Dietary Protein Level Affects the Growth Performance of Large Male Genetically Improved Farmed Tilapia, Oreochromis niloticus , Reared in Fertilized Freshwater Cages

An 8‐wk experiment was conducted to evaluate the effects of dietary protein on the growth performance, body composition, and serum biochemical indices of large male genetically improved farmed tilapia ( GIFT ), Oreochromis niloticus , reared in fertilized freshwater cages. Six semi‐purified diets were formulated, containing 18.05, 22.39, 25.97, 31.62, 35.97, and 39.89% protein (designated as P18 , P22 , P26 , P30 , P34 , and P38 , respectively). Each diet was fed to randomly selected fish (initial mean weight 216.7 g) in four cages. At the end of the experiment, the final body weights varied significantly ( P < 0.05), ranging from 520 to 580 g. Fish receiving the P30 diet had the highest final weight value (580.5 ± 5.4 g). Weight gains of fish fed the P26 and P30 diets were significantly higher ( P < 0.05) than those fed the P18 and P38 diets. The apparent feed efficiency of fish fed the P18 diet was significantly lower ( P < 0.05) than that of fish in the other groups except the P22 group ( P > 0.05). The apparent protein efficiency ratio decreased gradually with increasing dietary protein. Whole‐body moisture and ash showed no significant difference ( P > 0.05), but protein and fat content were significantly affected by dietary protein level ( P < 0.05). No significant differences were found for serum total protein, triglyceride, total cholesterol, glutamic‐oxaloacetic transaminase, and glutamic‐pyruvic transaminase levels among different diets ( P > 0.05); only serum glucose level was affected by dietary protein levels ( P < 0.05). Second‐order polynomial regression of weight gain suggested 29.3% dietary protein is adequate content for maximum growth of large male GIFT , O. niloticus .