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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.

For a Chinook salmon restoration project in the lower Yuba River in California, we designed and deployed a wireless sensor network to monitor river temperatures at micro-habitat scales. The study required that temperatures be measured along a 3 km study reach, across the channel, and into off-channel areas. To capture diel and seasonal fluctuations, sensors were sampled quarter-hourly for the full duration of the six-month juvenile salmon winter residency. This sampling duration required that nodes minimize power-use. We adopted event-based software on MSP430 micro-controllers with 433 MHz radio and minimized the networking duty-cycle. To address link failures, we included network storage. As the network lacked real-time clocks, data were timestamped at the destination. This, coupled with the storage, yielded timestamp inaccuracies, which we re-aligned using a novel algorithm. We collected over six months of temperature data from 35 sensors across seven nodes. Of the packets collected, we identified 21% as being incorrectly timestamped and were able to re-align 41% of these incorrectly timestamped packets. We collected temperature data through major floods, and the network uploaded data until the flood destroyed the sensors. The network met an important need in ecological sampling with ultra-low power (multi-year battery life) and low-throughput.

Extirpation of the Central Valley spring-run Chinook salmon ( Oncorhynchus tshawytscha) evolutionary significant unit (ESU) from the San Joaquin River is emblematic of salmonid declines across the western seaboard of the United States. Habitat restoration and fish reintroduction efforts are ongoing, but recent telemetry studies have revealed low outmigration survival of juveniles to the ocean. Previous investigations have focused on modeling survival relative to river discharge and geographic regions, but have largely overlooked the effects of habitat variability. To evaluate the link between environmental conditions and survival of juvenile spring-run Chinook salmon, we combined high spatial resolution habitat mapping approaches with acoustic telemetry along a 150 km section of the San Joaquin River during the spring of 2019. While overall outmigration survival was low (5%), our habitat-based classification scheme described variation in survival of acoustic-tagged smolts better than other candidate models based on geography or distance. Two regional mortality sinks were evident along the longitudinal profile of the river, revealing poor survival in areas that shared warmer temperatures but that diverged in chlorophyll a, fluorescent dissolved organic matter (fDOM), turbidity, and dissolved oxygen levels. These findings demonstrate the value of integrating river habitat classification frameworks to improve our understanding of survival dynamics of imperiled fish populations.

Little is known about the distribution and ecology of juvenile green sturgeon throughout their range and specifically within California’s Central Valley and the San Francisco Bay Estuary. The objective of our study was to identify rearing movement patterns of juvenile green sturgeon and their interaction with Delta and Bay habitats. Acoustic biotelemetry was used to record juvenile movements and residence within different regions of this highly modified watershed. We document a diversity of movements exhibited by 31 acoustically tagged juvenile green sturgeon monitored over a 9-month period. The juvenile sturgeon either (1) remained in the Delta or moved (2) into the Carquinez Straits, (3) into San Pablo Bay, (4) into San Pablo Bay but returned to Delta, (5) through the estuary and likely left through the mouth of the bay, (6) or left the estuary only to return later. Spatiotemporal habitat quality varies within and between Delta and Bay habitats. Variation in water year and hydrologic connectivity are likely to influence habitat quality and quantity, and behaviors may be differentially optimized for fitness in response to environmental variation. Our results provide initial evidence supporting multiple behavioral movement patterns and a broad use of regional habitats for green sturgeon in the California Central Valley. This apparent behavioral diversity observed through movement patterns may ensure long-term success of all population segments over time in an extremely variable environment, ultimately supporting overall population stability (i.e., the portfolio effect). Enhanced conservation efforts may be needed to protect juvenile green sturgeon and the habitats they rely upon.

Stream restorations are increasingly critical for managing and recovering freshwater biodiversity in human-dominated landscapes. However, few studies have quantifiedhow 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 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 warmwater 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. Richness of nonnative species decreased at every site following the Accord, while 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.

Food availability is a key determinant of the nursery value of a given habitat for larval and juvenile fishes. Growth, survival, and recruitment success are often inter-correlated and influenced by prey availability and associated feeding success. This is likely true for the threatened population of Longfin Smelt (Spirinchus thaleichthys) in the San Francisco Estuary (SFE) which has collapsed in recent decades along with its preferred prey. In years with high precipitation and freshwater outflow, larval Longfin Smelt are found in shallow wetland habitats throughout the SFE, but variation in the availability of food and feeding success in these habitats remains unexplored. To examine spatial variation in the trophic value of different rearing habitats, we quantified variation in prey availability, feeding success, and prey selection for larval and juvenile Longfin Smelt captured in restored tidal marshes, sloughs, and open-water habitats in the northern and southern SFE. Prey abundance varied spatially, with densities approximately tenfold greater in southern sloughs and restored tidal ponds relative to northern and open-water habitats. Feeding success of larval Longfin Smelt was positively correlated with both fish length and prey density. Larval Longfin Smelt fed selectively on the copepod Eurytemora affinis, with larger individuals (> 25 mm total length) exhibiting an ontogenetic diet shift to larger mysid shrimps. Our results suggest that wetland habitats across the SFE vary greatly in their trophic value, with previously unexplored habitats exhibiting the highest densities of prey and the highest foraging success for larval Longfin Smelt.

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.

In estuaries, fluctuating environmental conditions exact strenuous physiological demands on the fishes that inhabit these oft-impacted areas, including the Critically Endangered delta smeltHypomesus transpacificusin California (USA). Using an archive of otoliths spanning 2011-2019, we examined how growth rates of wild subadult delta smelt vary ontogenetically, regionally, and in relation to variation in the physical environment during late-summer and fall in the upper San Francisco Estuary. Recent growth rates were quantified using otolith increment analysis and modeled as functions of both intrinsic (age) and extrinsic (temperature, salinity, clarity, and region) factors using a suite of generalized additive models. Age explained 60% of the variation in log10-transformed growth rates, which peaked at 50-80 d post hatch. Overall, age-adjusted growth rates declined at temperatures >20°C, increased with practical salinity values of 0-4, and exhibited interactive patterns with water clarity. Growth rates appeared highest in the West and Central Delta, and lowest in the North Delta, also corresponding with patterns in environmental conditions. Here, we provide new evidence for how vital rates of wild delta smelt vary spatially and in relation to abiotic environmental variation. Such otolith-based growth reconstructions often provide the first direct look at how the vital rates of wild fish respond to environmental variationin situ, and how future changes are likely to affect the dynamics of wild populations.

Consistent individual differences in behaviour, or personality, likely influence patterns of space use and resource use in wild animals. However, studies on personality-dependent space use in natural ecosystems remain rare due to the difficulty of obtaining paired data sets on spatial dynamics and repeated personality measures from marked animals. We used repeated standardized assays (open field, mirror image stimulation, flight initiation distance and behaviour in trap) to perform the first characterization of personality in a free-ranging population of golden-mantled ground squirrels, Callospermophilus lateralis. We then used multilevel modelling to determine whether personality influenced 95% home range size, 50% core area size, movement speed or use of a preferred resource (‘perches’, vision-enhancing prominences such as rocks, which enhance survival) in nature. Data collected over 3 years showed that individual squirrels consistently differed in activity, sociability, boldness and aggressiveness (adjusted repeatability 0.16–0.44) and that activity was correlated with sociability (posterior mean correlation [95% credible interval] = 0.65 [0.39, 0.87]). We did not find an effect of personality on home range size, but bolder individuals maintained larger core areas than shyer individuals. More active and bolder individuals moved faster under natural conditions compared to their less active and shyer conspecifics. Individuals that scored higher for all four personality traits had more perches in both their home ranges and core areas compared to individuals with lower personality scores. Our results are indicative of personality-dependent space use and resource use in this study system. We hope our study will inspire future research that links animal personality with spatial ecology to inform wildlife management in natural ecosystems.

Understanding intraspecific variation in habitat use, particularly of long-lived fishes across multiple life history stages, is core to improved conservation management. Here, we present results from a synthesis of acoustic telemetry data for sub-adult and adult white sturgeon (Acipenser transmontanus) from 2010 to 2017 in the San Francisco Estuary and Sacramento River ecosystems. We focused primarily on uncovering spatial patterns of inferred habitat occupancy across life stages, and on linking habitat use to extant anthropogenic threats. We found substantial differences in habitat use across individuals and over time that was related to fish size classes defined relative to the slot limit (102-152 cm) used to regulate recreational fishing. However, differences in habitat use were not explained by fish sex or water year flow conditions. We also estimated indices of overall exposure for two major threats: capture by anglers and habitat modification. Fish of harvestable size were detected less often than others in areas where many are caught. Future monitoring and management of white sturgeon might benefit from examining multiple phases of white sturgeon life history. For example, additional tracking studies could improve our understanding of juvenile habitat use, adult survival rates, patterns of anadromy, and cross-basin habitat utilization.

The combustion of fossil fuels is currently causing rapid rates of ocean warming and acidification worldwide. Projected changes in these parameters have been repeatedly observed to stress the physiological limits and plasticity of many marine species from the molecular to organismal levels. High latitude oceans are among the fastest changing ecosystems; therefore, polar species are projected to be some of the most vulnerable to climate change. Antarctic species are particularly sensitive to environmental change, having evolved for millions of years under stable ocean conditions. Otoliths, calcified structures found in a fish’s inner ear used to sense movement and direction, have been shown to be affected by both warming and CO2-acidified seawater in temperate and tropical fishes but there is no work to date on Antarctic fishes. In this study, juvenile emerald rockcod (Trematomus bernacchii) were exposed to projected seawater warming and CO2-acidification for the year 2100 over 28 days. Sagittal otoliths were analyzed for changes in area, perimeter, length, width and shape. We found ocean warming increased the growth rate of otoliths, while CO2-acidified seawater and the interaction of warming and acidification did not have an effect on otolith development. Elevated temperature also altered the shape of otoliths. If otolith development is altered under future warming scenarios, sensory functions such as hearing, orientation, and movement may potentially be impaired. Changes in these basic somatic abilities could have broad implications for the general capabilities and ecology of early life stages of Antarctic fishes.

Outmigrating, juvenile Chinook salmon Oncorhynchus tshawytscha, with access to floodplains (e.g., Yolo Bypass California, USA), grow faster than those restricted to the main channel of the Sacramento River. How these young salmon might use rooted, vegetative structure (e.g., to decrease energy expenditures) while holding positions in flowing water on floodplains and flooded riparian zones is unknown. We conducted daytime experiments in a large (24.4 m long) flume containing a planted area (9.76 m × 1.22 m) of sandbar willows, Salix interior. Flume water was maintained at 1.5 m depth and 16 °C over a 15–90 cm s−1 test velocity range. Fish were videoed using 19 cameras to determine positional behavior, including their depth, use of vegetation, and tail-beat (body-undulation) frequencies (TBFs). These TBFs were replicated with similarly-sized salmon in a calibrated, Brett-type swimming respirometer, where oxygen consumption rates were measured. Using these laboratory measurements, we estimated their swimming velocities and energetic costs associated with occupying sandbar willow habitats in the flume. As flume velocities increased and the leafy canopies of the willows were bent over from the flow, salmon occupied deeper water, among the thick stems of the willows, and maintained their positions. Even at the highest (90 cm s−1) nominal flume velocities, their estimated swimming velocities were only 35.6 cm s−1, within the bottom 15 cm of the water column. This resulted in unchanged energetic costs, compared with those estimated at lower nominal water velocities. The use of vegetated (e.g., with sandbar willow common to the riparian zone) floodplains, rather than non-vegetated ones, can potentially provide energy-saving, growth-promoting daytime habitat for migrating juvenile salmonids during river-flow periods that include floodplain inundation.

Pacific salmonids, cold-water fishes native to the northern hemisphere, span a massive geographic range (~ 33° latitude) and are exposed to a wide variety of environmental conditions regionally and temporally. California is home to the greatest concentration of at-risk anadromous salmonids and warming river temperatures pose both current and future threats to numerous populations. Thermal standards for management of California populations are currently based on guidelines for multiple salmonid species and from populations across the Pacific Coast. However, a growing body of literature suggests that salmonid populations exhibit population-specific thermal requirements. Furthermore, in California, salmonid populations regularly encounter temperatures that exceed current thermal standards based upon performance of outside populations. This review focuses on Chinook salmon (Oncorhynchus tshawytscha), providing evidence for interpopulation variation in thermal performance across life stages, and explores the drivers of variation. To describe the formation of interpopulation variation, we define fundamental and ecological thermal physiologies. Fundamental thermal physiology is the composite of intrinsic physiological traits and abiotic factors that define a species’ thermal window. Ecological and environmental interactions constrain this fundamental thermal physiology, yielding an ecological thermal physiology. Thermal physiology, viewed through this lens, provides researchers and managers avenues for salmonid research and conservation at the population scale. A more nuanced approach to west-coast salmonid conservation will be required to protect the most at-risk and vulnerable populations. Successful salmonid management must incorporate population-specific traits and present and future watershed conditions.

Capacity to cope with warming temperatures is a key determining factor of species' persistence under global climate change. Many successful invasive species have heightened thermal tolerance relative to their native counterparts, which may provide competitive advantages for habitat utilization and resource acquisition under warming scenarios, ultimately contributing to radically altered community composition. Enhanced transcriptional plasticity may be an important factor conferring superior abilities to cope with environmental stress, but the molecular mechanisms driving key differences of organismal traits in invasive versus native species are not well known. Although it is predicted that established invaders will evolve canalized phenotypes well‐adapted to new environments, it is not clear whether the same expectations are true for invaders of variable thermal environments or under climate warming scenarios where abilities to cope with fluctuating and increasing temperatures may provide fitness advantages. Here, we compare a highly successful invasive fish and a sympatric endangered native fish living in a dynamic estuarine environment that is projected to warm under climate change. We linked organismal physiological limits with global transcriptional responses at multiple common relative and absolute temperature thresholds and determined that heightened thermal tolerance of invasive Inland Silversides (Menidia beryllina) is associated with transcriptional changes that are greater both in the number of genes and the magnitude of response relative to native Delta Smelt (Hypomesus transpacificus). Modulated genes contributed to the enrichment of biological processes that differed between species and generally increased with temperature. These results are in concordance with the hypothesis that transcriptional plasticity may play a key role in determining population persistence, species interactions, and shaping community assemblages under climate change. Future studies encompassing a wider range of species and taxa are needed to determine whether this is a general pattern found between native and invasive species more broadly.

White Sturgeon (Acipenser transmontanus) are a long-lived, slow-growing, and late-reproducing anadromous fish common in estuaries and coastal habitats along the North American West Coast. These life history characteristics make populations vulnerable to human impacts and a challenge to study and manage. Previous studies in the San Francisco Estuary, California have provided insights into rearing habitats and migratory patterns but are limited in spatial and temporal scope. Fin ray geochemical analysis can provide a non-lethal approach to reconstruct migratory patterns and environmental conditions experienced throughout an individual fish’s lifespan. However, it is not known how soon post hatch age-0 White Sturgeon fin rays begin to calcify, reducing confidence in early life history temporal resolution using geochemical approaches. We used osteological (clear and stain) and geochemical techniques (laser-ablation-ICP-MS) to describe calcification initiation and completion, and element incorporation in the leading fin ray of known-age White Sturgeon reared at constant water temperature (18.6 °C) from 1 to 76 days post hatch (dph). We found that fin rays begin calcifying as early as ~20 dph (~27 mm total length) and are >95% calcified by ~72 dph (~70 mm total length). Consequently, the first ~20 dph are not likely to be recorded in the fin ray. Observed element (Li, Mg, Cu, Zn, Rb, Sr, Ba, Pb, U) incorporation patterns suggest that fin rays can provide a powerful tool to study White Sturgeon early movement and migratory patterns, habitat use, and environmental exposure.

Riverine ecosystems around the world have undergone extensive anthropogenic alterations, often to the detriment of native aquatic biodiversity. Migratory fishes are particularly vulnerable to habitat fragmentation and degradation. For example, Chinook Salmon populations in the Sacramento River, CA have been declining for nearly a century. Its Mediterranean climate, position in a landscape dominated by urban and agricultural land use, and the presence of an inland delta that serves as the hub of California’s vast water conveyance system, makes this a particularly perilous region for migrating juvenile salmon. This study used acoustic telemetry to investigate survival and routing of juvenile fall and spring-run Chinook Salmon through their riverine and estuarine migratory phases during the historic California drought of 2012–2016. Survival to the ocean in this river system is already lower relative to Chinook Salmon populations in North American. Survival to the Pacific Ocean ranged from 0.0–9.1% over the course of this three-year study. Use of four migratory routes through the Delta was variable, both within and between years. We documented extensive use and relatively high survival in Steamboat Slough (up to 41.5% routing through and up to 39.8% survival), part of the Sacramento River’s distributary network. Migration through this pathway precludes juvenile salmon from encountering junctions leading to low survival routes. Results from this modeling effort provide valuable information and highlight opportunities for directing management actions to benefit these ecologically and economically valuable runs of Chinook Salmon.

Juvenile fall-run Chinook salmon (

Juvenile fall-run Chinook salmon (Oncorhynchus tshawytscha) in the Sacramento–San Joaquin River Basin experience temporally and spatially heterogenous temperature regimes, between cool upper tributaries and the warm channelized Delta, during freshwater rearing and outmigration. Limited water resources necessitate human management of dam releases, allowing temperature modifications. The objective of this study was to examine the effect of temperature on specific dynamic action (SDA), or the metabolic cost associated with feeding and digestion, which is thought to represent a substantial portion of fish energy budgets. Measuring SDA with respect to absolute aerobic scope (AAS), estimated by the difference between maximum metabolic rate (MMR) and standard metabolic rate (SMR), provides a snapshot of its respective energy allocation. Fish were acclimated to 16°C, raised or lowered to each acute temperature (13°C, 16°C, 19°C, 22°C or 24°C), then fed a meal of commercial pellets weighing 2% of their wet mass. We detected a significant positive effect of temperature on SMR and MMR, but not on AAS. As expected, there was no significant effect of temperature on the total O2 cost of digestion, but unlike other studies, we did not see a significant difference in duration, peak metabolic rate standardized to SMR, time to peak, percent of meal energy utilized, nor the ratio of peak O2 consumption to SMR. Peak O2 consumption represented 10.4–14.5% of AAS leaving a large amount of aerobic capacity available for other activities, and meal energy utilized for digestion ranged from 5.7% to 7.2%, leaving substantial remaining energy to potentially assimilate for growth. Our juvenile fall-run Chinook salmon exhibited thermal stability in their SDA response, which may play a role in maintaining homeostasis of digestive capability in a highly heterogeneous thermal environment where rapid growth is important for successful competition with conspecifics and for avoiding predation.

Swimming behavior of Chinook Salmon (Oncorhynchus tshawytscha) smolts affects transit time, route selection and survival in complex aquatic ecosystems. Behavior quantified at the river reach and junction scale is of particular importance for route selection and predator avoidance, though few studies have developed field-based approaches for quantifying swimming behavior of juvenile migratory fishes at this fine spatial scale. Two-dimensional acoustic fish telemetry at a river junction was combined with a three-dimensional hydrodynamic model to estimate in situ emigration swimming behavior of federally-threatened juvenile Chinook salmon smolts. Fish velocity over ground was estimated from telemetry, while the hydrodynamic model supplied simultaneous, colocated water velocities, with swimming velocity defined by the vector difference of the two velocities. Resulting swimming speeds were centered around 2 body lengths/second, and included distinct behaviors of positive rheotaxis, negative rheotaxis, lateral swimming, and passive transport. Lateral movement increased during the day, and positive rheotaxis increased in response to local hydrodynamic velocities. Swim velocity estimates were sensitive to the combination of vertical shear in water velocities and vertical distribution of fish.

Longfin smelt (Spirinchus thaleichthys) is a threatened anadromous fish species that spawns in freshwater to moderately brackish (i.e. 5–10 ppt) reaches of the upper San Francisco Estuary and has declined to ~1% of its pre-1980s abundances. Despite 50+ years of population monitoring, the efficacy of 10+ years of conservation efforts for longfin smelt remain uncertain due to a limited understanding of how the species responds to environmental variation, such as salinity. For example, high mortality during larval stages has prevented culture efforts from closing the life cycle in captivity. Here, we investigated the effects of salinity on longfin smelt yolk-sac larvae. Newly hatched larvae from four single-pair crosses were acutely transferred to and reared at salinities of 0.4, 5, 10, 20 or 32 ppt. We compared whole-body water and sodium ion (Na+) content, notochord length and yolk-sac volume at 12, 24, 48, 72, and 96 hours post-transfer for each salinity treatment. We found that larvae maintained osmotic and ionic balance at 0.4–10 ppt, whereas salinities ˃10 ppt resulted in decreased water and increased whole-body Na+ content. We also found that larvae grew largest and survived the longest when reared at 5 and 10 ppt, respectively, and that yolk resorption stalled at 0.4 ppt. Finally, there were significant but small interclutch variations in responses to different salinities, with clutch accounting for

Longfin smelt (Spirinchus thaleichthys) is a threatened anadromous fish species that spawns in freshwater to moderately brackish (i.e. 5–10 ppt) reaches of the upper San Francisco Estuary and has declined to ~1% of its pre-1980s abundances. Despite 50+ years of population monitoring, the efficacy of 10+ years of conservation efforts for longfin smelt remain uncertain due to a limited understanding of how the species responds to environmental variation, such as salinity. For example, high mortality during larval stages has prevented culture efforts from closing the life cycle in captivity. Here, we investigated the effects of salinity on longfin smelt yolk-sac larvae. Newly hatched larvae from four single-pair crosses were acutely transferred to and reared at salinities of 0.4, 5, 10, 20 or 32 ppt. We compared whole-body water and sodium ion (Na(+)) content, notochord length and yolk-sac volume at 12, 24, 48, 72, and 96 hours post-transfer for each salinity treatment. We found that larvae maintained osmotic and ionic balance at 0.4–10 ppt, whereas salinities ˃10 ppt resulted in decreased water and increased whole-body Na(+) content. We also found that larvae grew largest and survived the longest when reared at 5 and 10 ppt, respectively, and that yolk resorption stalled at 0.4 ppt. Finally, there were significant but small interclutch variations in responses to different salinities, with clutch accounting for

In the original online version of this article, there were some errors in the values in Table 7. The original article was corrected.

The Delta Smelt (Hypomesus transpacificus), once an abundant fish endemic to the Sacramento-San Joaquin Estuary, is now on the brink of extinction. Due to the high sensitivity of this species, knowledge of their stress response will be vital to their future survival and sustainability. Understanding the magnitude and kinetics of cortisol induction in Delta Smelt will provide valuable information when interpreting the degree of environmentally relevant stressors, such as warming and predator exposure. As little is known about the primary stress response and cortisol dynamics in Delta Smelt, the first aim of this study was to measure basal and maximal whole-body cortisol prior to and following exposure to a sublethal and significant netting stress at 17 and 21 °C. Our findings reveal that juvenile Delta Smelt held at 21 °C display an exacerbated stress response and a reduction in available energy compared to fish held at 17 °C. There was no evidence of the secondary stress response to the netting stress as whole-body glucose and lactate levels in treatment groups remained similar to basal values. The second aim of this study was to investigate the effect of a largemouth bass (Micropterus salmoides) predator cue, which was found to induce a significant increase in cortisol relative to control levels in juvenile Delta Smelt. Indices such as cortisol can be used as bioindicators of stress in the field and results from this study suggest that moderate temperatures and reduced predation are optimal release conditions during hatchery-based supplementation to minimize stress to this highly sensitive species.

Climate change is warming up water all over the world, including in the San Francisco Estuary. This has caused fish who live there to change their behavior in unexpected ways. All animals, including fish, have regular and specialized behaviors that help them to survive. Fish swim, by themselves or in groups, to move to safer habitats, to find food or mates, or just to avoid danger. Scientists worry that changes to these behaviors brought on by warming water will make estuaries less safe for rare and endangered fish like the delta smelt. In the San Francisco Estuary, we found that warmer waters caused delta smelt to swim faster and further away from their neighbors, and to be eaten more often by predators. All these changes could hurt the ability of delta smelt to survive in the future.

Predation is a common cause of early life stage mortality in fishes, with reduced risk as individuals grow and become too large to be consumed by gape‐limited predatory fishes. Large‐bodied species, such as sturgeon, may reach this size‐refuge within the first year. However, there is limited understanding of what this size threshold is despite the value of this information for conservation management. We conducted laboratory‐based predation experiments on juvenile green sturgeon, Acipenser medirostris, to estimate vulnerability to predation during outmigration from their natal reaches in California to the Pacific Ocean. Two highly abundant and non‐native predatory fish species (largemouth bass, Micropterus salmoides, and striped bass, Morone saxatilis) were captured in the wild to be tested with developing juvenile green sturgeon from the UC Davis Green Sturgeon Broodstock Program. Experimental tanks, each containing five predators, received thirty prey for 24‐hr exposures. Between sturgeon prey trials, predators were exposed to alternative prey species to confirm predators were exhibiting normal feeding behaviors. In addition to green sturgeon mortality data, trials were video recorded and predatory behaviors were quantified. Overall, these predator species displayed much lower rates of predation on juvenile green sturgeon than alternate prey. Predation decreased with green sturgeon size, and predation risk diminished to zero once sturgeon reached a length threshold of roughly 20–22 cm total length, or between 38% and 58% of predator total length. Behavioral analyses showed low motivation to feed on green sturgeon, with both predators attempting predation less frequently as sturgeon grew. Results of this study imply that optimizing growth rates for larval and juvenile sturgeon would shorten the time in which they are vulnerable to predation. Future experiments should assess predation risk of juvenile green sturgeon by additional predator species common to the Sacramento‐San Joaquin watershed.

Author(s): Davis, BE; Hansen, MJ; Cocherell, DE; Nguyen, TX; Sommer, T; Baxter, RD; Fangue, NA; Todgham, AE | Abstract: The effects of water temperature on individual and group movement behaviour in prey fish can affect ecological interactions such as competition and predation, but how variability in temperature influence fish behaviour is less understood. Of particular concern is how increased warming in tidally fluctuating estuaries may impact the native and endangered delta smelt (Hypomesus transpacificus, Osmeridae). To help address this issue, we tested the effects of increased water temperature (fluctuating [17–21°C] and warm [21°C] acclimated treatments) on juvenile delta smelt individual and group behaviour, response to chemical alarm and predator cues, as well as capacity to evade predation. In addition, predation of delta smelt was tested in the presence of a dominant invasive competitor, Mississippi silversides (Menidia beryllina, Atherinopsidae), as well as comparative predation mortality on Mississippi silversides when isolated. After 7ndays of increased temperature treatments, delta smelt in the warm treatment increased swimming velocity, decreased turning angle, and altered group structure with larger inter-individual distances compared to fish in the control (17°C) and fluctuating temperature treatments. Following conspecific and predator chemical alarm cues, delta smelt showed anti-predator responses. Control and fluctuating treatment fish responded to conspecific cues with increased swimming speeds, decreased inter-individual distances and near-neighbour distances, and, after 15nmin, fish recovered back to baseline behaviours. In contrast, fish in the warm treatment had not recovered after 15nmin, and swimming speeds were maintained at roughly 25ncm/s, close to maximum capabilities. Fish in control and fluctuating treatments showed minimal responses to predator cues, whereas delta smelt exposed to warm conditions significantly increased swimming speeds and decreased turning angle. Predation of delta smelt by largemouth bass (Micropterus salmoides, Centrarchidae) was greatest under the warm treatment, correlating with altered behaviours of delta smelt; however, predation of Mississippi silversides was greater than delta smelt, independent of temperature. This study provides novel insight into the group behaviour of delta smelt, their response to predation, and how prolonged exposure to elevated temperature may induce negative individual and group behaviours causing alterations in predator–prey dynamics. This work highlights the importance of testing ecologically realistic temperature fluctuations in experiments as delta smelt had significantly altered responses to elevated temperature, dependent on variability of warming.

The entrainment and impingement of fish into water diversion infrastructure is one of the several factors contributing to their decline. Here, controlled experiments assessed the potential for a behavioral guidance device [a light-emitting diode (LED) light array] strobing at various spectra to reduce the entrainment of juvenile Chinook salmon (Oncorhynchus tshawytscha) into a water diversion pipe. Fish were tested during the day and night, and under control conditions (light off) and red, blue, and white spectra strobing at 2 Hz. Fish entrainment into the diversion pipe was evaluated. Results indicated greater entrainment at night compared to day. All trials at night with the LED light strobing had higher entrainment than the control, with blue and white spectra corresponding to greater entrainment than red spectra. During the day, the white spectra treatment was different from the red treatment, with lower entrainment. LED lights employed to repel migratory juvenile salmon away from water intake structures may be ineffectual but there is potential for the light to be used as an attractant to guide fish towards desirable features such as “safe” areas (bypass channels or fishways).

Innovative telemetry and biologging technology has increased the amount of available movement data on aquatic species. However, real-time information on the environmental factors influencing animal movements can be logistically challenging to obtain, particularly in habitats where tides and currents vary locally. Hydrodynamic models are capable of simulating complex tidal flow, and may thus offer an alternative method of contextualizing animal movement in coastal habitats. Here we use this tool to examine the influence of tide on the movement of broadnose sevengill sharks (Notorynchus cepedianus) in the San Francisco Bay estuary. Three sharks were actively tracked using acoustic transmitters for 3 to 4days. We then generated a hydrodynamic model of the estuary and calculated current vectors along each track. We hypothesized that the sharks would adjust their swimming speed and direction depending on current strength when passing through the channel underneath the Golden Gate Bridge. Our results indicate that sharks did tend to follow the current flow in the channel, but their overall displacement did not significantly correlate with tidal amplitude. We conclude that the sharks may respond to environmental factors other than tidal flow, altering their movement at a finer scale than initially considered. Overall, this suggests that hydrodynamic simulation models can be used to visualize and quantify environmental factors that may affect movement patterns in aquatic organisms. We recommend future studies combine these models with other biologging techniques to measure energy expenditure at a finer spatial scale.

In highly regulated systems, like large dammed rivers, conservation legislation requires that systems are managed, in part, to avoid adverse impacts on endangered species. However, multiple endangered species can occur in the same system, and management actions that benefit one species may be detrimental to another species. The current water management strategies in the Sacramento River basin are an example of this conflict. Cold-water releases from Shasta Reservoir during the summer and fall months are aimed at protecting Sacramento River winter-run Chinook (SRWRC) salmon by providing suitable incubation temperatures for their eggs. However, the effects of these regulated water temperature releases on another threatened species, green sturgeon, are less well understood. In this study, we applied a simplified dynamic energy budget (DEB) model (aka DEBkiss) to explore the effect of food limitation and water temperature on the growth rates of green sturgeon. This model captures these effects and able to predict the growth of green sturgeon at different food levels and temperature conditions. We then linked the DEB model with a physically‐based water temperature model. We applied the DEB - water temperature linked model for green sturgeon along with a temperature-dependent egg to fry survival model for SRWRC salmon to quantify the consequences of managing water temperatures to improve salmon eggs survival on the growth rate of green sturgeon. We found that mean temperature-dependent egg-to-fry survival of salmon increased across a modeled environmental gradient from critically dry to wet water year types, while the fractional growth rate of juvenile green sturgeon showed the opposite trend, and decreased as water years transitioned from dry to wet conditions. We also found a non-linear negative correlation between temperature-dependent mean growth rate of green sturgeon and mean temperature-dependent egg-to-fry survival of salmon, which indicated there is a river temperature related trade-off between early growth rate of green sturgeon and embryonic stage survival of salmon. However, the relatively small gains in the growth rate of green sturgeon achieved in years when temperature criteria for SRWRC salmon eggs were not met came at the cost of large reduction in temperature-dependent egg-to-fry survival of salmon. Thus, we concluded the current Sacramento River water-temperature management for the eggs of the endangered SRWRC salmon eggs have a relatively small impact on the growth rate of green sturgeon.

Extirpation of the Central Valley spring-run Chinook Salmon ESU (Oncorhynchus tshawytscha) from the San Joaquin River is emblematic of salmonid declines across the Pacific Northwest. Habitat restoration and fish reintroduction efforts are ongoing, but recent telemetry studies have revealed low outmigration survival of juveniles to the ocean. Previous investigations have focused on modeling survival relative to river discharge and geographic regions, but have largely overlooked the effects of habitat variability. To evaluate the link between environmental conditions and survival of juvenile spring-run Chinook Salmon, we combined high spatial resolution habitat mapping approaches with acoustic telemetry along a 150 km section of the San Joaquin River during the spring of 2019. While overall outmigration survival was low (5%), our habitat-based classification scheme described variation in survival of acoustic-tagged smolts better than other candidate models based on geography or distance. There were two regional mortality sinks evident along the longitudinal profile of the river, revealing poor survival in areas that shared warmer temperatures but that diverged in chlorophyll-α, fDOM, turbidity and dissolved oxygen levels. These findings demonstrate the value of integrating river habitat classification frameworks to improve our understanding of survival dynamics of imperiled fish populations. Importantly, our data generation and modeling methods can be applied to a wide variety of fish species that transit heterogeneous and diverse habitat types.

Loss of estuarine and coastal habitats worldwide has reduced nursery habitat and function for diverse fishes, including juvenile Chinook salmon (Oncorhynchus tshawytscha). Underutilized off-channel habitats such as flooded rice fields and managed ponds present opportunities for improving rearing conditions and increasing habitat diversity along migratory corridors. While experiments in rice fields have shown enhanced growth rates of juvenile fishes, managed ponds are less studied. To evaluate the potential of these ponds as a nursery habitat, juvenile Chinook salmon (~ 2.8 g, 63 mm FL) were reared in cages in four contrasting locations within Suisun Marsh, a large wetland in the San Francisco Estuary. The locations included a natural tidal slough, a leveed tidal slough, and the inlet and outlet of a tidally muted managed pond established for waterfowl hunting. Fish growth rates differed significantly among locations, with the fastest growth occurring near the outlet in the managed pond. High zooplankton biomass at the managed pond outlet was the best correlate of salmon growth. Water temperatures in the managed pond were also cooler and less variable compared to sloughs, reducing thermal stress. The stress of low dissolved oxygen concentrations within the managed pond was likely mediated by high concentrations of zooplankton and favorable temperatures. Our findings suggest that muted tidal habitats in the San Francisco Estuary and elsewhere could be managed to promote growth and survival of juvenile salmon and other native fishes.

Floodplains represent critical nursery habitats for a variety of fish species due to their highly productive food webs, yet few tools exist to quantify the extent to which these habitats contribute to ecosystem-level production. Here we conducted a large-scale field experiment to characterize differences in food web composition and stable isotopes (δ¹³C, δ¹⁵N, δ³⁴S) for salmon rearing on a large floodplain and adjacent river in the Central Valley, California, USA. The study covered variable hydrologic conditions including flooding (1999, 2017), average (2016), and drought (2012–2015). In addition, we determined incorporation rates and tissue fractionation between prey and muscle from fish held in enclosed locations (experimental fields, cages) at weekly intervals. Finally, we measured δ³⁴S in otoliths to test if these archival biominerals could be used to reconstruct floodplain use. Floodplain-reared salmon had a different diet composition and lower δ13C and δ³⁴S (δ¹³C = -33.02±2.66‰, δ³⁴S = -3.47±2.28‰; mean±1SD) compared to fish in the adjacent river (δ¹³C = -28.37±1.84‰, δ³⁴S = +2.23±2.25‰). These isotopic differences between habitats persisted across years of extreme droughts and floods. Despite the different diet composition, δ¹⁵N values from prey items on the floodplain (δ¹⁵N = 7.19±1.22‰) and river (δ¹⁵N = 7.25±1.46‰) were similar, suggesting similar trophic levels. The food web differences in δ13C and δ³⁴S between habitats were also reflected in salmon muscle tissue, reaching equilibrium between 24–30 days (2014, δ¹³C = -30.74±0.73‰, δ³⁴S = -4.6±0.68‰; 2016, δ¹³C = -34.74 ±0.49‰, δ³⁴S = -5.18±0.46‰). δ³⁴S measured in sequential growth bands in otoliths recorded a weekly time-series of shifting diet inputs, with the outermost layers recording time spent on the floodplain (δ³⁴S = -5.60±0.16‰) and river (δ³⁴S = 3.73±0.98‰). Our results suggest that δ¹³C and δ³⁴S can be used to differentiate floodplain and river rearing habitats used by native fishes, such as Chinook Salmon, across different hydrologic conditions and tissues. Together these stable isotope analyses provide a toolset to quantify the role of floodplains as fish habitats.

Author(s): Lehman, Brendan M.; Johnson, Rachel C.; Adkison, Mark; Burgess, Oliver T.; Connon, Richard E.; Fangue, Nann A.; Foott, J. Scott; Hallett, Sascha L.; Martinez–Lopez, Beatriz; Miller, Kristina M.; Purcell, Maureen K.; Som, Nicholas A.; Valdes–Donoso, Pablo; Collins, Alison L. | Abstract: Chinook Salmon (Oncorhynchus tshawytscha) are increasingly vulnerable to anthropogenic activities and climate change, especially at their most southern species range in California’s Central Valley. There is considerable interest in understanding stressors that contribute to population decline and in identifying management actions that reduce the impacts of those stressors. Along the west coast of North America, disease has been linked to declining numbers of salmonids and identified as a key stressor resulting in mortality. In the Central Valley, targeted studies have revealed extremely high prevalence of infectious agents and disease. However, there has been insufficient monitoring to understand the effect that disease may have on salmon populations. In order to inform future research, monitoring, and management efforts, a two-day workshop on salmon disease was held at UC Davis on March 14-15, 2018. This paper summarizes the science presented at this workshop, including the current state of knowledge of salmonid disease in the Central Valley and current and emerging tools to better understand its impacts on salmon. We highlight case studies from other systems where successful monitoring programs have been implemented. First, in the Klamath River where the integration of several data collection and modeling approaches led to the development of successful management actions, and second in British Columbia where investment in researching novel technologies led to breakthroughs in the understanding of salmon disease dynamics. Finally, we identify key information and knowledge gaps necessary to guide research and management of disease in Central Valley salmon populations.

Delta smelt (Hypomesus transpacificus) is a critically endangered species endemic to the San Francisco Bay Delta (SFBD). Important for the conservation of this species is understanding the physiological and ecological impacts contributing to their population decline, and current studies lack information on embryonic development. Changes in patterns of salinity across the SFBD may be a particularly important environmental stressor contributing to the recruitment and survival of the species. Throughout their ontogeny, delta smelt may exhibit unique requirements and tolerances to environmental conditions including salinity. Here, we describe 22 stages of embryonic development of H. transpacificus that characterize early differentiation from the fertilized egg until hatching, allowing the identification of critical morphological features unique to this species. Additionally, we investigated aspects of physiological tolerance to environmental salinity during development. Embryos survived incubation at salinity treatments between 0.4 and 20 ppt, yet had lower hatch success at higher salinities. Prior to hatching, embryos exposed to higher salinities had increased osmolalities and reduced fractions of yolk implying that the elevated external salinity altered the physiology of the embryo and the environment internal to the chorion. Lastly, egg activation and fertilization appear to also be impacted by salinity. Altogether, we suggest that any potential tolerance to salinity during embryogenesis, a common feature in euryhaline teleost species, impacts life cycle transitions into, and out of, embryonic development. Results from this investigation should improve conservation and management practices of this species and further expand our understanding of the intimate relationship between an embryo and its environment.

As part of a restoration project designed to enhance Chinook salmon populations in the lower Yuba River, we sought to characterize river temperatures at micro-habitats scales relevant to juveniles. Given that temperatures vary more across the channel than they do longitudinally, temperature had to be assayed both along the length of a study reach as well as across the channel. These temperatures needed to be measured at least hourly in order to capture the totality of the diel fluctuations. It was necessary to sample for the full duration of juvenile residence in the lower Yuba River – a six-month period over the winter. Though it was desirable to design a system that could sample for at least a year to capture the totality of seasonal fluctuations. In this paper, we present the design, implementation and deployment of a wireless sensor network that was installed to monitor temperature data on the Lower Yuba River. The mesh network was implemented using nodes that contained a MSP430 chipset with a radio operating at 433 MHz. We describe the network architecture which included a network storage function to address intermittent link failures. We describe the event-based software architecture of the mesh network mode specifically in relation to energy optimization. We also give details of the deployment and provide measurement data.

Author(s): Singer, GP; Hansen, MJ; Ho, KV; Lee, KW; Cocherell, DE; Peter Klimley, A; Rypel, AL; Fangue, NA | Abstract: Acoustic telemetry, a commonly used tool for examining movements and survival of aquatic species, is often applied without a comprehensive understanding of transmitter implantation effects. This can be problematic when the goal of the study is to use telemetry results to make inferences regarding broader populations. Here, we examined juvenile Chinook Salmon Oncorhynchus tshawytscha at varying time intervals after transmitter implantation to assess the behavioral implications of tagging. The following behavioral metrics in response to a novel environment were compared across treatment and control groups: time to emergence from shelter into the open portion of the test arena, rheotactic response, total activity, and rates of exploration. Tagged fish (114–132nmm FL) were tested at 0, 1, or 4nd postsurgery (day-0, day-1, and day-4 groups, respectively), and their behavior was compared to that of similarly handled control fish. Emergence from refuge was the only metric that differed significantly between treatment and control groups. Fish tested on the same day as the surgery were less likely to emerge from the refuge, with only 46% of the day-0 tagged fish emerging compared to 88, 93, and 80% of the day-1, day-4, and control groups, respectively. However, day-0 fish that did emerge from the refuge had rheotactic responses, total activity, and exploration rates similar to those of fish from the other treatment and control groups. This study may have fisheries research and management implications, especially for telemetry studies and monitoring efforts. We encourage researchers using this technology to consider (1) observing a post-transmitter-implantation recovery period of at least 24nh prior to release, adjusting study plans and logistics accordingly; (2) applying sufficient scientific rigor to emerging tagging technology prior to wide-scale adoption; and (3) when possible, conducting concurrent battery life and tag effects studies with any field release of tagged fishes, as differing relationships between fish size, tag size, and tagging techniques may yield variable results.

Using a system optimized for propagating human keratinocytes, culture of skin samples from white and green sturgeons generated epithelial cells capable of making cross-linked protein envelopes. Two distinct forms of TGM1-like mRNA were molecularly cloned from the cells of white sturgeon and detected in green sturgeon cells, accounting for their cellular envelope forming ability. The protein translated from each displayed a cluster of cysteine residues resembling the membrane anchorage region expressed in epidermal cells of teleosts and tetrapods. One of the two mRNA forms (called A) was present at considerably higher levels than the other (called B) in both species. Continuous lines of white sturgeon epidermal cells were established and characterized. Size measurements indicated that a substantial fraction of the cells became enlarged, appearing similar to squames in human epidermal keratinocyte cultures. The cultures also expressed CYP1A, a cytochrome P450 enzyme inducible by activation of aryl hydrocarbon receptor 2 in fish. The cells gradually improved in growth rate over a dozen passages while retaining envelope forming ability, TGM1 expression and CYP1A inducibility. These cell lines are thus potential models for studying evolution of fish epidermis leading to terrestrial adaptation and for testing sturgeon sensitivity to environmental stresses such as pollution.

Early developmental stages of fishes are particularly sensitive to changes in environmental variables that affect physiological processes such as metabolism and growth. Both temperature and food availability have significant effects on the growth and survival of larval and juvenile fishes. As climate change and anthropogenic disturbances influence sensitive rearing environments of fishes it is unlikely that they will experience changes in temperature or food availability in isolation. Therefore, it is critical that we determine the effects of each of these potential stressors on larval growth and development, as well as understand the additive, synergistic or antagonistic effects of both. We reared threatened green sturgeon Acipenser medirostris (initial age ca. 32 days post hatch) at four temperatures (11, 13, 16 and 19°C) and two food availability rates (100% and 40% of optimal) to assess the effects of these stressors and their interactions on larval growth. We compared the overall size (fork length, total length and mass), growth rates (cm day-1 and g day-1 ) and relative condition factor of these larval and juvenile fish at 3 week intervals for up to 12 weeks. Our results indicated that temperature and food availability both had significant effects on growth and condition and that there was a significant interaction between the two. Fish reared with limited food availability exhibited similar patterns in growth rates to those reared with elevated food rates, but the effects of temperature were greatly attenuated when fish were food-limited. Also, the effects of temperature on condition were reversed when fish were reared with restricted food, such that fish reared at 19°C exhibited the highest relative condition when fed optimally, but the lowest relative condition when food was limited. These data are critical for the development of relevant bioenergetics models, which are needed to link the survival of larval sturgeons with historic environmental regimes, pinpoint temperature ranges for optimal survival and help target future restoration sites that will be important for the recovery of sturgeon populations.

Understanding physiological limits and the ability to acclimatize to changing conditions will determine whether species will be able to cope with further increases in water temperature. Changes in temperature may be considered to become stressful for an ectotherm when it results in reduced performance, which can lead to fitness level consequences. The relative intensity of the stressor as well as the duration of the exposure to the stressor will determine the response observed. Transcriptomic responses can potentially indicate thresholds where physiological performance begins to decline. An understanding of the cellular shifts throughout the temperature range that an organism experiences in the wild is often lacking, especially for species of conservation concern such as the delta smelt (Hypomesus transpacificus). We examined the expression of 15 genes that represented cellular responses related to stress, growth, cell proliferation and osmoregulation to show how the response patterns change to acute increases in temperatures that occur throughout the thermal distribution of the species. Several genes showed U-shaped or inverted U-shaped response patterns suggesting the presence of sub-lethal thresholds as temperatures increase. We also highlight the importance of including a temporal component to exposure studies as several genes showed a delay in the recovery to control levels at extreme temperatures. We propose that the non-linear response patterns represent sub-lethal thermal thresholds that can predict the severity of the response to thermal stressors. Identifying these sub-lethal thresholds can help differentiate between responses to routine increases in water temperature and responses that can lead to longer-term fitness impacts.

Anthropogenic water management projects and facilities that alter the local and regional hydrology of riverine environments greatly influence the behavior, physiology, and survival of native fishes. To mitigate for losses of native fishes at these structures, many are outfitted with fish-exclusion screens to reduce entrainment. The effect of fish size and age on behavior near fish screens, however, is largely unknown. Therefore, we tested two size classes of juvenile green sturgeon (Acipenser medirostris; small, early juveniles: 9.2 ± 0.2 cm fork length [FL], 6.9 ± 0.3 g; intermediate juveniles: 18.8 ± 0.2 cm FL, 36.9 ± 0.8 g) near fish-exclusion screens in a laboratory swimming flume. Although size was a significant factor influencing the way in which fish contacted the screens (i.e., proportion of body contacts, p = 2.5 × 10−9), it did not significantly influence the number of times fish contacted screens or the amount of time fish spent near screens. We also compared the performance of these two size classes to that of older and larger sturgeon that were tested previously (29.6 ± 0.2 cm FL, 147.1 ± 3.1 g), and documented a clear difference in the behavior of the fish that resulted in disparities in how the large fish contacted screens relative to small- or intermediate-sized juveniles (p = 0.005, 5.4 × 10−4, respectively). Our results further our understanding of how ontogeny affects fish behavior near anthropogenic devices, and are informative for managers seeking to identify the most susceptible size and age class of juvenile green sturgeon to water-diversion structures to potentially develop size-specific conservation strategies.

Fish health can be affected by many environmental factors including infection with pathogens. Infectious disease can lead to high mortality in wild populations and in aquaculture, resulting in significant economic losses. Teleost fishes share a highly conserved response to pathogen infections, starting with an immediate activation of the innate immune response before shifting to an adaptive immune response. Fishes also show altered transcriptomic expression in stress-related genes during acute and chronic infections. The ciliate Ichthyophthirius multifiliis ( Ich ) is one of the most widely distributed ectoparasites for freshwater fish, providing the possibility to identify infection severity via phenotypical assessments. Molecular markers are increasingly incorporated in health assessments and to determine sublethal physiological responses to stressors. We evaluated the effects of an infection with Ich using the pelagic fish species delta smelt ( Hypomesus transpacificus ) as a model. We investigated transcriptional changes of ten genes belonging to the innate immune response and five heat shock genes related to thermal and general stress response. Molecular assessments were conducted in three tissues (gill, spleen and kidney), and collected from fish with different infection intensities. The combination of molecular markers, including immune and stress-related genes, resulted in significant differentiation between infection levels in all tissue types. The strongest results were observed in gill and kidney, however, the detection of infection severity was most effective when combining the transcriptomic results from all tissues. Chemokine cxcb was the most responsive gene in all tissues and provided significant upregulation in fish with visible Ich -infection. The identification of molecular markers targeting fundamental host responses associated with infections is contributing to the establishment of techniques to assess fish health in natural habitats and in aquaculture.