Juvenile Steelhead Response to Spatio-Temporal Dynamics in Thermal Refugia On the Klamath River

As climate change and land-use impacts drive increases in river temperatures, thermal refugia created by coolwater tributary inputs are becoming increasingly important habitat for coldwater fish species. Summer mainstem temperatures on the Klamath River in northern California often reach physiologically stressful levels (~22-26°C) for native salmonid populations, driving juvenile salmonids into thermal refugia. These refugia are dynamic environments, and habitat quality may fluctuate in the refuge as well as in the surrounding sub-optimal mainstem habitat. The goal of this study was to determine which environmental factors are driving juvenile steelhead use of thermal refugia, and the timescale at which fish are shifting between thermal environments, in order to gain a better understanding of when these areas become critical habitat and how to effectively manage and restore thermal refugia on warming rivers. I used an innovative approach, taking advantage of relatively new radio tag technology to infer fish location from body temperature. I tagged juvenile steelhead at thermal refugia sites with temperature-sensitive radio tags to track their body temperature throughout the summer, and correlated fish body temperature with simultaneous water temperatures to ascertain habitat use. I found that juvenile steelhead used both refugia and mainstem habitat, even when mainstem temperatures reached sub-lethal levels. The strongest drivers of juvenile steelhead thermal refugia use were mainstem water temperature, thermal heterogeneity, and time of day. Steelhead primarily stayed in thermal refugia at night, whereas daytime refugia use increased with rising mainstem temperature and higher levels of thermal heterogeneity. These results raise interesting questions regarding what factors may be driving juvenile steelhead to leave refugia at thermally stressful mainstem temperatures, and suggest that there may be a trade-off between thermal benefits and other resources, such as prey availability.