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

We investigated how acclimation to 8, 4 and 1 degrees C, and acute cooling from 8 to 1 degrees 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 similar to 28-65 beats min(-1)), with values for 1 degrees C-acclimated fish slightly higher than those of acutely exposed conspecifics, the resting cardiac output (Q_) of 1 degrees C-acclimated fish was much lower and compensated for by a higher resting blood oxygen extraction ( M. (O2)/Q_). In contrast, the acutely exposed fish had a similar to 2-fold greater resting stroke volume (VS) compared with that of the other groups. Second, increases in f(H) (1.2-to 1.4-fold) contributed little to Q_ during the U-crit test, and the contributions of _Q (V-S) versusM. (O2)/Q_ to aerobic scope (AS) were very different in the two groups tested at 1 degrees C (1 degrees C-acclimated and 8-1 degrees C fish). Finally, U-crit was 2.08 and 1.69 body lengths (BL) s(-1) in the 8 and 4 degrees C-acclimated groups, but only 1.27 and 1.44 BL s(-1) in the 1 degrees C-acclimated and 8-1 degrees C fish, respectively - this lower value in 1 degrees C versus 8-1 degrees C fish despite higher values for maximum metabolic rate and AS. These data: support recent studies which suggest that the capacity to increase fH 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.