Temperature effects on metabolic rate, swimming performance and condition of Pacific cod Gadus macrocephalus Tilesius

Critical swimming speed (U_crit) and rate of oxygen consumption of Pacific cod Gadus macrocephalus acclimated to 4 and 11°C were determined to assess the influence of water temperature on performance. The physiological effect of exercise trials on fish held at two temperatures was also assessed by comparing haematocrit and plasma concentrations of cortisol, metabolites and ions collected from fish before and after testing. The U_crit of fish acclimated and exercised at 4°C did not differ from those acclimated and exercised at 11°C [1.07 body lengths (total length) s^-1]. While the standard metabolic rate of 11°C acclimated fish was 28% higher than that of 4°C fish, no significant difference was observed between fish acclimated at the two temperatures. Plasma concentrations of cortisol, glucose and lactate increased significantly from pre- to post-swim in both groups, yet only concentrations of cortisol differed significantly between temperature treatments. Higher concentrations of cortisol in association with greater osmoregulatory disturbance in animals acclimated at the lower temperature indicate that the lower water temperature acted as an environmental stressor. Lack of significant differences in U_crit between temperature treatments, however, suggests that Pacific cod have robust physiological resilience with respect to swimming performance within temperature changes from 4 to 11°C.