Behavioral thermoregulation and associated mortality trade-offs in migrating adult steelhead (Oncorhynchus mykiss): variability among sympatric populations

We used radiotelemetry to assess thermoregulatory behaviors for 14 Populations (n = 3985) of adult summer steelhead (Oncorhynchus mykiss) as they passed through the Columbia River migration corridor. Steelhead use of small cool-water tributaries (thermal refugia) rapidly increased when the Columbia River reached a temperature threshold of about 19 degrees C. When main stein temperatures were warmest (i.e., >21 degrees C), more than 70% of the tagged fish used refugia sites and these fish had median refugia residence times of 3-4 weeks. Thermoregulatory responses were similar across populations, but there were large among-population differences in the incidence and duration of refugia use likely linked to population-specific migration timing patterns. In survival analyses using 1285 known-origin steelhead, fish that used thermal refugia were significantly less likely to survive to natal basins, were harvested at relatively high rates in refugia tributaries, and had greater unknown mortality in the main stein. These results highlight the trade-off between the presumed physiological benefits of thermal refugia use and a likely increase in harvest and other mortality risks that arise when preferred thermal habitats are severely constricted.