RESPIRATORY GAS-EXCHANGE, NITROGENOUS WASTE EXCRETION, AND FUEL USAGEDURING AEROBIC SWIMMING IN JUVENILE RAINBOW-TROUT

The types of fuel burned by juvenile rainbow trout (17 g) during a 58- h period of aerobic sustained exercise were studied by respirometry. A ttempts to measure fuel usage by depletion (the compositional approach ) in these same fish were unsuccessful due to lack of detectable chang es in proximate body composition. O-2 consumption, CO2 excretion, and nitrogenous waste excretion (ammonia-N plus urea-N) were measured in i ndividual fish swum continuously at 55% and 80% of maximum sustainable swimming speed and in non-swimming controls. O-2 consumption and CO2 excretion increased with swimming speed, and decreased over time. Abso lute rates of N excretion were independent of swimming speed and time. Instantaneous aerobic fuel use, as calculated from the respiratory qu otients and nitrogen quotients, was approximately 47% lipid, 30% prote in, and 23% carbohydrate in non-swimmers at the start of the experimen t. With increased swimming speed there was no change in absolute rates of protein oxidation, while lipid and carbohydrate oxidation both inc reased. Therefore, the relative protein contribution decreased with in creasing speed but increased with swimming duration as the oxidation o f other fuels declined over time. However, lipid oxidation predominate d at all speeds and at all times. The relative contribution of carbohy drate increased with swimming speed and decreased over time. These res ults suggest that swimming becomes more efficient over time and help r esolve uncertainties in the literature. We conclude that lipid is the main fuel of aerobic exercise, that protein catabolism is kept at mini mum levels necessary for maintenance, and that carbohydrate oxidation becomes more important with increased white muscle recruitment at high er speed.