In the present study, we aimed to evaluate the optimum dietary protein level in hybrid pufferfish (Takifugu obscurus female x T. rubripes male). To this end, we formulated five isoenergetic (16 MJ/kg gross energy) diets with casein with different levels of protein-namely, 35%, 45%, 50%, 55% and 65%. Then, fish with the average weight of 5.68 +/- 0.05 g (mean +/- SD) were randomly distributed into 15 tanks, 20 animals in each tank. The fish received their diet twice a day (09:00 and 17:00) ad libitum. The results showed that, after the 8-weeks feeding trial, the fish that received 45% and 50% protein diets had significantly higher final weight, WG, and SGR that the fish that received 35% protein diet (p < 0.05). However, final weight, WG and SGR did not differ significantly among the fish that received 35%, 55% and 65% protein diets. Next, the fish on 35% protein diet had a significantly higher PER that their counterparts on 50%, 55% and 65% protein diets (p < 0.05). However, no such difference was observed between the two groups of fish that received 35% and 45% protein diets. Furthermore, the fish on 50% and 55% protein diets had a significantly higher whole body protein content as compared to the fish that received 35% and 65% protein diets (p < 0.05). In addition, the fish on 65% protein diet had a significantly higher whole body lipid content as compared to the group that received 35% protein diet (p < 0.05). In addition, the highest arginine, histidine, phenylalanine, threonine, valine, glutamic acid, proline and tyrosine content were observed in the fish that received 50% protein diet. The fish on 35% and 65% protein diets had a significantly higher hemoglobin than the group on 55% protein diet (p < 0.05); however, no differences were observed among the groups on 45%, 50%, and 55% protein diets. Finally, the fish fed 35% has a significantly higher AST activity that the groups that received 45% and 55% protein diets (p < 0.05); yet, no significant differences were found among the groups on 45%, 50%, and 55% protein diets. The results of two-slope straight broken-line and quadratic broken-line analysis of WG revealed that, for the maximum growth of juvenile hybrid pufferfish, the optimal dietary protein should be 46.3% and 51.5%, respectively.
