Respiratory gas exchange, nitrogenous waste excretion, and fuel usage during aerobic swimming in juvenile rainbow trout

Abstract

The types of fuel burned by juvenile rainbow trout (17 g) during a 58-h period of aerobic sustained exercise were studied by respirometry. Attempts to measure fuel usage by depletion (thecompositional approach) in these same fish were unsuccessful due to lack of detectable changes in proximate body composition. O₂ consumption, CO₂ excretion, and nitrogenous waste excretion (ammonia-N plus urea-N) were measured in individual fish swum continuously at 55% and 80% of maximum sustainable swimming speed and in non-swimming controls. O₂ consumption and CO₂ excretion increased with swimming speed, and decreased over time. Absolute rates of N excretion were independent of swimming speed and time.Instantaneous aerobic fuel use, as calculated from the respiratory quotients and nitrogen quotients, was approximately 47% lipid, 30% protein, and 23% carbohydrate in non-swimmers at the start of the experiment. With increased swimming speed there was no change in absolute rates of protein oxidation, while lipid and carbohydrate oxidation both increased. Therefore, the relative protein contribution decreased with increasing speed but increased with swimming duration as the oxidation of other fuels declined over time. However, lipid oxidation predominated at all speeds and at all times. The relative contribution of carbohydrate increased with swimming speed and decreased over time. These results suggest that swimming becomes more efficient over time and help resolve uncertainties in the literature. We conclude that lipid is the main fuel of aerobic exercise, that protein catabolism is kept at minimum levels necessary for maintenance, and that carbohydrate oxidation becomes more important with increased white muscle recruitment at higher speed.