The evaluation of energy intake adjustments and preferences in juvenile rainbow trout fed increasing amounts of lipid.

This study aimed at a better understanding of the feed intake (FI) regulation in rainbow trout by dietary digestible energy (DE). The DE contents of the three diets (20.5, 23.0 and 24.7 kJ per g dry diet) were modified by supplementing different amounts of fish oil. The crude lipid and protein levels were 13%, 26% and 34% and 64%, 54% and 48%, respectively (% dry diet). The daily FI was measured by means of self-feeders in groups of rainbow trout (32-55 g, initial BW) during two 5 to 6-week trials. Their eventual preference for one of the lipid levels was evaluated by offering the choice between a low and higher lipid diet. The results indicated that fish of a similar body mass had a similar FI without apparent energy intake compensations. It is believed that the excessive energy intakes with the higher lipid diets were not caused by a higher palatability of these diets since the trout did not express any particular preference. The trout fed the lipid rich diets had a higher level of body adiposity, but a similar protein growth. The observation that the trout did not reduce FI when fed the high lipid diet implies a low negative feedback by the ingested fat or by the increase in body adiposity. The similarities in lean carcass growth favour the idea that growing animals regulate their FI in order to meet the demand for maximal protein growth rather than to satisfy a predetermined energy requirement.