Responses of juvenile and sexually mature two-summer-old salmon ( Salmo salar L.) to prolonged swimming

Abstract

Two-summer-old juvenile and sexually mature Atlantic salmon were forced to swim for 0.5, 1, 2, 4, 8 or 12 h at 50 cm/s (ca. 2.5 body length/s). Collapsed fish and fish swimming the whole test time were sampled for physiological analyses. Both the juvenile and mature fish exhibited an initial stress response to increased flow, as shown by the elevated plasma cortisol and lactate concentrations and by the changes in erythrocytic ATP and chloride concentrations. The physiological state of the mature males recovered well within 2–4 h of swimming. In contrast, the juveniles had continuously high plasma cortisol and lactate values with a large individual variation, indicating that the energy production for swimming was partially anaerobic. The collapsed fish showed clear signs of anaerobic metabolism. The results indicate that the aerobic capacity for prolonged swimming is lowered in juvenile salmon also beyond the time of smolt migration, whereas the mature males have a swimming capacity similar to wild parr. However, the responses of juvenile salmon to swimming in autumn are not similar to those observed on migrating smolt.