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In an effort to explore the thermal limitations of Colorado River cutthroat trout Oncorhynchus clarkii pleuriticus, the critical thermal maxima (Tcmax) of 1+ year Lake Nanita strain O. c. pleuriticus were evaluated when acclimated to 10, 15 and 20°
C. The mean ±s.d.Tcmax for O. c. pleuriticus acclimated to 10° C was 24·6 ± 2·0°C (n = 30), for 15° C‐acclimated fish was 26·9 ± 1·5° C (n = 23) and for 20°
C‐acclimated fish was 29·4 ± 1·1° C (n = 28); these results showed a marked thermal acclimation effect (Q10 = 1·20). Interestingly, there was a size effect within treatments, wherein the Tcmax of larger fish
was significantly lower than that of smaller fish acclimated to the same temperature. The critical thermal tolerances of age 0 year O. c. pleuriticus were also evaluated from three separate populations: Lake Nanita, Trapper Creek and Carr Creek reared under ‘common‐garden’
conditions prior to thermal acclimation. The Trapper Creek population had significantly warmer Tcmax than the Lake Nanita population, but that of the Carr Creek fish had Tcmax similar to both Trapper Creek and Lake Nanita fish. A comparison of these O.
c. pleuriticus Tcmax results with those of other stream‐dwelling salmonids suggested that O. c. pleuriticus are less resistant to rapid thermal fluctuations when acclimated to cold temperatures, but can tolerate similar temperatures when acclimated to warmer temperatures.