Effects of River Temperature on Stock- and Sex-Specific Survival of Adult Migrating Fraser River Sockeye Salmon

Mean summer water temperatures in the Fraser River (British Columbia, Canada) have increased by ~1.5-2.0^oC since the 1950’s.  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 1,474 adults from four Fraser River sockeye salmon stock-aggregates (Chilko, Quesnel, Stellako-Late Stuart and Late Shuswap).  Survival of Late Shuswap 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. We also investigated how temperature affected river section- and sex-specific survival of Late Shuswap fish. When the fish encountered cool temperatures (i.e. 14^oC), survival was lowest in the section where constricted passages and complex flows occur (i.e. Hells Gate). In contrast, survival of fish that encountered warm temperatures (i.e. 19^oC) decreased as the migration progressed upstream. No differences were detected in survival of males and females in any section when they encountered cool temperatures, but when the river was warm females survived poorer than males towards the end of the migration. Our findings emphasize the need to consider stock- and sex-specific responses to temperature and climate warming into fisheries management and conservation strategies.