Dietary protein requirement of white sea bream ( Diplodus sargus ) juveniles

A trial was undertaken to estimate the protein requirement of white sea bream ( Diplodus sargus ). Five fish meal‐based diets were formulated to contain graded levels of protein (from 60 to 490 g kg −1 ). Each diet was assigned to triplicate groups of 25 fish with a mean individual body weight of 22 g. Fish fed the 60 g kg −1 protein diet lost weight during the trial, while growth improved in the other groups as dietary protein level increased up to 270–370 g kg −1 . Feed efficiency improved as dietary protein level increased. Maximum protein efficiency ratio (PER) was observed with the 17% protein diet. N retention (NR) (% N intake) was not different among groups fed diets with 17% protein and above. Ammonia excretion (g kg −1 ABW day −1 ) increased as dietary protein level increased, while no differences in urea excretion were noted. An exponential model was used to adjust specific growth rate and NR (g kg −1  day −1 ) to dietary protein level. Based on that model, dietary protein required for maximum retention was 330 g kg −1 , while for maximum growth it was 270 g kg −1 . On a wet weight basis, there were no differences in whole body composition of fish‐fed diets with 170 g kg −1 protein and above, except for the protein content, which was lower in group fed the 170 g kg −1 protein diet than the 490 g kg −1 protein diet. Specific activities of hepatic amino acid catabolism enzymes (glutamate dehydrogenase, alanine aminotransferase and aspartate aminotransferase) increased as dietary protein levels increased. There were no differences among groups in fatty acid synthetase and malyc enzyme but 6‐phosphate dehydrogenase (G6PDH) was significantly lower in fish fed the 60 g kg −1 protein diet than the 170 and 490 g kg −1 protein diets.