Evaluating Factors Related to Inriver Adult Mortality in Copper River Sockeye Salmon

This project evaluated potential biological and environmental factors shaping spawning migration success by sockeye salmon of the Copper River in southcentral Alaska – a highly glaciated watershed that supports commercial, subsistence, and personal-use sockeye fisheries. We tagged a total of 322 individuals and detected a positive relationship between sockeye body size and energy density and the likelihood of successful passage through Wood Canyon, suggesting that long-term declines in sockeye body size and energetic condition may translate into lower migratory success in-river. We also found that energy provisioning among individual river populations is commensurate with the length of their migration. Compared to our results in 2021, the sockeye salmon radio tagged during 2019 had markedly lower migratory success (i.e., elevated enroute mortality losses), elevated transcriptomic responses for hypoxia stress (possibly from physical exertion), viral disease development (apparently associated with Infectious Hematopoietic Necrosis Virus, IHNV, infection), and a lower elevated transcriptomic response for thermal stress. Using output from an existing hydrology model, we predicted an increasing likelihood of impassable conditions in the lower river as a result of accelerated glacial melting leading to critically high flows impeding adult sockeye salmon migrants.

As the climate changes, addressing escapement quality, in addition to escapement quantity, is becoming an increasingly important component of contemporary fisheries management. The evaluation of the risk of inriver mortality and reproductive performance in CR sockeye will help managers determine if the discrepancy between lower and upper river escapement estimates could have a biological or ecological basis. This analysis could result in allowances for additional levels of escapement through harvest management to ensure sustainability of run-timing groups or populations that may be a concern, thus ensuring that subsistence opportunities are protected and maximized.