Carbohydrate level in the diet of silver barb, Puntius gonionotus (Bleeker) fingerlings: effect on growth, nutrient utilization and whole body composition

Five iso-nitrogenous (300 g crude protein kg^?1 diet) semi-purified diets with graded levels of carbohydrate at 220 (D-1), 260 (D-2), 300 (D-3), 340 (D-4) and 380 (D-5) g kg^?1 diet were fed ad libitum to Puntius gonionotus fingerlings (average weight 0.59±0.01 g) in triplicate groups (20 fish replicate^?1) for a period of 90 days to determine the effect of the dietary carbohydrate level on the growth, nutrient utilization, digestibility, gut enzyme activity and whole-body composition of 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. The maximum weight gain, specific growth rate, protein efficiency ratio, RNA:DNA ratio, whole-body protein content, protease activity, protein and energy digestibility and minimum feed conversion ratio (FCR) were found in the D-2 group fed with 260 g carbohydrate kg^?1 diet. The highest protein and energy retention was also recorded in the same group. However, from the second-order polynomial regression analysis, the maximum growth and nutrient utilization of P. gonionotus fingerlings was 291.3–298.3 g carbohydrate kg^?1 diet at a dietary protein level of 300 g kg^?1 with a protein/energy (P/E) ratio of 20.58 ?20.75 g protein MJ^?1.