Disease Potential in Coho Salmon: Effects of Water Temperature and Fisheries Capture/Release

The continued propagation of Pacific salmon fisheries is currently in question due to uncertainty related to disease, rising river temperatures, and fisheries demand. The life cycle of Pacific salmon requires them to leave the ocean as adults and return to their natal rivers where they spawn and die. Viruses, bacteria and other microorganisms present in the ocean, river and estuary infect salmon throughout their life cycle but can become pathogenic and potentially cause disease and premature mortality of adult fish during freshwater migration. Elevated water temperature has been shown to decrease the resiliency of salmon in freshwater and advance pathogen infections and disease. Capture and release from in-river fisheries also presents physiological challenges to fish. Entanglement and handling injuries create sites of opportunity for infection in addition to eliciting stress responses in fish. Disease-related delayed mortality yields an unknown contribution to total mortality within returning salmon stocks currently counted as escapement. This knowledge gap creates the potential for the harvest of Pacific salmon to be presently unsustainable and requires immediate attention. This study is the first quantitative laboratory assessment of the combined effects of elevated water temperature and fisheries capture/release on the natural pathogen load and disease progression in Fraser River Pacific salmon. The first installment of this comprehensive evaluation of multiple salmon species evaluated the response of Fraser River coho salmon (Oncorhynchus kisutch) to these stressors and has yielded preliminary results supportive of expected mortality. Overall, mortality was greater among fish held at high temperature (40%) than those held at optimum temperature (6%), and was further amplified in fish exposed to fisheries simulation at high temperature (65% within treatment). These results are suggestive of an interaction between these two stressors. The information gained from this research will be widely applicable across species and regions with direct implications for management.