Protein requirement of silver barb, Puntius gonionotus fingerlings

Five iso-energetic (15.05 MJ kg(-1)) semi-purified diets with graded levels of crude protein, i.e. 200 (D-1), 250 (D-2), 300 (D-3), 350 (D-4) and 400 (D-5) g kg(-1) diet were fed to Puntius gonionotus fingerlings (average weight 0.88 +/- 0.03 g) in triplicate groups (15 healthy fish per replicate) for a period of 90 days to determine the optimum protein requirement of the fish. Fifteen flow-through cement tanks of 100-L capacity with a flow rate of 0.5 L min(-1) were used for rearing the fish. Specific growth rate (SGR), food conversion (food gain) ratio (FCR), nutrient digestibility and retention, digestive enzyme activity, RNA : DNA ratio and tissue composition were used as response parameters with respect to dietary protein levels and feed intake. The mean weight gains of fish after 90 days were 10.84 +/- 0.27, 11.07 +/- 0.12, 14.09 +/- 0.20, 11.27 +/- 0.12 and 10.91 +/- 0.25 g for D-1, D-2, D-3, D-4 and D-5, respectively. Maximum SGR (3.13 +/- 0.02% per day), RNA : DNA ratio (10.09 +/- 0.09), tissue protein content (160 +/- 0.1 g kg(-1) wet weight), protease activity (25.27 +/- 0.47 mu g of leucine liberated mg tissue per protein h(-1) at 37 degrees C) and minimum FCR (1.60 +/- 0.02) was found in D-3 group fed with 300 g kg(-1) protein level. All these parameters were negatively affected with the further increase in protein level in the diet. Digestibility of protein, lipid and energy was not affected because of variation in dietary protein levels and nitrogen intake of fish. Maximum energy retention (27.68 +/- 0.12%) was recorded at 300 g kg(-1) dietary crude protein fed group. However, using broken line regression analysis, the maximum growth was found to be at 317.7 g kg(-1) dietary protein. Hence, it may be concluded that the protein requirement of P. gonionotus fingerling is 317.7 g kg(-1) diet with a resultant P/E ratio of 21.1 g protein MJ(-1).