Impacts of cumulative thermal and fishery stressors and infection development on the health and survival of adult Pacific salmon during freshwater residence




Teffer, Amy

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Cumulative stressors influence the infection development, health and survival of wild Pacific salmon (Oncorhynchus spp.). Infectious disease is generally assumed to be the ultimate cause of death of wild adult salmon, but empirical evidence demonstrating links between infections and early mortality (i.e., prior to spawning) is lacking, especially as a function of cumulative migratory stressors. The influences of high river temperature and fishery capture and release on infection development and early mortality was explored in three Pacific salmon species. Adults were captured at river entry and held in freshwater tanks for the duration of river migration (days–weeks). Tank temperatures reflected either optimal (cool), warm (climate change scenario), or dynamic (changes in river temperature, behavioral thermoregulation) thermal conditions during migration. A subset of fish in all temperature groups was treated with a fishery bycatch release simulation (gillnet entanglement, air exposure) at the start of the holding period. We tracked shifts in physiology, immune activity and multiple infections using repeated biopsy (gill, blood) and molecular tools. Laboratory experiments were complimented by a telemetry study to assess impacts on behavior in the river. Novel application of high-throughput qPCR on nonlethally-sampled gill measured infections (bacteria, viruses, protozoa) concurrently with host immune gene expression, and was complemented by blood plasma chemistry to assess physiology. Ecologically relevant high temperatures increased mortality, infection development and stress metabolites and impaired host osmoregulatory function. Fishery stress reduced survival, especially after long entanglements and at high temperature, which reduced the capacity of individuals to resolve stress and infections. Females were more drastically affected, and mortality was delayed by more than a week. Fish with heavy infections in the river migrated more rapidly but traveled less distance. Sublethal effects of stressors included reduced migration rates and suppressed maturation indices that could delay maturity and extend river residence. Finally, river-exposed fish carried heavier infections and died sooner than those that bypassed the lower river, suggesting a causal influence of infections on early mortality. These findings support river-derived infections as causal factors contributing to the early mortality of adult Pacific salmon in fresh water and clarify its mechanisms, which comprise influences of multiple infections, sex, species, water temperature and fishery stress.



Pacific salmon, Temperature, Stress, Gene expression, Pathogens, Physiology, Disease ecology, Fisheries