Dietary nutrient composition affects digestible energy utilisation for growth : a study on Nile tilapia (Oreochromis niloticus) and a literature comparison across fish species
Résumé
The effect of the type of non-protein energy (NPE) on energy utilisation in Nile tilapia was studied, focusing on digestible energy utilisation for growth (k gDE). Furthermore, literature data on k gDE across fish species were analysed in order to evaluate the effect of dietary macronutrient composition. A total of twelve groups of fish were assigned in a 2 × 2 factorial design: two diets (‘fat’ v. ‘starch’) and two feeding levels (‘low’ v. ‘high’). In the ‘fat’-diet, 125 g fish oil and in the ‘starch’-diet 300 g maize starch were added to 875 g of an identical basal mixture. Fish were fed restrictively one of two ration levels (‘low’ or ‘high’) for estimating k gDE. Nutrient digestibility, N and energy balances were measured. For estimating k gDE, data of the present study were combined with previous data of Nile tilapia fed similar diets to satiation. The type of NPE affected k gDE (0·561 and 0·663 with the ‘starch’ and ‘fat’-diets, respectively; P < 0·001). Across fish species, literature values of k gDE range from 0·31 to 0·82. Variability in k gDE was related to dietary macronutrient composition, the trophic level of the fish species and the composition of growth (fat:protein gain ratio). The across-species comparison suggested that the relationships of k gDE with trophic level and with growth composition were predominantly induced by dietary macronutrient composition. Reported k gDE values increased linearly with increasing dietary fat content and decreasing dietary carbohydrate content. In contrast, k gDE related curvilinearly to dietary crude protein content. In conclusion, energy utilisation for growth is influenced by dietary macronutrient composition.