Polka dot grouper Cromileptes altivelis fingerlings require high protein and moderate lipid diets for optimal growth and nutrient retention

An 8-week comparative slaughter experiment was carried out to determine the effect of dietary protein and lipid on growth, apparent digestibility (AD) and nutrient retention of polka dot grouper Cromileptes altivelis. Fingerlings were fed diets that varied in crude protein (CP) at 55 g kg(-1) increments between 410 and 630 g kg(-1) dry matter (DM) and at either a moderate (150 g kg(-1) DM) or high (240 g kg(-1)DM) lipid concentration. Each diet was fed to satiety twice daily to four replicate tanks (110 L) of fish. One replicate block of tanks comprised 150 fish of mean (+/-SD) initial weight of 9.6 +/- 0.29 g, which were distributed equally to 10 tanks. The other three replicate blocks of tanks comprised 300 fish of 12.6 +/- 0.45 g, which were distributed equally to 30 tanks. Tanks were provided with filtered and heated (29 +/- 0.5 degreesC) seawater in a flow-through system within a laboratory where photoperiod was maintained at 12 : 12 h light-dark cycle. Voluntary food intake was not significantly affected by either the CP or lipid concentration of the diet (mean +/- SD of 1.93 +/- 0.146 g week(-1)) but there was a trend for intake to be higher on the moderate compared with the high lipid diets (mean +/- SEM of 1.97 versus 1.89 +/- 0.033 gweek(-1), respectively). Daily growth coefficient (DGC) and food conversion ratio (FCR) improved linearly (P < 0.01) with increasing dietary CP (from 0.94 to 1.35% day(-1) for DGC and 1.58 to 1.00 g DM g(-1) wet gain for FCR) and these responses were almost coincident for each of the lipid series. The AD of CP increased linearly with increasing dietary CP (from 46.8 to 74.1%) and was independent of dietary lipid. Apparent digestibility of energy increased curvilinearly with increasing dietary CP, with the quadratic component being more prominent for the high-lipid series. Increasing the amount of lipid in the diet markedly increased the lipid content of the fish from an initial composition (mean +/- SD) of 173 +/- 7.3 g kg(-1) to a final composition (mean +/- SEM) of either 217 or 250 +/- 5.9 g kg(-1) for moderate and high-lipid series, respectively. Total body lipid content tended to increase linearly with increasing dietary CP for the high-lipid series but with an opposite effect for the moderate-lipid series. The retention of digestible nitrogen decreased linearly with increasing dietary CP but at a steeper rate for the moderate, compared with the high, lipid series (from 62.7 to 35.7%, slope -0.115 for moderate-lipid and 54.6 to 41.9%, slope -0.050 for high-lipid). A quadratic function of dietary CP concentration best explained the retention of digestible energy with the curvilinearity being more marked for the high, compared with the moderate, lipid diet series. While there was some indication that ingested lipid spared dietary protein, the results showed a far greater propensity of polka dot grouper fingerlings to use protein as the prime dietary energy source. Diets for juvenile polka dot grouper should contain not less than 440 g digestible protein kg(-1) DM and at least 150 g lipid kg(-1) DM.