The effect of thermal experience on the survival of sockeye salmon (Oncorhynchus nerka) during their spawning migration in The Fraser River, BC

Sockeye salmon (Oncorhynchus nerka) hold tremendous cultural, economic, and ecological value. The Fraser River, in British Columbia supports the largest return of sockeye salmon in Canada. The water temperature in the Fraser River has been increasing in recent decades, with an estimated increase of 1.5°C and 3°C in mean and maximum temperatures, respectively, since the 1950s and a significant increase in the number of days above critical thermal threshold for sockeye salmon. Sockeye salmon are ectotherms, meaning their physiology is highly dependent on their external environment and driven by changes in external temperature. Previous studies have shown that sockeye salmon experience significant impairment and mortality between 18°C and 21°C, and that the impacts of water temperature on survival vary between sexes and populations. Data for this study were collected by LGL Limited, UBC, Kintama, and DFO researchers and their partners. 3265 sockeye salmon were captured and either tagged with radio or acoustic transmitters in 2002, 2003, 2006, 2010, and 2011 and their migration was tracked using receiver arrays. Field-based studies that aim to investigate how water temperature affects survival during the adult spawning migration pose several challenges: 1) what aspect of the thermal experience is assessed, 2) imperfect detection can influence the outcome of studies relying on tagged fish, and 3) populations with small numbers of tagged fish can create unreliable estimates. I aim to address these challenges by 1) comparing the effect of three components of thermal experience, 2) by using an integrated travel time model and state-space version of the Cormack Jolly Seber (CJS) model to differentiate survival and detection, and 3) using a hierarchical mixed effects model to improve the survival probability estimates of populations with low sample sizes. The three aspects of thermal experience that were tested were: 1) the average temperature of the first 10 days following entry into the Fraser River, 2) the moving average of river temperature up to the final detection, and 3) the number of days above 18°C up to the final detection. The best fitting model, of the moving average river temperature prior to the final detection, was selected. The highest overall migration survival probability was found in the Harrison River Conservation Unit (CU), while the lowest overall migration survival probability was estimated in the Anderson-Seton CU. Across all CUs the overall migration survival probability estimate was lower for females than males. Male and female sockeye salmon reached 50% survival probability at ~18.5-20°C and ~18°C-19°C average temperature, respectively. These results align with previous research indicating sockeye salmon survival is significantly impaired between 18°C-21°C. These results also provide evidence to support conservation and enhancement of thermal refuges as well as conservation unit and sex-specific escapement targets and management adjustments.