Apparent Optimal Dietary Protein Level and Growth Response of Grow-Out Hybrid Grouper (Epinephelus akaara ♀ × E. lanceolatus ♂) Under Practical Formulation Conditions

This study evaluated the effects of graded dietary protein levels on growth performance, feed utilization, physiological responses, and antioxidant status in grow-out-stage hybrid grouper (Epinephelus akaara ♀ × E. lanceolatus ♂) under practical formulation conditions. A total of 450 fish (initial body weight 240 ± 1 g) were randomly assigned to 18 tanks and fed six practical near-isocaloric diets formulated to provide graded crude protein levels of 40% (P40), 45% (P45), 50% (P50), 55% (P55), 60% (P60), or 65% (P65) for 11 weeks. Growth performance indices, including final body weight (FBW), weight gain (WG), specific growth rate (SGR), and feed efficiency (FE), increased from P40 to P60, with the highest values observed in the P60 group. A decline in these indices at P65 indicated reduced nutrient utilization at excessive dietary protein levels within the present diet series. Broken-line regression analysis, using weight gain as the response variable, suggested an apparent optimum around 58–59% crude protein within the present high-fish meal practical formulation matrix, with relatively wide confidence intervals around the breakpoint estimate. Hematological and plasma biochemical parameters, including hematocrit (Hct), hemoglobin (Hb), glucose (GLU), total protein (TP), total cholesterol (TCHO), and blood urea nitrogen (BUN), were not significantly affected by dietary protein level, suggesting stable physiological status across treatments. Dorsal muscle proximate composition and amino acid profiles were similarly unaffected. Antioxidant responses showed moderate variation, with plasma catalase (CAT) activity highest in the P55 group, while plasma glutathione (GSH) levels remained stable across treatments. These findings suggest that, under the present high fish meal practical formulation conditions, an apparent dietary crude protein level of approximately 58–59% may improve growth and feed utilization in grow-out-stage hybrid grouper without markedly altering physiological and antioxidant status.