Using Stock-Specific Thermal Physiology to Simulate In-River Mortality of Fraser River Sockeye Salmon.

Annual estimates of mortality en-route to spawning grounds for certain populations of Fraser River Sockeye salmon (Oncorhynchus nerka) can routinely exceed 50% posing challenges for harvest management and salmon conservation. High estimates of mortality are also correlated with high migration temperatures, suggesting temperature is one of the main factors. However, these estimates of apparent mortality are uncertain as they currently rely on discrepancies between up-river spawning ground estimates and lower river escapement estimates adjusting for estimates of in-river catch. Therefore, applying thermal physiology may serve as a tool to explain the potential contribution temperature-related mortality could have in calculating annual apparent mortality estimates for specific populations. I incorporated short- and long-term mortality functions based on population-specific aerobic scope temperature thresholds and an aggregate accumulation of degree-days threshold to a simulation model to estimate en-route mortality associated with temperature exposure for six Fraser River Sockeye salmon populations: Early Stuart, Gates Creek, Stellako, Chilko, and Weaver Creek. I compared simulated temperature based mortality rates to apparent mortality estimates (i.e. difference between estimates) and tested model sensitivity to uncertainty in short- and long-term LD50, arrival timing, and movement rate parameters. Results show that high temperature is likely a key driver of large en-route loss as both simulated mortality and apparent mortality estimates were higher in warmer years and lower in cooler years. I attribute the simulated mortality rates being generally lower than apparent mortality to the role that other sources of mortality can play (e.g. source error, high discharge). Simulated mortality rates were most sensitive to the short-term LD50 parameter, followed by the long-term LD50 and arrival timing parameters. However, simulated mortality rates were not sensitive to changes in movement rates. The model can explain temperature-related population-specific differences in apparent mortality between co-migrating populations (e.g., up to 80% absolute differences between Chilko and co-migrating populations) and provides evidence that these differences are driven by differences in aerobic scope. My results could inform managers of the relative importance of key parameters (short- and long-term mortality, and arrival timing) when estimating population-specific temperature-related mortality.