Studies have predicted declines in trout habitat of over 50% given projected climate warming scenarios. Therefore, knowledge of the performance of trout strains at elevated temperature is critical for management of trout fisheries and trout strains under climate change. Our objectives were to compare the performance of three strains of Rainbow Trout Onchorynchus mykiss at elevated temperatures. We compared growth, consumption, and feed efficiency at 20°C and 22°C, and critical thermal maxima (CTM) among the strains. The Case Western strain is registered as a warm‐tolerant strain with the National Fish Strain Registry, and it did achieve a significantly higher CTM (31.29°C) than the Kamloops (31.14°C) but not the Wytheville (31.20°C) strain. Such small differences are likely not biologically significant. While the CTM of the Case Western strain was higher than that of the Kamloops, the Case Western strain consistently performed poorer than the other strains in feeding and growth experiments at 20°C and 22°C. Our results add to the growing body of literature suggesting that differences in thermal performance do exist between strains of trout. While there is debate about whether these differences are due to evolutionary adaptation to local environmental conditions or phenotypic plasticity, the differences being found in thermal performance of trout suggests future research aimed at improving thermal performance via thermal testing and selective breeding may yield stock with improved tolerance and growth at warmer temperatures. Received April 17, 2014; accepted June 24, 2014

Assessing the Thermal Tolerance and Growth Potential at Elevated Temperatures of Three Different Strains of Rainbow Trout (Oncorhynchus mykiss) Michael A. Porto This thesis examines how critical thermal maxima (CTM) differs between different strains of rainbow trout (Oncorhynchus mykiss) and explores the relationship between thermal tolerance and consumption, conversion efficiency, and growth rates at elevated temperatures. This thesis is comprised of three chapters; (1) an introduction and literature review on studies of temperature tolerance and growth in salmonids, (2) an experimental study investigating the CTM of three different strains of rainbow trout, and (3) an experimental study investigating the correlation between thermal tolerance and consumption, food conversion efficiency (FCE) and growth at elevated temperatures. For the first experimental study, we quantified the CTM of the Kamloop, Wytheville and Case Western strains of rainbow trout. The Case Western strain was of particular interest to us because it was thought to possess a higher thermal tolerance than other common strains of trout. We found that there were significant differences between the CTMs of the three strains and that the Case Western stain did have a higher thermal tolerance than the Kamloop and Wytheville strains (Mann-Whitney U; p < 0.003). We also found a significant positive correlation between CTM and condition factor (linear regression; p < 0.001). In the second study, we determined consumption, FCE, and growth rates at temperatures of 20 and 22:C for individuals who's CTMs had previously been determined. We found that there were significant differences between the consumption, FCE, and growth rates of the three strains at both temperatures (Two-way ANOVA; p < 0.001). In general, the Case Western strain consumed, converted less efficiently, and grew less than the other two strains at both temperatures. We also found no significant correlations between individual CTM and consumption, FCE, or growth at either temperature. There was a slight trend in these correlations however. As individual CTM increased, consumption, FCE, and growth rates tended to decrease. This may indicate some tradeoff between the ability to survive and grow at these stressful temperatures.