Effect of dietary starch source (normal versus waxy) and protein levels on the performance of white sea bream Diplodus sargus (Linnaeus) juveniles

An experiment was performed to evaluate the growth performance and feed utilization of white sea bream juveniles (initial weight, 14 g) fed diets of cornstarch of different origins (normal and waxy). Four experimental diets were formulated to be isolipidic and to contain normal and waxy starch (26% or 42%) at two protein (36% and 48%) levels. The growth trial lasted 15 weeks and, at the end of the trial, there were no differences in the growth rate among groups. At the highest dietary starch level - but not at the lowest level - the feed efficiency ratio and PER were significantly lower in fish fed the waxy starch diet. Protein efficiency ratio and N retention (% N intake) were not affected by starch source but were significantly higher in the diets with a lower protein content. No differences in energy retention (% energy intake) were observed among groups. Except for the protein content, which was significantly higher in fish fed diets with a high protein level, no other differences were observed among groups in whole-body composition. Hepatosomatic Index (HSI) was significantly higher in fish fed high-starch diets, but there were no differences in visceral indices among groups. The apparent digestibility coefficients of protein and energy were not affected by the dietary starch level, but were significantly lower in diets including waxy starch. Glutamate dehydrogenase activity was higher in fish fed high-protein diets, but it was not affected by dietary starch source. Alanine aminotransferase and aspartate aminotransferase activities were not different among groups. Glucose 6-phosphate dehydrogenase, malic enzyme and fatty acid synthetase activities were not affected by the dietary starch level, but were significantly lower in fish fed waxy starch. The results of this study indicate that diets for white sea bream juveniles may include up to 42% starch without negative effects on fish performance. Moreover, normal starch appears to be more efficiently used as an energy source than waxy starch.