When measuring whole body composition of fish at increasing sizes, each unit weight gain is assumed to equal the body composition at a certain size. Protein content was relatively constant and independent of fish species and size and was on average 176F8.4, 171F7.9 and 168F7.3 mg g 1 for gilthead sea bream, European sea bass and white grouper, respectively. On the other hand, energy concentrations changed with increasing fish weight and showed different values according to species, as depicted in Fig. 2. The actual equations describing the change of energy density with fish size are shown below: