Adult Salmon and Steelhead in Hot Water: Insights from Individual-Based Biotelemetry Paired with Archival Temperature Loggers

Rising river temperatures in western North America have increased the vulnerability of many Pacific salmon (Oncorhynchus spp.) populations, and there is a need to predict and manage risk for these groups.  In a series of studies, we estimated thermal exposure of adult summer steelhead (O. mykiss) and spring- and fall-run Chinook Salmon (O. tshawytscha) as they migrated through the Columbia, Willamette, and Snake River basins (Washington-Oregon).  Archival temperature loggers and radiotelemetry were used to reconstruct spatially-explicit thermal histories for several hundred migrants and the histories were associated with migration behaviors and survival.  Along mainstem migration corridors, many fish behaviorally thermoregulated during the warmest periods, using non-natal, cool-water tributaries as thermal refuge sites.  Threshold temperatures of 19-21°C prompted refuge use, with some fish (especially steelhead) using sites for several weeks.  In the Willamette River, most (~65%) spring-run salmon encountered potentially stressful conditions (≥18°C) in lower river reaches, whereas Snake River steelhead and fall-run salmon encountered very warm conditions (means >19.5°C), primarily inside dam fishways.  Adult exposure to warm water was often associated with reduced survival to spawning. These individual-based studies have helped address long-standing questions regarding how warmer rivers may affect salmon and steelhead migration phenology, physiology, survival, and reproductive success.