Effect of dietary protein level, initial body weight, and their interaction on the growth, feed utilization, and physiological alterations of Nile tilapia, Oreochromis niloticus (L.)

A 10-week feeding trial was conducted to assess the interaction between dietary protein level and fish weight on the growth, feed utilization, and physiological alterations of Nile tilapia, Oreochromis niloticus (L.). Fish were categorized into three weights; 0.4–0.5 g (fry), 17–22 g (fingerling), and 37–43 g (advanced juvenile). Diets containing 25, 35, or 45% crude protein (CP) were fed by triplicate to each fish weight. Fish growth, feed utilization, and protein turn-over were significantly affected by dietary protein level and fish weight, meanwhile their interaction significantly affected specific growth rate and protein efficiency ratio (PER) only. Unionized ammonia was significantly affected by dietary protein level, fish weight, and their interaction. Moreover, protein and lipid contents in whole-body of fish were significantly affected by dietary protein level and fish weight, while their interaction significantly affected total lipids content only. Ash content significantly differed with fish weight only. The optimum feed conversion ratio (FCR) was obtained with fry tilapia fed the 45%-CP diet; whereas, the poorest FCR was observed for advanced juveniles fed the 25%-CP diet. The lowest PER and protein productive value (PPV) values were obtained with the 45%-CP diet fed to advanced juveniles; whereas, the highest values were obtained with the 25%-CP diet fed to fry. The highest protein growth rate (PGR) was obtained with fry tilapia fed the 45%-CP diet, while the lowest one was obtained with advanced juvenile fed the 25%-CP diet. Hematological variables were significantly affected by protein level, fish weight, and their interaction except for serum lipids which was not significantly affected by the interaction. Activities of aspartate amninotransferase (AST) and alanine aminotransferase (ALT) in serum, liver, and muscles were significantly affected by dietary protein level and fish weight. The interaction significantly affected enzyme activities except for serum AST, which was not significant. The optimum growth of fry tilapia was obtained at 45% CP, while fingerling and advanced juvenile showed optimum growth performance with the 35%-CP diet. Excess protein in fingerling and advanced juvenile might be deaminated and used as energy source resulting in increased blood glucose, protein, and lipids as well as increased unionized ammonia in the environment.