EFFECTS OF TRAINING ON RESPIRATORY GAS-EXCHANGE, NITROGENOUS WASTE EXCRETION, AND FUEL USAGE DURING AEROBIC SWIMMING IN JUVENILE RAINBOW-TROUT (ONCORHYNCHUS-MYKISS)

Fuel utilization during aerobic exercise was determined in juvenile ra inbow trout that had previously undergone 2 weeks of continuous aerobi c training at 25% of their maximum sustainable speed (U-crit) and was compared with that in untrained trout. Instantaneous fuel usage was ca lculated from simultaneous measurements of oxygen consumption (M-O2) c arbon dioxide excretion (M-CO2), and nitrogenous waste excretion (M-N) . Over 58 h of sustained aerobic exercise at 55% of U-crit, M-O2 and M -CO2 remained virtually constant in the trained fish, in contrast to t he significant fall over time in untrained fish, but total gas exchang e was similar in the two groups. Aerobic respiratory quotient remained constant in the two treatments. M-N was lower in the trained fish, re sulting in a much lower (and stable) nitrogen quotient; the nitrogen q uotient increased with swimming duration in the untrained fish. Lipid was the major fuel source powering aerobic exercise in both groups. Ca rbohydrate also played a significant role, whereas protein was of lowe st quantitative importance. Training resulted in a decrease in relianc e on protein and an increase in reliance on lipid metabolism. The role of protein oxidation in supporting aerobic swimming in fish, which is smaller than commonly believed, is further reduced by training.