Optimal dietary protein level and protein to energy ratio for hybrid grouper (Epinephelus fuscoguttatus ♀ × Epinephelus lanceolatus ♂) juveniles

• Optimal dietary protein level of hybrid grouper juveniles was estimated to be 53.5% of dry matter.
• The optimal value of dietary P/E ratio at 53.5% dietary protein is 157 mg/kcal for hybrid grouper juveniles.
• Lower P/E ratios not only resulted in lower weight gain but also produced higher lipid accumulation in the abdominal cavity.

Two consecutive growth trials were undertaken to study optimal dietary protein level and protein to energy ratio of hybrid grouper (Epinephelus fuscoguttatus ♀ × Epinephelus lanceolatus ♂) juveniles. In the first trial (trial 1), seven isolipidic and isoenergetic experimental diets were formulated to contain 38%, 42%, 46%, 50%, 54%, 58%, or 62% dietary crude protein (CP) level (dry-matter basis), being abbreviated as P38, P42, P46, P50, P54, P58 and P62, respectively. Each diet was given to triplicate groups (initial average body weight, 3.58 ± 0.05 g/fish) for 7 weeks. Weight gain (WG%) of fish fed with P38 and P42 were significantly lower (P < 0.05) than fish fed with P46, P50, P54, P58 or P62. Fish fed with P46 had significantly lower WG% than fish fed with P62. Fish fed with P38 exhibited significantly higher feed conversion ratio (FCR) than fish fed with P46, P50, P54, P58 or P62. Based on the quadratic regression model at 95% of maximum response of WG%, optimal dietary protein level of hybrid grouper juveniles was estimated to be 53.5% of dry matter. In trial 2, the other seven isonitrogenous (53.5% of dry matter) experimental diets were formulated to contain various dietary protein/energy ratios of 191, 173, 157, 145, 134, 124 and 116 mg/kcal (P/E191, P/E173, P/E157, P/E145, P/E134, P/E124 and P/E116), respectively. Each diet of trial 2 was given to triplicate groups (initial average body weight, 10.98 ± 0.15 g/fish) for 8 weeks. WG% of fish fed the diet with P/E191 was significantly lower than that of fish fed the diet with P/E173 or P/E157, and there were no significant differences in WG% values among fish fed diets with P/E191, P/E145, P/E134, P/E124 and P/E116. Values of DFI of fish fed P/E191 and P/E173 were significantly higher than those of fish fed P/E134, P/E124 and P/E116. Fish fed P/E191 exhibited significantly higher FCR than fish fed P/E157, P/E145, P/E134, P/E124 or P/E116, and fish fed P/E173 showed significantly higher FCR than fish fed P/E124 or P/E116. IPF together with lipid contents of whole-body and liver were increased as dietary P/E ratio decreased. Results of trial 2 indicated that dietary P/E ratio of 157 mg/kcal was optimal for hybrid grouper juveniles.