Effects of dietary protein to energy ratios on growth and body composition of juvenile Japanese seabass, Lateolabrax japonicus

Abstract

A growth experiment was conducted to determine the optimal dietary protein to energy (P/E) ratio for juvenile Japanese seabass (initial average weight 6.26 ± 0.10 g). Nine practical diets were formulated to contain three protein levels (36%, 41% and 46%), each with three lipid levels (8%, 12% and 16%), in order to produce a range of P/E ratios (from 19.8 to 28.6 mg protein kJ - 1). Each diet was randomly assigned to triplicate groups of 60 fish in sea floating cages (1.5 × 1.5 × 2.0 m). Fish were fed twice daily (06:30 and 16:30) to apparent satiation for 8 weeks. The water temperature fluctuated from 26.5 to 32.5 °C, the salinity from 32 % to 36 % and dissolved oxygen content was approximately 7 mg l -1 during the experimental period. The results showed that the growth was significantly affected by dietary P/E ratio ( P <0.05). Fish fed the diets with 46% protein (12% and 16% lipid, P/E ratio of 26.7 and 25.7 mg protein kJ -1) had the highest specific growth rates (SGR) (4.26 and 4.24% day -1). However, fish fed the diet with 41% protein and 12% lipid showed comparable growth (4.20% day -1), and had higher protein efficiency ratio (PER), protein productive value (PPV) and energy retention than other groups ( P < 0.05). No significant differences in survival were found among dietary treatments. Carcass lipid content positively correlated with dietary lipid level irrespective of protein level and inversely correlated with carcass moisture content. Carcass protein and ash contents increased with increasing dietary lipid at each protein level. These results suggest that the diet containing 41% protein and 12% lipid with P/E of 25.9 mg protein kJ -1 is optimal for Japanese seabass.