7.89% SE). These findings suggest agricultural floodplains require no additional modifications to promote fish growth and survival. Results illustrate the benefits of integrating working landscapes with conservation initiatives, as this approach creates more accessible and beneficial habitat for native fish species.

Freshwater fishes are increasingly facing extinction. Some species will require conservation intervention such as habitat restoration and/or population supplementation through mass-release of hatchery fish. In California, USA, a number of conservation strategies are underway to increase abundance of the endangered Delta Smelt (Hypomesus transpacificus); however, it is unclear how different estuarine conditions influence hatchery fish. The goal of this study was to evaluate a year of Delta Smelt field deployments to inform species conservation strategies of suitable conditions for smelt physiology. Hatchery-reared Delta Smelt was deployed in experimental cages (seven deployments) throughout the Estuary in the winter, summer and fall of 2019. Effects of season and location of cage deployments on fish health (condition factor and histological condition of liver and gill), growth, thermal tolerance and survival were evaluated. The results indicate both seasonal and location differences, with high survival in the winter (100%) and fall (88-92%) compared to lower survival in summer (67%). In the summer, one of the study sites had no surviving fish following high temperature exposure, which peaked similar to 26 degrees C. After 29 days in the cages, surviving Delta Smelt in summer and fall showed signs of nutritional stress that may be related to biofouling of the cages limiting passive food inputs, restriction of natural foraging behaviour by containment in the cages, and water temperatures that were too high given the chronically low pelagic productivity in the Estuary overall. Field measurements of upper thermal tolerance (CTmax) following caging exposures suggest that laboratory measures of CTmax may overestimate the realized tolerance in a more stochastic field environment. This study demonstrates the utility of using cages as an experimental tool to better understand aspects of Delta Smelt physiological responses to environmental changes across estuarine habitats in a more natural-field setting, while also highlighting potential limitations of using cages. Field deployments of caged Delta Smelt demonstrated that survival in the San Francisco Estuary was generally high but physiological responses varied by season and region. Warm temperatures make summer deployments particularly challenging, cages may limit behaviour and nutrition, and we uncovered a difference of thermal tolerance between laboratory and field measures.

Conservation of endangered fishes commonly includes captive breeding, applied research, and management. Since 1996, a captive breeding program has existed for the federally threatened and California endangered Delta Smelt Hypomesus transpacificus, an osmerid fish endemic to the upper San Francisco Estuary. Although this program serves as a captive refuge population, with experimental releases being initiated to supplement the wild population, it was uncertain how individuals would survive, feed, and maintain condition outside hatchery conditions. We evaluated this and the effects of three enclosure designs (41% open, 63% open, and 63% open with partial outer mesh wrap) on growth, survival, and feeding efficacy of cultured Delta Smelt at two locations (Sacramento River near Rio Vista, CA and in Sacramento River Deepwater Ship Channel) in the wild. Enclosures exposed fish to semi-natural conditions (ambient environmental fluctuations and wild food resources) but prevented escape and predation. After four weeks, survival was high for all enclosure types (94-100%) at both locations. The change in condition and weight was variable between sites, increasing at the first location but decreasing at the second location. Gut content analysis showed that fish consumed wild zooplankton that came into the enclosures. Cumulatively, results show that captive-reared Delta Smelt can survive and forage successfully when housed in enclosures under semi-natural conditions in the wild. When comparing enclosure types, we observed no significant difference in fish weight changes (p = 0.58-0.81 across sites). The success of housing captive-reared Delta Smelt in enclosures in the wild provides preliminary evidence that these fish may be suitable to supplement the wild population in the San Francisco Estuary. Furthermore, these enclosures are a new tool to test the efficacy of habitat management actions or to acclimate fish to wild conditions as a soft release strategy for recently initiated supplementation efforts.

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.

Stream restorations are increasingly critical for managing and recovering freshwater biodiversity in human-dominated landscapes. However, few studies have quantified how rehabilitative actions promulgate through aquatic communities over decades. Here, a long-term dataset is analyzed for fish assemblage change, incorporating data pre- and post-restoration periods, and testing the extent to which native assemblage stability has increased over time. In the late 1950s, a large capacity dam was installed on Putah Creek (Solano County, CA, USA), which altered the natural flow regime, channel structure, geomorphic processes, and overall ecological function. Notably, downstream flows were reduced (especially during summer months) resulting in an aquatic assemblage dominated by warm-water nonnative species, while endemic native species subsisted at low levels as subordinates. A court-mediated Accord was ratified in 2000, providing a more natural flow regime, specifically for native and anadromous fishes in the stream. The richness of nonnative species decreased at every site following the Accord, while the richness of native species increased or stayed constant. At the three most upstream sites, native species richness increased over time and ultimately exceeded nonnative richness. Native assemblage recovery was strongest upriver, closer to flow releases and habitat restoration activities, and decreased longitudinally downstream. Rank-abundance curves through time revealed that, while species evenness was low throughout the study, dominance shifted from nonnative to native species in the upstream sites coincident with rehabilitation efforts. Mean rank shifts decreased following flow rehabilitation; thus the assemblage became increasingly stable over time following flow rehabilitation. Putah Creek's rehabilitation may represent a model for others interested in improving endemic freshwater communities in degraded ecosystems.

Abstract The San Francisco Estuary (SFE) is one of the most degraded ecosystems in the United States, and organisms that inhabit it are exposed to a suite of environmental stressors. The delta smelt (Hypomesus transpacificus), a small semi-anadromous fish endemic to the SFE and considered an indicator species, is close to extinction in the wild. The goal of this study was to investigate how environmental alterations to the SFE, such as reductions in turbidities, higher temperatures and increased prevalence of invasive predators affect the physiology and stress response of juvenile delta smelt. Juvenile delta smelt were exposed to two temperatures (17 and 21°C) and two turbidities (1–2 and 10–11 NTU) for 2 weeks. After the first week of exposure, delta smelt were exposed to a largemouth bass (Micropterus salmoides) predator cue at the same time every day for 7 days. Fish were measured and sampled on the first (acute) and final (chronic) day of exposures to predator cues and later analyzed for whole-body cortisol, glucose, lactate, and protein. Length and mass measurements were used to calculate condition factor of fish in each treatment. Turbidity had the greatest effect on juvenile delta smelt and resulted in reduced cortisol, increased glucose and lactate, and greater condition factor. Elevated temperatures reduced available energy in delta smelt, indicated by lower glucose and total protein, whereas predator cue exposure had negligible effects on their stress response. This is the first study to show reduced cortisol in juvenile delta smelt held in turbid conditions and adds to the growing data that suggest this species performs best in moderate temperatures and turbidities. Multistressor experiments are necessary to understand the capacity of delta smelt to respond to the multivariate and dynamic changes in their natural environment, and results from this study should be considered for management-based conservation efforts.

Understanding interpopulation variation is important to predicting species responses to climate change. Recent research has revealed interpopulation variation among several species of Pacific salmonids; however, the environmental drivers of population differences remain elusive. We tested for local adaptation and countergradient variation by assessing interpopulation variation among six populations of fall-run Chinook Salmon from the western United States. Juvenile fish were reared at three temperatures (11, 16 and 20°C), and five physiological metrics were measured (routine and maximum metabolic rate, aerobic scope, growth rate and critical thermal maximum). We then tested associations between these physiological metrics and 15 environmental characteristics (e.g. rearing temperature, latitude, migration distance, etc.). Statistical associations between the five physiological metrics and 15 environmental characteristics supported our hypotheses of local adaptation. Notably, latitude was a poor predictor of population physiology. Instead, our results demonstrate that populations from warmer habitats exhibit higher thermal tolerance (i.e. critical thermal maxima), faster growth when warm acclimated and greater aerobic capacity at high temperatures. Additionally, populations with longer migrations exhibit higher metabolic capacity. However, overall metabolic capacity declined with warm acclimation, indicating that future climate change may reduce metabolic capacity, negatively affecting long-migrating populations. Linking physiological traits to environmental characteristics enables flexible, population-specific management of disparate populations in response to local conditions.