Energy utilization in mullet,Rhinomugil corsula, exercised continuously for 5 h at different swimming speeds ranging between 20 and 77 cm s−1 was studied in a tunnel type apparatus. Oxygen consumption, carbon dioxide output and respiratory quotient decreased with increase in the duration of exercise. The initial (1st h) respiratory quotient, which was always above unity, increased with intensity of swimming speed, but the steady (5th h) respiratory quotient remained below unity, at about the same level irrespective of the increase in activity.
Nitrogen (NH3 and total) excretion, ammonia quotient and nitrogen quotient increased with both duration and intensity of exercise, indicating the increased protein utilization during the later phase of exercise.
An attempt has been made to estimate the relative energy derivation from different substrates with some assumptions for the entire 5 h of exercise at 20 and 77 cm s−1. At 20 cm s−1 the energy derived aerobically from proteins, carbohydrates and fats and that from anaerobic source (carbohydrate) were estimated as 13040·3, 1378·2, 2367·3 and 1048·5 J kg−1 fish, being 73·1, 7·7, 13·3 and 5·9% of a total of 17834·3 J kg−1 fish, whereas at a swimming speed of 77 cm s−1 it was estimated as 33587·9, 2841·4 and 1305·4 J kg−1 fish from proteins, fats and anaerobic source respectively, being 89·0, 7·5, 3·5% of a total of 37734·7 J kg−1 fish.