Climate change-induced increases in summer water temperature have been associated with elevated mortality of adult sockeye salmon (Oncorhynchus nerka) during river migration. We show that cardiorespiratory physiology varies at the population level among Fraser River sockeye salmon and relates to historical environmental conditions encountered while migrating. Fish from populations with more challenging migratory environments have greater aerobic scope, larger hearts, and better coronary supply. Furthermore, thermal optima for aerobic, cardiac, and heart rate scopes are consistent with the historic river temperature ranges for each population. This study suggests that physiological adaptation occurs at a very local scale, with population-specific thermal limits being set by physiological limitations in aerobic performance, possibly due to cardiac collapse at high temperatures.

Mean summer water temperatures in the Fraser River (British Columbia, Canada) have increased by $1.5 1C since the 1950s. In recent years, record high river temperatures during spawning migrations of Fraser River sockeye salmon (Oncorhynchus nerka) have been associated with high mortality events, raising concerns about long-term viability of the numerous natal stocks faced with climate warming. In this study, the effect of freshwater thermal experience on spawning migration survival was estimated by fitting capture-recapture models to telemetry data collected for 1474 adults (captured in either the ocean or river between 2002 and 2007) from four Fraser River sockeye salmon stockaggregates (Chilko, Quesnel, Stellako-Late Stuart and Adams). Survival of Adams sockeye salmon was the most impacted by warm temperatures encountered in the lower river, followed by that of Stellako-Late Stuart and Quesnel. In contrast, survival of Chilko fish was insensitive to the encountered river temperature. In all stocks, in-river survival of ocean-captured sockeye salmon was higher than that of river-captured fish and, generally, the difference was more pronounced under warm temperatures. The survival-temperature relationships for ocean-captured fish were used to predict historic and future (2010-2099) survival under simulated lower river thermal experiences for the Quesnel, Stellako-Late Stuart and Adams stocks. A decrease of 9-16% in survival of all these stocks was predicted by the end of the century if the Fraser River continues to warm as expected. However, the decrease in future survival of Adams sockeye salmon would occur only if fish continue to enter the river abnormally early, towards warmer periods of the summer, as they have done since 1995. The survival estimates and predictions presented here are likely optimistic and emphasize the need to consider stock-specific responses to temperature and climate warming into fisheries management and conservation strategies.

Recent studies have shown that warm temperatures reduce survival of adult migrating sockeye salmon ( Oncorhynchus nerka ), but knowledge gaps exist on where high-temperature-related mortality occurs along the migration and whether females and males are differentially impacted by river temperature. In this study, we monitored 437 radio-tagged Fraser River sockeye salmon and used capture–mark–recapture modelling approaches to investigate whether river thermal conditions differentially influence (i) spatial patterns of survival along a 413-km stretch of migration and (ii) survival of the sexes. Regardless of water temperature, survival decreased in the river section containing the most hydraulically difficult passages of the migration. However, when water temperature was warm (19 °C), survival decreased even further in the final 186 km of the migration prior to reaching the spawning grounds, particularly in females. Female and male survival differed but only when they experienced warm river temperatures. Under such conditions, the overall freshwater migration survival of males was 1.6 times higher (0.79 ± 0.09 standard error, SE) than that of females (0.50 ± 0.11 SE). As maturing female sockeye salmon maintain higher levels of plasma cortisol compared with males, we suspect that females could be immuno-compromised and thus less resistant to pathogens whose rates of development are accelerated by warm temperatures.

The impact of freshwater environmental factors on spawning migration mortality was modeled to provide a predictive tool for fisheries management of four run timing groups of Fraser River sockeye salmon Oncorhynchus nerka: early Stuart (Stuart Lake), early summer, summer, and late. We tested the significance of different measures of water temperature, discharge, fish abundance, and entry timing for forecasting discrepancies between lower-river and upriver escapement estimates using multiple regressions of principal component scores. Descriptive discrepancy models (i.e., ''management adjustment'' models) identified using Akaike's information criterion were consistent with the known biology of each group. For example, temperature and discharge thresholds were selected for early Stuart run discrepancy models, reflecting the extremes in both variables experienced by these early migrants. Predictive discrepancy models were also generated for each run timing group by using the limited number of environmental variables that are available in-season to fisheries managers. Even predictive discrepancy models using simple environmental metrics of average river temperature, flow, and river entry timing provide a valuable tool for forecasting relative indices of spawning migration mortality. This study provides an example of how environmentally based predictive tools can be used to inform fisheries management decisions and improve the probability of achieving spawning escapement targets.

Evolutionary adaptation affects demographic resilience to climate change but few studies have attempted to project changes in selective pressures or quantify impacts of trait responses on population dynamics and extinction risk. We used a novel individual-based model to explore potential evolutionary changes in migration timing and the consequences for population persistence in sockeye salmon Oncorhynchus nerka in the Fraser River, Canada, under scenarios of future climate warming. Adult sockeye salmon are highly sensitive to increases in water temperature during their arduous upriver migration, raising concerns about the fate of these ecologically, culturally, and commercially important fish in a warmer future. Our results suggest that evolution of upriver migration timing could allow these salmon to avoid increasingly frequent stressful temperatures, with the odds of population persistence increasing in proportion to the trait heritability and phenotypic variance. With a simulated 2°C increase in average summer river temperatures by 2100, adult migration timing from the ocean to the river advanced by ∼10 days when the heritability was 0.5, while the risk of quasi-extinction was only 17% of that faced by populations with zero evolutionary potential (i.e., heritability fixed at zero). The rates of evolution required to maintain persistence under simulated scenarios of moderate to rapid warming are plausible based on estimated heritabilities and rates of microevolution of timing traits in salmon and related species, although further empirical work is required to assess potential genetic and ecophysiological constraints on phenological adaptation. These results highlight the benefits to salmon management of maintaining evolutionary potential within populations, in addition to conserving key habitats and minimizing additional stressors where possible, as a means to build resilience to ongoing climate change. More generally, they demonstrate the importance and feasibility of considering evolutionary processes, in addition to ecology and demography, when projecting population responses to environmental change.

Short episodic high temperature events can be lethal for migrating adult Pacific salmon (Oncorhynchus spp.). We downscaled temperatures for the Fraser River, British Columbia to evaluate the impact of climate warming on the frequency of exceeding thermal thresholds associated with salmon migratory success. Alarmingly, a modest 1.0 1C increase in average summer water temperature over 100 years (1981-2000 to 2081-2100) tripled the number of days per year exceeding critical salmonid thermal thresholds (i.e. 19.0 1C). Refined thresholds for two populations (Gates Creek and Weaver Creek) of sockeye salmon (Oncorhynchus nerka) were defined using physiological constraint models based on aerobic scope. While extreme temperatures leading to complete aerobic collapse remained unlikely under our warming scenario, both populations were increasingly forced to migrate upriver at reduced levels of aerobic performance (e.g. in 80% of future simulations, !90% of salmon encountered temperatures exceeding populationspecific thermal optima for maximum aerobic scope; T opt 5 16.3 1C for Gates Creek and T opt 5 14.5 1C for Weaver Creek). Assuming recent changes to river entry timing persist, we also predicted dramatic increases in the probability of freshwater mortality for Weaver Creek salmon due to reductions in aerobic, and general physiological, performance (e.g. in 42% of future simulations !50% of Weaver Creek fish exceeded temperature thresholds associated with 0-60% of maximum aerobic scope). Potential for adaptation via directional selection on run-timing was more evident for the Weaver Creek population. Early entry Weaver Creek fish experienced 25% (range: 15-31%) more suboptimal temperatures than late entrants, compared with an 8% difference (range: 0-17%) between early and late Gates Creek fish. Our results emphasize the need to consider daily temperature variability in association with population-specific differences in behaviour and physiological constraints when forecasting impacts of climate change on migratory survival of aquatic species.

Warming rivers and an improved knowledge of thermal impacts on fish are fueling a need for simple tools to generate water temperature forecasts that aid in decision making for the management of aquatic resources. Although there is strong evidence for temperature‐dependent mortality in freshwater fish populations, the application of water temperature models for in‐season fisheries management is still limited due to a lack of appropriate temperature thresholds and due to uncertainty in forecasts. We evaluated the ability of statistical models based on seasonal trends, air temperature, and discharge to produce daily forecasts of water temperature in the Fraser River, British Columbia, including explicit quantification of uncertainty in predictor variables. For all models evaluated (with and without air temperature and/or discharge predictor variables), the top model choice varied as a function of environmental conditions, uncertainty in the air temperature forecasts used to predict water temperature, and the selection of quantitative performance criteria (i.e., defining the “best” model based on the smallest mean raw error or based on the ability to accurately forecast extreme water temperatures). Water temperature forecasts averaged across 10 d produced by simple models that were fitted only to historical seasonal water temperature trends were as accurate as forecasts generated from uncertain air temperature predictions. Models fitted to air temperature were critical for forecasting high temperature thresholds; even the use of uncertain air temperature forecasts predicted high water temperatures with greater accuracy than models that lacked an air temperature covariate. In contrast, models that were fitted to discharge variables lowered the rate of false‐negative and false‐positive errors associated with estimating below‐average temperatures. On the basis of our findings, we suggest that fisheries managers should quantify the effect of uncertainties in model predictor variables when assessing water temperature models and should evaluate model performance in the context of system‐specific conditions and management objectives.Received May 15, 2013; accepted September 13, 2013Published online January 31, 2014

Uncertainties prevalent in fisheries systems result in deviations between management targets and observed outcomes. As an example of attempting to deal with such uncertainty, fishery managers of sockeye salmon Oncorhynchus nerka from the Fraser River, British Columbia, use environmentally based management adjustment (MA) models to forecast indices of in-river loss of adults as they migrate upstream to spawn. Losses forecasted by MA models are directly incorporated into estimates of total allowable catch, resulting in harvest reductions that aim to increase the probability of achieving spawning escapement targets. However, the relative forecasting success of different MA models has not been rigorously assessed. Therefore, we used a suite of forecasting and hindcasting metrics to rank the performance of numerous MA models. We found that the rank of each model varied across sockeye salmon stock aggregates (i.e., run timing groups) and depended on the performance measures chosen for evaluation. Although model selection in fisheries research is often determined solely by model-fitting criteria, such as R 2 and Akaike's information criterion (corrected for small-sample bias), in our case the models with the largest mean R 2 value, the smallest mean corrected Akaike's information criterion, or both often ranked poorly for measures of model forecast performance (i.e., mean raw error, mean absolute error, and root mean square error). Although no single model performed best across all run timing groups, failure to apply an MA produced the worst outcome (for 3 of the 4 run timing groups) or second-worst outcome (for the fourth group). We provide a framework for model selection based on the relative importance of different model selection criteria and their associated performance measures. We urge scientists and managers to work closely together to develop appropriate metrics for assessing model performance and for objectively selecting forecast models that will best meet management objectives.Fisheries managers are tasked with meeting society's competing demands, including opportunities for income, employment, cultural identification, productive ecosystems, recreation, and sustenance. However, given the variability in natural systems and variation in effectiveness of management efforts, there