Selection for upper thermal tolerance in rainbow trout (Oncorhynchus mykiss Walbaum)

Rainbow trout (Oncorhynchus mykiss Walbaum) in southern Western Australia have undergone passive selection for over 19 generations to survive high water temperatures. Based on the conceptual model of 'oxygen-and capacity-limited thermal tolerance', we measured critical thermal maximum (CTmax), maximum heart rate (f(H,max)) and aerobic scope to test the hypothesis that these rainbow trout can maintain aerobic scope at high temperatures through a robust cardiac performance supporting oxygen delivery. Across five family groups CTmax averaged 29.0 +/- 0.02 degrees C. Aerobic scope was maximized at 15.8 +/- 0.3 degrees C (T-opt), while the upper pejus temperature (T-pej, set at 90% of maximum aerobic scope) was 19.9 +/- 0.3 degrees C. Although aerobic scope decreased at temperatures above T-opt, the value at 25 degrees C remained well over 40% of the maximum. Furthermore, pharmacologically stimulated f(H,max) increased with temperature, reaching a peak value between 23.5 +/- 0.4 and 24.0 +/- 0.4 degrees C(T-max) for three family groups. The Arrhenius breakpoint temperature (T-AB) for f(H,max) was 20.3 +/- 0.3 to 20.7 +/- 0.4 degrees C, while the average Q(10) breakpoint temperature (T-QB, when the incremental Q(10)<1.6) for f(H,max) was 21.6 +/- 0.2 to 22.0 +/- 0.4 degrees C. Collectively, f(H,max) progressively became less temperature dependent beyond 20 degrees C (T-AB andT(QB)), which coincides with the upper T-pej for aerobic scope. Although upper thermal performance indices for both aerobic scope and f(H,max) were compared among family groups in this population, appreciable differences were not evident. Compared with other populations of rainbow trout, the present assessment is consistent with the prediction that this strain has undergone selection and shows the ability to tolerate higher water temperatures.