Dietary manipulations affecting growth and nitrogenous waste production of red drum, Sciaenops ocellatus I.: Effects of dietary protein and energy levels

Although considerable progress has been made in determining nutritional requirements for maximum growth of red drum, it is important to consider that dietary manipulations also may influence other responses that are of aquacultural importance such as nitrogenous waste production. Therefore, an 8-week feeding trial was conducted with juvenile red drum (7.7 g/fish initial weight) in recirculating 110-l aquaria, to determine the effects of dietary protein and energy manipulations on growth, body composition and aspects of nitrogenous waste production of these fish. A total of eight dietary treatments, with six of the treatments constructed in a 3 x 2 factorial array, were evaluated. Factorial treatments had either 35, 40 or 45% crude protein (CP) with medium or high digestible energy levels ranging from 35.2 to 46.4 kJ/g protein at each protein level based on previous studies with this species. These diets were fed to fish at a rate approaching apparent satiation. Two additional treatments consisted of feeding fish the two 40% CP diets at precisely half the rate of fish fed to apparent satiation. Weight gain of fish significantly (P < 0.05) increased in response to increases in dietary protein, and within a protein level, was reduced by high dietary energy. Feeding at a reduced rate resulted in poorer weight gain, except for fish fed the diet containing higher energy. For all treatments, fat deposition was increased in the peritoneal cavity and liver by high dietary energy. However, muscle lipid was not impacted by the dietary treatments. Plasma ammonia and urea levels, as well as hepatic glutamate dehydrogenase and glutamine synthetase were not affected by dietary energy or feed rate manipulations. However, ammonia production and activity of the ammoniagenic enzyme glutaminase were reduced in fish fed the higher dietary energy levels. This study suggests that high dietary energy is able to reduce ammonia production of juvenile red drum but also results in fish with higher fat deposition and lower growth rates. (C) 1999 Elsevier Science B.V. All rights reserved.