Stock assessment models are simplifications of a stock’s population dynamics. One commonly simplified component of population dynamics is natural mortality, which refers to all components of mortality that are not attributed to harvest, including predation, starvation, disease, and senescence. Although recent modeling advances have improved our ability to estimate natural mortality, most research has focused on either estimating aggregate natural mortality or predation mortality rates, while continuing to simplify other components. We contend that starvation-induced mortality, particularly for mature marine animals, represents an overlooked component of natural mortality and should be more frequently considered when developing scientific advice for ecosystem-based fisheries management. To defend this contention, we (1) describe marine animal energy maintenance and when starvation may be important, (2) highlight three case studies where starvation-induced mortality of mature animals may have affected population dynamics, (3) describe methods for measuring and estimating starvation to improve ecosystem-based fisheries management, and (4) describe how starvation risks may increase with climate change. Overall, improving stock assessment modeling requires that we continually reassess whether model simplifications are sufficient to estimate how populations are changing in support of fisheries management.

Pacific salmon currently experience temperatures in freshwater that can reach or exceed their upper thermal limits, but little is known about these limits for early life stages. As such, this study aimed to (i) evaluate the role of thermal acclimation on the upper thermal tolerance of stream-type juvenile Chinook salmon (Oncorhynchus tshawytscha) fry and parr and (ii) determine how different methods for assessing thermal limits affect the measured outcome. Both parr and fry life stages were acclimated for 2 weeks at temperatures spanning present (15, 18 and 20°C) and expected future summer levels (24°C). Using fish from each acclimation temperature, we determined the critical thermal maximum at rest (CT max ) and during swimming (CT swim; temperature at which fish fatigue from swimming) for both life stages, and further assessed prolonged swim performance and post-swim mortality. CT swim was a more sensitive indicator of upper thermal limits compared to CT max , for both life stages. Fish acclimated to higher temperatures generally exhibited higher thermal tolerance though acclimation capacity was diminished between 20 and 24°C. There was high post-swim mortality for the 24°C acclimation groups in both parr and fry, indicating an upper thermal limit where recovery was impaired. Overall, fry had higher upper thermal tolerance (shown by CT swim , prolonged swim completion and post-swim mortality rates) compared to parr, demonstrating that life stage thermal tolerance corresponded with seasonal temperature exposure differences. Warm water temperatures exceeding 20°C are now routinely occurring in the study population region and many salmonid habitats worldwide, so we expect increasing mortality rates of early life stages in coming years.

Post-release survival in catch-and-release fisheries is highly variable and context dependent, yet management agencies often apply uniform survival estimates. A significant source of this uncertainty is our limited understanding of how the capture event itself drives physiological stress and recovery. Fight dynamics reflect how much effort a fish expends and the exhaustion it experiences. Non-invasive accelerometers attached to the fishing line to record the capture event provide a powerful yet underused tool for quantifying these fight dynamics. Traditional analytical approaches reduce accelerometry data to summary statistics, obscuring fine-scale temporal patterns. Here, we introduce a toolbox based on dynamic time warping (DTW) to preserve the temporal structure of capture events and link fight behaviour to physiological disturbance. We attached tri-axial jerk accelerometers ('jerk' tags) to fishing lines and captured Chinook salmon (Oncorhynchus tshawytscha) and coho salmon (O. kisutch) using rod-and-reel angling off the Pacific coast of Vancouver Island in British Columbia, Canada. We quantified physiological disturbance by sampling blood pH and plasma lactate 1 h post-capture. To determine whether biologging tags can predict physiological outcomes, we systematically evaluated 13 analytical pipelines, including jerk summary metrics (fight duration, burst frequency, intensity patterns) and DTW-based approaches using raw, filtered and differentiated jerk data. These complementary approaches captured different behavioural dimensions linking fight to physiology. Summary metrics described broad fight patterns and clustered fish with temporally similar fight signatures, while DTW detected Y-axis dimensions linked to individual recovery (plasma lactate). This toolbox is transferable across species and logger types, requiring only accelerometers attached to angling gear. By systematically evaluating processing pipelines rather than defaulting to conventional metrics only, researchers can optimize inference and identify behavioural signatures that predict physiological disturbance. This approach provides a scalable tool for developing evidence-based best practices to improve conservation outcomes.

ABSTRACT Aim Climate‐driven declines in fish body size suggest direct negative impacts on larger individuals and indirect impacts on species interactions, population dynamics and ultimately ecosystem functions and services. This study uses metabolic scaling as a mechanistic framework to identify aerobic limitations in fish across size to provide a foundation for a better understanding of the ecological consequences of size‐specific species vulnerability in a warming world. Location Global. Time Period Present. Major Taxa Studied Fish. Methods We synthesized over 12,000 individual maximum and resting metabolic rates (MMR, RMR) across optimal and supra‐optimal warm temperatures to determine how they scale with mass and calculated factorial aerobic scope (FAS = MMR/RMR) to assess a potential aerobic constraint in fish taxa globally and across species groups with shared ecologies and morphologies. Additionally, we identified interspecific and intraspecific variation in mass‐independent metabolic rates across diverse fish species. Results Across fish taxa globally, metabolic scaling slopes differed between MMR and RMR and across optimal and warm temperature conditions, but within both temperature categories, aerobic scope scaling slopes decreased as temperature increased. Metabolic scaling relationships varied among diverse species groups and across life stages. Generally, MMR and RMR scaling slopes were higher under warm compared to optimal temperatures, while the scaling slopes of FAS were more negative under warm conditions. Main Conclusions Factorial aerobic scope was more negative under warm conditions, suggesting that larger fish face greater metabolic constraints under warming. The synthesis highlights variation in scaling relationships across species groups and a general pattern in which larger fish experience aerobic limitations. Evidence for size‐specific vulnerability will improve by incorporating currently underrepresented data, such as larger‐bodied and freshwater fish, especially under warming.

There has been much interest in understanding the mechanisms that determine the thermal tolerance of fishes. Given the importance of swimming for fish survival, it is critical to understand the mechanisms that determine why fish fatigue from exercise when temperatures increase to improve our ability to predict the impacts of climate change on fish populations. For the same reason, it is also necessary to understand the drivers of inter-individual and life stage variation in warming tolerance. Here, we used the temperature at which a fish fatigues from swimming under acute warming (CTswim) to examine how and why individuals and life stages differ in their warming tolerance. Specifically, we tested whether muscle lactate accumulation and enzyme activities, indicative of aerobic and anaerobic capacity, predict inter-individual variation in CTswim at two life stages. We used Chinook salmon (Oncorhynchus tshawytscha) fry and parr acclimated to four temperatures for several weeks to further explore the effect of acclimation temperature on these mechanisms. Our findings indicate that the capacity to remain aerobic for as long as possible while swimming (and thus maintain low lactate) during acute warming is likely to be a factor determining fatigue in young fry, but that other factors may become more important as salmon age.

Rivers, lakes, and wetlands are facing threats that continue to grow in intensity and frequency from climate change, habitat fragmentation, invasive species, changes in food availability, natural disasters, various forms of pollution (e.g., trace metals, light, noise), and emerging infectious diseases. These disruptions to freshwater environments are driving population declines in freshwater fishes as well as threatening migratory species that need freshwater habitats to complete their life cycle. To improve freshwater fish conservation efforts, it is essential to understand the magnitude and nature of the threats fish are currently facing. Here, we present a series of case studies that illustrate the utility of employing physiological methods to assess both the threats facing freshwater fishes, and the conservation efforts being used to help preserve freshwater biodiversity. We present an array of physiological tools that can be used across multiple levels of biological organization, from molecular to population-level, to address a variety of questions. Finally, we share what we view to be pressing questions in freshwater fish conservation physiology and highlight strategies to help bridge gaps across different user groups.

Genomic tools are becoming increasingly necessary for mitigating biodiversity loss and guiding management decisions in the context of climate change. Freshwater fish species are particularly susceptible to the impacts of changing environments, including kokanee, the resident form of sockeye salmon (Oncorhynchus nerka), which has already been negatively impacted by increases in extreme temperature throughout its distribution. A previous study using whole genome resequencing of wild kokanee stocks identified 1412 environmentally associated SNPs and demonstrated genomic offset, a measure of climate vulnerability, to be significantly correlated with higher increases in extreme warm temperatures across much of the species' range in western Canada. Here, we aimed to operationalize this information for fisheries management by first developing a Genotyping-in-Thousands by sequencing (GT-seq) panel populated exclusively with environment associated SNPs. We then evaluated the robustness of the GT-seq panel relative to the signal in the whole genome resequencing baseline and demonstrated a novel application of donor and recipient importance (DI/RI) analysis to inform recreational fisheries stocking decisions. We found that a reduced GT-seq panel of 616 SNPs exhibited a significant positive correlation with those calculated from the full set of 1412 SNPs across the climate change scenarios tested; similar results were obtained when adding new reference populations not included in the original whole genome resequencing baseline. The DI/RI analysis revealed clear spatial trends, with populations situated in the warmest regions of southern interior British Columbia (Canada) having the highest probability for successful translocations to different recipient locations to the north. Similarly, candidate recipient lakes for stocking at the center of the distribution had higher recipient importance values than those located towards the eastern and western range peripheries. Although further refinement is required, pairing targeted genotyping with genomic offset and DI/RI predictions holds great promise for informing freshwater fisheries management moving forward.

Salmonid fishes are a focal point of conservation physiology due to their high value to humans and ecosystems, and their susceptibility to decline from climate change. A significant challenge in conserving these fishes is that populations of the same species can be locally adapted to vastly different habitats within their wild ranges and can therefore have unique tolerance or vulnerability to environmental stressors within those habitats. Within the state of Oregon, USA, summer steelhead (Oncorhynchus mykiss) inhabit both cool, coastal waters most typically associated with Pacific salmonids and arid, inland environments where temperatures are more extreme. Here, we utilized streamside physiological experiments paired with habitat temperature monitoring to assess the thermal tolerance and vulnerability of four populations of summer steelhead from distinct thermal habitats. All populations had unique responses of critical thermal maximum, aerobic scope and exercise recovery to temperature. Despite populations from warm habitats exhibiting higher thermal tolerance than populations from cooler habitats, summer steelhead from warm habitats appear to be more vulnerable to the physiological consequences of warming based on the extreme temperatures they already experience during the summer. These results demonstrate an example of thermal physiology varying between populations within the same portion of their latitudinal range and highlight the need for habitat-specific conservation strategies for this species.

The thermal sensitivity of heart rate (fH) in fishes has fascinated comparative physiologists for well over a century. We now know that elevating fH is the primary mechanism through which fishes increase convective oxygen delivery during warming to meet the concomitant rise in tissue oxygen consumption. Thus, limits on fH can constrain whole-animal aerobic metabolism. In this Review, we discuss an increasingly popular methodology to study these limits, the measurement of pharmacologically induced maximum fH (fH,max) during acute warming of an anaesthetized fish. During acute warming, fH,max increases exponentially over moderate temperatures (Q10∼2-3), but this response is blunted with further warming (Q10∼1-2), with fH,max ultimately reaching a peak (Q10≤1) and the heartbeat becoming arrhythmic. Because the temperatures at which these transitions occur commonly align with whole-animal optimum and critical temperatures (e.g. aerobic scope and the critical thermal maximum), they can be valuable indicators of thermal performance. The method can be performed simultaneously on multiple individuals over a few hours and across a broad size range (<1 to >6000 g) with compact equipment. This simplicity and high throughput make it tractable in lab and field settings and enable large experimental designs that would otherwise be impractical. As with all reductionist approaches, the method does have limitations. Namely, it requires anaesthesia and pharmacological removal of extrinsic cardiac regulation. Nonetheless, the method has proven particularly effective in the study of patterns and limits of thermal plasticity and holds promise for helping to predict and mitigate outcomes of environmental change.

Salmonids are ecologically, economically, and culturally significant throughout the Northern Hemisphere, yet they are imperiled by warming habitat temperatures. Cardiac collapse has been implicated as a key mechanism determining salmonid thermal tolerance. Both adaptation and plasticity are central mechanisms allowing salmon cardiac morphology and physiology to respond to climate change. Notably, vulnerability to warming varies within species: across populations, sexes, and life stages. Managers and conservation practitioners can use cardiac thermal performance thresholds to inform on climate-adaptive fisheries management approaches in support of sustainable fisheries and healthy ecosystem services.

The environment is changing rapidly, and considerable research is aimed at understanding the capacity of organisms to respond. Changes in environmental temperature are particularly concerning as most animals are ectothermic, with temperature considered a key factor governing their ecology, biogeography, behaviour and physiology. The ability of ectotherms to persist in an increasingly warm, variable, and unpredictable future will depend on their nutritional status. Nutritional resources (e.g. food availability, quality, options) vary across space and time and in response to environmental change, but animals also have the capacity to alter how much they eat and what they eat, which may help them improve their performance under climate change. In this review, we discuss the state of knowledge in the intersection between animal nutrition and temperature. We take a mechanistic approach to describe nutrients (i.e. broad macronutrients, specific lipids, and micronutrients) that may impact thermal performance and discuss what is currently known about their role in ectotherm thermal plasticity, thermoregulatory behaviour, diet preference, and thermal tolerance. We finish by describing how this topic can inform ectotherm biogeography, behaviour, and aquaculture research.

Climate change is affecting freshwater systems, leading to increased water temperatures, which is posing a threat to freshwater ecological communities. In the Nechako River, British Columbia, a water management program has been in place since the 1980s to maintain water temperatures at 20 °C during the migration of adult Sockeye salmon. However, the program's effectiveness in mitigating the impacts of climate change on resident species like Chinook salmon's thermal exposure is uncertain. In this study, we utilised the CEQUEAU hydrological model and life stage-specific physiological data to evaluate the consequences of the current program on Chinook salmon's thermal exposure under two contrasting climate change and socio-economic scenarios (SSP2-4.5 and SSP5-8.5). The results indicate that the thermal exposure risk is projected to be above the optimal threshold for parr (intermediate juvenile) and adult life stages under both scenarios relative to the 1980s. Under the SSP5-8.5 scenario, these life stages could experience an increase in thermal exposure ranging from two to five times higher by the 2090s compared to the 1980s. This exposure is projected to occur during the months in which these life stages emerge, including the period when the program is active (July 20th to August 20th). Additionally, our study shows that climate change will result in a substantial rise in cumulative heat degree days, ranging from 1.9 to 5.8 times (2050s) and 2.9 to 12.9 times (2090s) in comparison to the 1980s under SSP5-8.5. Our study highlights the need for a holistic approach to reviewing the current Nechako management plan, ensuring that all species in the Nechako River system are considered especially in the face of climate change.

Understanding the adaptive potential of populations and species is pivotal for minimizing the loss of biodiversity in this era of rapid climate change. Adaptive potential has been estimated in various ways, including based on levels of standing genetic variation, presence of potentially beneficial alleles, and/or the severity of environmental change. Kokanee salmon, the non-migratory ecotype of sockeye salmon (Oncorhynchus nerka), is culturally and economically important and has already been impacted by the effects of climate change. To assess its climate vulnerability moving forward, we integrated analyses of standing genetic variation, genotype-environment associations, and climate modeling based on sequence and structural genomic variation from 224 whole genomes sampled from 22 lakes in British Columbia and Yukon (Canada). We found that variables for extreme temperatures, particularly warmer temperatures, had the most pervasive signature of selection in the genome and were the strongest predictors of levels of standing variation and of putatively adaptive genomic variation, both sequence and structural. Genomic offset estimates, a measure of climate vulnerability, were significantly correlated with higher increases in extreme warm temperatures, further highlighting the risk of summer heat waves that are predicted to increase in frequency in the future. Levels of standing genetic variation, an important metric for population viability and resilience, were not correlated with genomic offset. Nonetheless, our combined approach highlights the importance of integrating different sources of information and genomic data to formulate more comprehensive and accurate predictions on the vulnerability of populations and species to future climate change.

Female Pacific salmon often experience higher mortality than males during their once-in-a-lifetime up-river spawning migration, particularly when exposed to secondary stressors (e.g. high temperatures). However, the underlying mechanisms remain unknown. One hypothesis is that female Pacific salmon hearts are more oxygenlimited than those of males and are less able to supply oxygen to the body's tissues during this demanding migration. Notably, female hearts have higher coronary blood flow, which could indicate a greater reliance on this oxygen source. Oxygen limitations can develop from naturally occurring coronary blockages (i.e. coronary arteriosclerosis) found in mature salmon hearts. If female hearts rely more heavily on coronary blood flow but experience similar arteriosclerosis levels as males, they will have disproportionately impaired aerobic performance. To test this hypothesis, we measured resting (RMR) and maximum metabolic rate (MMR), aerobic scope (AS) and acute upper thermal tolerance in coho salmon (Oncorhynchus kisutch) with an intact or artificially blocked coronary oxygen supply. We also assessed venous blood oxygen and chemistry (cortisol, ions and metabolite concentrations) at different time intervals during recovery from exhaustive exercise. We found that coronary blockage impaired MMR, AS and the partial pressure of oxygen in venous blood (PvO2) during exercise recovery but did not differ between sexes. Coronary ligation lowered acute upper thermal tolerance by 1.1 degrees C. Although we did not find evidence of enhanced female reliance on coronary supply, our findings highlight the importance of coronary blood supply for mature wild salmon, where migration success may be linked to cardiac performance, particularly during warm water conditions.

Aquatic systems are warming and exceeding upper thermal limits (UTLs) for many fish species, yet understanding how they inform resilience to climate change is challenging. Using Pacific salmon (Oncorhynchus spp.) as a model, we conducted a systematic review involving 168 papers investigating UTL in five species. We found considerable variation in UTL among species, within species and across life stages; largely attributed to methodological approaches (e.g. CTmax/UILT, Aerobic/Cardiac Scope, Thermal Migration Barriers, Rearing Mortality, Thermal Preference/Avoidance). Given that each method has strengths and weaknesses owing to logistics, time scale and ecological realism, we offer a new framework for assessing vulnerability to warming that stresses the importance of considering UTL metrics in the context of intended use (i.e. the development of management guidelines, projections of future persistence and survival) and what aspect (physiological or behavioural) of thermal response a metric investigates. Comparing studies with identical UTL approaches revealed that within species, UTL was higher for populations historically encountering higher temperatures-suggesting local thermal adaptation. Within populations, UTL differed across the lifecycle, being highest in fry and lowest in embryo and migrating adults, but this was not universal. For spawning Pacific salmon, UTL has not been assessed and few studies have examined estuarine and marine stages. Ultimately, this data gap limits the life-history thermal perspectives that can be drawn and may indicate a broader gap for all fishes, given that Pacific salmon are among the most well-studied species. Our framework illustrates the inherent and methodological inconsistencies in UTL and offers a guide for how thermal limits can best be used to assess the warming tolerance and vulnerability of fishes.

Climate change is intensifying extreme weather events, including marine heatwaves, which are prolonged periods of anomalously high sea surface temperature that pose a novel threat to aquatic animals. Tropical animals may be especially vulnerable to marine heatwaves because they are adapted to a narrow temperature range. If these animals cannot acclimate to marine heatwaves, the extreme heat could impair their behavior and fitness. Here, we investigated how marine heatwave conditions affected the performance and thermal tolerance of a tropical predatory fish, arceye hawkfish (Paracirrhites arcatus), across two seasons in Moorea, French Polynesia. We found that the fish’s daily activities, including recovery from burst swimming and digestion, were more energetically costly in fish exposed to marine heatwave conditions across both seasons, while their aerobic capacity remained the same. Given their constrained energy budget, these rising costs associated with warming may impact how hawkfish prioritize activities. Additionally, hawkfish that were exposed to hotter temperatures exhibited cardiac plasticity by increasing their maximum heart rate but were still operating within a few degrees of their thermal limits. With more frequent and intense heatwaves, hawkfish, and other tropical fishes must rapidly acclimate, or they may suffer physiological consequences that alter their role in the ecosystem.

Natural and anthropogenic stressors are dramatically altering environments, impacting key animal physiological traits, including cardiac performance. Animals require energy and nutrients from their diet to support cardiac performance and plasticity; however, the nutritional landscape is changing in response to environmental perturbations. Diet quantity, quality and options vary in space and time across heterogeneous environments, over the lifetime of an organism and in response to environmental stressors. Variation in dietary energy and nutrients (e.g. lipids, amino acids, vitamins, minerals) impact the heart's structure and performance, and thus whole-animal resilience to environmental change. Notably, many animals can alter their diet in response to environmental cues, depending on the context. Yet, most studies feed animals ad libitum using a fixed diet, thus underestimating the role of food in impacting cardiac performance and resilience. By applying an ecological lens to the study of cardiac plasticity, this Commentary aims to further our understanding of cardiac function in the context of environmental change.

In thermally variable ecosystems, temperatures can change extensively on hourly and seasonal timescales requiring ectotherms to possess a broad thermal tolerance (critical thermal minima [CTmin] and maxima [CTmax]). However, whether fish acclimate in the laboratory similarly as they acclimatize in the field under comparable thermal variation is unclear. We used temperature data from a tidal salt marsh to design 21-day lab-acclimation treatments (static: 12, 17, 22, 27 °C; daily variation with mean 22 °C: i) range 17-27 °C, ii) range 17-27 °C with irregular extremes within 12-32 °C). We compared thermal limits in lab-acclimated and field-acclimatized eurythermal arrow goby (Clevelandia ios). Variable temperature-acclimated and acclimatized fish had similar CTmin and CTmax. Notably, arrow gobies showed rapid plasticity in their absolute thermal tolerance within one tidal cycle. The daily mean and max temperatures experienced were positively related to CTmax and CTmin, respectively. This study demonstrates that ecologically informed lab acclimation treatments can yield tolerance results that are applicable to wild fish.