Optimal dietary protein level for the growth and metamorphosis of bullfrog (Lithobates catesbeianus) tadpoles

This study investigated the effects of dietary protein levels on growth performance, antioxidant capacity, digestive enzyme activity, and metamorphosis rate of the bullfrog (Lithobates catesbeianus) tadpoles. Seven isolipidic and isoenergetic diets were formulated with 24.61%, 28.49%, 33.74%, 37.50%, 44.06%, 48.39% and 53.92% crude protein. Triplicate replicates of tadpoles (growth stage 25 and 0.009 g) were fed with experimental diets two times daily for 11 weeks, and the daily feeding amount was 6.50% of the tadpole's body weight. Weight gain (WG), specific growth rate (SGR), the total antioxidant capacity (T-AOC) and catalase (CAT) activity in the liver, the intestinal trypsin activity (TPS) and metamorphosis rate at stage 42 of bullfrog tadpoles all increased with increasing dietary protein level from 24.61% to 44.06% and then decreased. Based on the second-order polynomial regression analysis of the weight gain and metamorphosis rate at stage 41 vs. dietary protein level, the optimal dietary protein level for bullfrog tadpoles was 42.49% and 46.50%, respectively. In conclusion, the optimum dietary protein level of bullfrog tadpoles was estimated to be 42.49%-46.50% of the diet.