Effects of dietary protein level and form on production characteristics and ammonia excretion of red drum Sciaenops ocellatus

The red drum traditionally has been an important commercial and recreational fish species in the Gulf of Mexico; therefore, its aquacultural production for food and for stock enhancement continues to develop. This laboratory has addressed refinements in protein and amino acid nutrition of this species in an effort to increase the efficiency of production while limiting nitrogenous wastes. In the present experiment, control diets at 35% and 45% crude protein (CP) contained only intact protein provided by a 50/50 mixture of red drum muscle and wheat gluten. Two experimental diets at each CP level contained the mixture (60% of CP) and crystalline amino acids (40% of CP) to provide lysine at the same level when expressed as a percent of crude protein. All diets contained 11% lipid by dry weight and an estimated 3.6 kcal digestible energy/g. The culture system consisted of 110-l aquaria in a recirculating mode containing brackish (7 per thousand) water at 27 ± °C. Twenty acclimated juvenile red drum initially averaging 3.4 g/fish were stocked in each aquarium. Diets were fed to fish in triplicate tanks at a fixed rate approaching apparent satiation twice daily for 6 weeks. At the end of the feeding trial, fish were weighed and three were euthanized for body composition analysis. Fish remaining in each tank were used to determine total ammonia nitrogen (TAN) excretion at 4-h postprandial. Diets supplemented with crystalline amino acids supported similar weight gain (94-98%) as that obtained by fish fed control diets with intact protein; however, fish fed the 45% CP diets had significantly greater weight gain than those fed the 35% CP diets. Fish fed the diets with 45% CP excreted significantly more ammonia than those fed 35% CP diets. The crystalline amino acid diets at each CP level also resulted in elevated ammonia excretion compared to intact protein diets. Thus, crystalline amino acid diets did not appear to be utilized as efficiently based on ammonia excretion, but reduced efficiency was not evident in weight gain or other production characteristics.