A Bioenergetics Approach to Assessing Steelhead Use of Thermal Refugia in the Klamath River, CA

Late summer and early fall water temperatures on the Klamath River can reach levels that are physiologically stressful to endangered and threatened salmonids. At the limits of their thermal tolerance, salmonids may behaviorally thermoregulate by moving to localized patches of colder water, or thermal refugia. The presence of these refugia may be key to salmonid survival, especially during periods of elevated mainstem temperatures when refugia may be the only usable habitat available. However, the dynamics and importance of thermal refugia are not thoroughly understood. While fish obtain thermal benefits by using refugia, potential trade-offs include lower food availability, decreased growth rates, and increased risk of disease due to high fish densities. The interplay between these factors influences how much time a fish chooses to spend in the mainstem river versus the refugia. My research focuses on defining the mechanisms driving steelhead thermal refugia use at both landscape and local scales, and the implications refugia use has for steelhead growth and survival. Previous studies on Klamath River refugia have not explored the potential importance of diurnal temperature variations in determining juvenile steelhead survival. The energetic costs of surviving at daily maximum temperatures may make these temperatures critical for determining overall salmonid survival. Knowledge of individual salmonid behavior surrounding refugia is key to understanding how factors such as diurnal temperature variations and heterogeneous food availability impact salmonid growth and survival. To address these issues, I conducted radio tracking studies to measure fine-scale spatiotemporal use of thermal refugia and the adjacent mainstem river by individual juvenile steelhead (Oncorhynchus mykiss). I will correlate these data with a high-resolution stream temperature model currently being developed by NMFS, which tracks Klamath mainstem diurnal temperature fluctuations. In addition, I collected diet and prey availability data, and the results of my fieldwork will be integrated into a temperature and food driven bioenergetics model. This will allow me to gain a mechanistic understanding of the relative importance and long-term consequences of the factors affecting salmonid use of thermal refugia.