Dietary manipulations affecting growth and nitrogenous waste production of red drum, Sciaenops ocellatus II. Effects of energy level and nutrient density at various feeding rates.
Two experiments were conducted with red drum in recirculating, 110-litre aquaria to examine the effects of increasing dietary energy levels, and altering dietary energy and protein levels in conjunction with feeding rates on weight gain, body composition and ammonia production. In the first experiment, juvenile red drum (35 g) were fed on diets containing 45% CP and DE 15.1 to 18.4 kJ/g for 6 weeks. In the second experiment, smaller (3.5 g) red drum were fed on diets with digestible protein and DE levels (DP/kJ) of 33/13, 40/15.5 and 50/18 at rates of 6, 5 and 4% of body weight (BW)/day, respectively, for 8 weeks. The 40/15.5 diet was also provided to apparent satiation. Each treatment was fed in triplicate and fish were weighed weekly to adjust feeding rates. Results from the first experiment indicated there were no significant differences in weight gain or feed conversion efficiency of red drum fed the various energy levels. However, intraperitoneal fat deposition increased with an increase in dietary energy above 15.1 kJ/g, but no trends were evident in other body indices or tissue composition. Ammonia production, measured at 2-h intervals postprandial, was significantly reduced at 6-h post-feeding in fish fed the highest energy level. Plasma glucose and activities of enzymes involved in ammonia metabolism were unaffected by dietary energy levels. In the second experiment, weight gain did not differ significantly between fish fed diets containing 40 or 50% DP, but was lower for fish fed the diet containing 33% DP. Fish fed the diet with least protein also had the lowest feed and protein efficiencies. While satiation feeding resulted in similar weight gain as feeding at a fixed rate, the efficiency of feed and protein utilization was improved with satiation feeding. There were no clear trends in body indices or ammonia production of fish subjected to the various regimes. In conclusion, higher dietary energy levels did not improve growth, but did lower nitrogenous waste production of red drum. Increasing dietary protein in conjunction with increasing energy levels also improved growth and efficiency of feed utilization.