We compared the thermal sensitivity of oxidative muscle function between the eurythermal Atlantic salmon (Salmo salar) and the more stenothermal Arctic char (Salvelinus alpinus; which prefers cooler waters). Power output was measured in red skeletal muscle strips and myocardial trabeculae, and efficiency (net work/energy consumed) was measured for trabeculae, from cold (6oC) and warm (15oC) acclimated fish at temperatures from 2-26oC. The mass-specific net power produced by char red muscle was greater than in salmon, by 2-5 fold depending on test temperature. Net power first increased, then decreased, when the red muscle of 6oC-acclimated char was exposed to increasing temperature. Acclimation to 15oC significantly impaired mass-specific power in char (by ∼40-50%) from 2 to 15oC, but lessened its relative decrease between 15 and 26oC. In contrast, maximal net power increased, and then plateaued, with increasing temperature in salmon from both acclimation groups. Increasing test temperature resulted in a ∼3-5 fold increase in maximal net power produced by ventricular trabeculae in all groups, and this effect was not influenced by acclimation temperature. Nonetheless, lengthening power was higher in trabeculae from warm acclimated char, and char trabeculae could not contract as fast as those from salmon. Finally, the efficiency of myocardial net work was approximately 2-fold greater in 15oC acclimated salmon than char (∼15 vs. 7%), and highest at 20oC in salmon. This study provides several mechanistic explanations as to their inter-specific difference in upper thermal tolerance, and potentially why southern char populations are being negatively impacted by climate change.

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