Cardiorespiratory physiology and swimming capacity of Atlantic salmon (Salmo salar) at cold temperatures

We investigated how acclimation to 8, 4 and 1°C, and acute cooling from 8 to 1°C, affected the Atlantic salmon's aerobic and anaerobic metabolism, and cardiac function, during a critical swim speed (U(crit)) test. This study revealed several interesting temperature-dependent effects. First, while differences in resting heart rate (f(H)) between groups were predictable based on previous research (range ∼28–65 beats min(−1)), with values for 1°C-acclimated fish slightly higher than those of acutely exposed conspecifics, the resting cardiac output ([Image: see text]) of 1°C-acclimated fish was much lower and compensated for by a higher resting blood oxygen extraction (Ṁ(O(2))/[Image: see text]). In contrast, the acutely exposed fish had a ∼2-fold greater resting stroke volume (V(S)) compared with that of the other groups. Second, increases in f(H) (1.2- to 1.4-fold) contributed little to [Image: see text] during the U(crit) test, and the contributions of [Image: see text] (V(S)) versus Ṁ(O(2))/[Image: see text] to aerobic scope (AS) were very different in the two groups tested at 1°C (1°C-acclimated and 8–1°C fish). Finally, U(crit) was 2.08 and 1.69 body lengths (BL) s(−1) in the 8 and 4°C-acclimated groups, but only 1.27 and 1.44 BL s(−1) in the 1°C-acclimated and 8–1°C fish, respectively – this lower value in 1°C versus 8–1°C fish despite higher values for maximum metabolic rate and AS. These data: support recent studies which suggest that the capacity to increase f(H) is constrained at low temperatures; show that cardiorespiratory function at cold temperatures, and its response to increased demands, depends on exposure duration; and suggest that AS does not constrain swimming capacity in salmon when chronically exposed to temperatures approaching their lower limit.