Metabolic adaptations of mullet Rhinomugil corsula (Hamilton) with special reference to energy utilization

While the respiratory quotient (R.Q.) of Rhinomugil corsula in air-saturated water remains near unity at all levels of random activity, the ammonia quotient (A.Q. = volume or mole: mole relation of ammonia excreted to oxygen consumed) is higher at lower levels of activity, indicating relatively higher protein utilization in less active fish. The mean routine R.Q. values of R. corsula in air-saturated water at temperatures of 30 and 35°C are 0.91 and 0.95 respectively. Assuming that the major sources of ammonia excreted are certain predominant free amino acids, an estimate of caloric break-up of routine meta-bolism of R. corsula suggests that the fish is completely aerobic in air-saturated water and that it derives 14, 44 and 42% of energy from proteins, carbohydrates and fats respectively from a total of 567 cal/kg fish/h at 30°C (15, 55 and 30% at 35°C). At oxygen concentrations below air saturation, R.Q. and A.Q. values of R. corsula increase with decrease in ambient oxygen indicating an increase in anaerobic metabolism and protein utilization. The correlation between the trends of R.Q. and A.Q. levels are similar to that observed earlier in Tilapia mossambica. During hypoxia (1.6 mg O₂/l at 30°C) R. corsula derives about half its energy for sustenance and activity anaerobically. The mullet does not appear to accumulate an oxygen debt during hypoxia.