The Effects of Semichronic Thermal Stress on Physiological Indicators in Steelhead

The physiological response of juvenile steelhead Oncorhynchus mykiss to prolonged heat stress was examined by exposing replicated groups of fish to three different temperatures, 15, 23, and 25 degrees C, for 25 consecutive days followed by a 55-d recovery period at 15 degrees C. We found that at temperatures 25 degrees C, steelhead consumed significantly less food per day and had elevated feed conversion rates but experienced slower growth, reduced body size, lower body fat, and elevated heat shock protein (hsp) 70 levels relative to fish exposed to 23 degrees C and 15 degrees C. Growth decreased 24.4% and 27.1% for length and mass, respectively, between 15 degrees C and 23 degrees C, and an additional 60% and 56.5%, respectively, between 23 degrees C and 25 degrees C during exposures. While growth increments and lipid levels recovered to control levels after water temperature was reduced, body size of the fish exposed to 25 degrees C lagged throughout the experiment. Our results indicate a temperature threshold after which steelhead exposed to semichronic thermal stress incur a physiological debt. Heat shock protein 70 levels were detectable up to 25 d poststress in fin and liver tissues. This result provides evidence that hsp 70 is a useful metric for measuring thermal stress that can be assessed nonlethally and used to monitor thermal-habitat restoration efforts for threatened and endangered salmonids. Received September 26, 2012; accepted May 13, 2013