Daily temperature experience and selection by adfluvial bull trout (Salvelinus confluentus)

Bull trout (Salvelinus confluentus) are a thermally sensitive cold-water species with a threatened conservation status across much of North America. To improve our understanding of bull trout thermal ecology, we tagged more than 150 adults with temperature-sensing acoustic biotelemetry transmitters and monitored the animals in a British Columbia reservoir for two years. Thermal resource selection was estimated in open water from the summer to autumn as the system transitioned to isothermal conditions. On average, bull trout thermal history tracked seasonal changes and remained close to the optimum temperatures for growth and metabolism. As summer progressed, bull trout increasingly selected temperatures of 11-15 degrees C as they became less available within the water column. Selection indicated the movement to shallower waters where 11-15 degrees C temperatures existed. The results show that bull trout mainly occupy a narrow range of temperatures which suggests the importance of cold-water refuge for the species.