Macronutrient selection through postingestive signals in sea bass fed on gelatine capsules

The role of the food orosensory properties on protein (P), fat (F) and carbohydrate (CH) self-selection was investigated in fish fed gelatine capsules containing pure macronutrients. A total of 40 sea bass (39.6+/-6.2 g initial body weight) distributed in eight 75-l tanks were used. In a sequence of experimental phases, sea bass were fed a pelleted complete diet, an encapsulated complete diet or a combination of separately encapsulated pure macronutrients. In order to induce associative learning, capsules containing a given macronutrient were paired with a particular colour. Our results demonstrate that fish are able to regulate food intake, so as to balance their energy intake, when they are fed a complete encapsulated diet and, therefore, without using the orosensory properties of the diet. Moreover, sea bass learn to discriminate and select among colour-coded, pure macronutrient capsules to compose a complete and balanced diet using colour as the only external cue. The composition of selected diet was 55% P, 23% CH and 22% F in terms of macronutrient percentage. The diet orosensory properties do not seem to be necessary to regulate macronutrient intake either, suggesting that an associative learning between capsule colour and content can be established through monitoring of macronutrient intake by postingestive and/or postabsorptive mechanisms. These results provide the first insight into energy and macronutrient self-selection by fish fed on gelatine capsules containing separate macronutrients.