Adult Pacific Salmonids in Hot Water: Behavior and Survival Studies from the Columbia River Basin

Rising temperatures and water management practices in western North America have decreased the extent of anadromous salmonid freshwater habitats.  Earlier spring warming, later fall cooling and higher mean and maximum temperatures affect salmonids at multiple scales, from individual fish behavior to fitness effects on regional populations.  From 1996-2010, we conducted a series of radiotelemetry and archival tag studies of adult Chinook salmon, sockeye salmon, and steelhead in the Columbia River and its major tributaries.  We used thousands of reconstructed migration histories to investigate the relationships among water temperatures, adult migration behaviors, and survival to spawning.  In the lower Columbia River migration corridor, many tagged salmon and steelhead behaviorally thermoregulated during the warmest periods by using non-natal cool-water tributaries as refugia from the warm main stem Columbia River.  Threshold temperatures of 19-21°C prompted extensive refugia use, with some fish (especially steelhead) holding for several weeks or more.  Thermal refugia along adult migration corridors likely provide a mix of physiological and energetic benefits for fish, and these small, patchily-distributed sites appear to be critical temporary habitats for many migrants.  However, dense fish concentrations at refugia sites attract intensive fisheries and refugia use has resulted in lower migration survival for some populations.  In several of our studies, we have found links between elevated prespawn mortality and exposure to high water temperature during migration and near spawning areas.  Negative temperature effects have typically been most severe for fish with additional risk factors, including poor initial physical condition, disease or parasite loading, and/or late migration timing.  Importantly, entire study populations had high prespawn mortality rates in some years. Annual estimates exceeded 55% for endangered Snake River sockeye salmon and have been more than 80% for threatened spring-run (Middle Fork Willamette River) and summer-run (South Fork Salmon River) Chinook salmon.  Our combined results highlight both the vulnerability and adaptability of adult salmonids to warming freshwater environments.  They also demonstrate the importance of understanding among-population differences, effects of refugia use on vulnerability to fisheries mortality, and a need to further investigate interactions among water temperature, behavior, physiology, and mortality risk factors in adult migrants.