Asian seabass Lates calcarifer perform well when fed pelleted diets high in protein and lipid

Under controlled (28 °C and 12:12 photoperiod) freshwater recirculation conditions, two 48-tank (180 l) experiments tested the hypothesis that the dietary crude protein (CP) specification for Asian seabass could be reduced by increasing the dietary concentration of lipid. In Experiment 1, 384 fish (230 g initial bodyweight) were fed air-dry extruded diets of 38%, 42.5%, 47.3% and 52% CP in combination with lipid concentrations of 7.0%, 12.8% and 18.3% (12 diets) for 8 weeks. In Experiment 2, 720 juvenile fish (80 g initial bodyweight) were fed for 6 weeks one of 12 air-dry diets that varied in lipid between 11.5% and 22.4% and in protein between 43.8% and 64.7%. Air-dry feed conversion ratio (FCR) and growth rate improved linearly with increasing dietary CP and improved step-wise with increasing dietary lipid from 1.48 to 0.86 g/g and 2.69 to 4.86 g/day, respectively, in Experiment 1 and similarly from 1.00 to 0.78 g/g and 2.27 to 3.10 g/day, respectively, in Experiment 2. In terms of growth rate and FCR, a small protein sparing effect of lipid was observed which was more pronounced for the smaller fish (Experiment 2). Increasing dietary CP resulted in very little change in the proportion of dietary N that was retained by the fish (35% to 42% in Experiment 1 and 25% to 31% in Experiment 2). In both experiments, retention of dietary gross energy improved as dietary lipid increased, with fat deposition appearing to be a major component of this energy conservation (total DM body lipid content increasing from 24.0% to 29.1% in Experiment 1 and from 20.3% to 32.7% in Experiment 2). However, DM lean gain increased linearly with increasing dietary CP over the range examined and was not influenced by the amount of lipid in the diet, indicating that any protein sparing by lipid was due to increased body fat deposition. The results show that productivity of Asian seabass can be markedly improved by increasing dietary protein and lipid concentrations. However, Asian seabass appear to have only a limited capacity to use lipid as a primary energy source. In this aspect, they differ markedly from salmonids.