Quantification of the optimum dietary protein level for juvenile A. nigrocauda in relation to nutrition is important for commercial fish culture operations, and to estimate the muscle composition and changes nitrogen excretion for the fish, the fishes were fed with changing protein levels. Six experimental diets containing 30.01%, 34.57%, 39.63%, 44.75%, 49.52%, and 54.47% protein with 17.24 MJ/kg diet, respectively, were fed to three-replicate six groups of forty fish (mean weight: 16.30±0.02 g) for 70 days. The results showed that the maximum specific growth rate (SGR) and the best protein efficiency ratio (PER) occurred at 44.75% dietary protein level (p<0.05). The polynomial equations regression analysis indicated that for maximal SGR, A. nigrocauda optimal required 44.32% dietary crude protein and the potential for the culture. The feed conversion ratio (FCR) was significantly decreased with increasing dietary protein levels up to 44.75% (p<0.05), but no significant differences were found between the groups with protein levels of 44.75% to 49.52% (p>0.05), and thereafter FCR was increased with further increases of dietary protein. Moreover, the fish accumulated the more protein and the less lipid in the muscle with the more dietary protein level. Muscle composition did not show difference on moisture and ash content between the groups with 30.01% to 54.47% protein level (p>0.05). The amount of Ammonia-N excreted by A. nigrocauda within 24 h was enhanced (p<0.05) as increasing dietary protein level, whereas Urea-N increased (p<0.05) except the highest dietary protein level of 54.47%.
