These differences are largely due to the growth potential and the composition of the growth. We have to assume that different fish have a genetically determined asymptotic body size and composition of weight gain and that they are capable of adjusting their feed or energy intake to realize the genetic potential. The growth rate under comparable conditions
and feeding per demand was the highest for white grouper, predicted to reach in 12 months 750 g compared to 380 g for gilthead sea bream and 325 g for European sea bass. The composition of the growth showed differences among the fish concerning the energy but not the protein content. According to Eq. (4), the energy content in a 400-g gilthead sea bream would be 10.7 kJ g 1 compared to an energy content of 7.0 kJ g 1 for white grouper (Eq. (6)) despite the fact that fish have been fed the same diet throughout their growth
period. Protein content on the other hand is fairly constant and was close to 170 mg g 1
independent of fish size and species. Therefore, total energy retention consists of two
components: a basic relatively constant protein retention and a variable additional energy
retention, mainly lipid, which increases with fish size and whose level differed with species.