Effects of isoenergetic diets with varying protein and lipid levels on the growth, feed utilization, metabolic enzymes activities, antioxidative status and serum biochemical parameters of black sea bream (Acanthopagrus schlegelii)

Six isoenergetic (20.9 kJ g(-1)) diets were fed to triplicate groups of 20 black sea bream, Acanthopagrus schlegelii (initial weight 11.20 +/- 0.20 g) to apparent satiation for eight weeks. The dietary protein (P) levels (28.85, 31.21, 34.15, 37.46, 40.22 and 42.87%) were combined with lipid (L) levels (26.32, 24.54, 22.73, 20.94, 19.13 and 17.35%), respectively, with the protein: energy (P/E) ratios ranging from 13.71 to 20.60 mg kJ(-1). The weight gain and specific growth rate of the experimental fish increased with increasing dietary P/E ratios up to fish fed the P37/L21 diet (P/E ratio 17.88 mg kJ(-1)), beyond which growth performance decreased (P < .05). Feed intake and feeding rate showed trends similar to the growth performance. Fish fed the P34/L23 diet had the highest values of viscerasomatic index and intraperitoneal fat ratio. The protein efficiency ratio, protein productive value and lipid contents of whole-body and muscle decreased with increasing dietary P/E ratios, whereas whole-body protein content had an inverse trend (P < .05). The fish fed P29/L26 and P31/L32 diets showed the highest contents of serum triglyceride and malondialdehyde, whereas fish fed P40/L19 and P43/L17 diets had the highest activities of liver alanine aminotransferase and aspartate aminotransferase (P < .05). Effective protein sparing effect of lipid occurred in the low protein diets. Based on quadratic regression analysis, the optimal protein requirement of black sea bream is estimated to be 37.10% (dietary energy level 20.9 kJ g(-1)) which is lower than previously reported.