Effects of pretreating a plant-based diet with phytase on diet selection and nutrient utilization in European sea bass

As most animals, fish possess an impressive array of behavioral mechanisms to deal with challenges posed by the composition of the food they consume. Such responses can be behavioral, through both food selection and regulation of the amounts eaten, which helps to alleviate the deleterious effects of such nutritional imbalances. The purpose of the two experiments presented was to examine the capacity of groups of European sea bass to differentiate between isoproteic diets containing soybean meal (SM0), soybean meal + 1500 FTU/kg of phytase (SM1) and fish meal (FM) to detect any preference and also the effect of each diet on nutrient utilization and growth in the carnivorous species studied. Ninety-nine fish (mean initial weight 265 ± 10.5 g, S.E.M) were divided into three self-selection treatments (three replicates) based on different diets (FM vs SM0, T1); (FM vs SM1, T2) and (SM0 vs SM1, T3) using self-feeders (Experiments 1). Preferences were expressed as relative changes in feed demands for a specific feeder and diet selection was expressed as the percentage of diet, considering the total of diets as 100%. Another seventy-two fish (mean initial weight 255.0 ± 11.0 g, S.E.M) were randomly distributed among nine tanks to analyze body composition (P and Ca retention) and growth parameters (Experiment 2). In this case, fish were fed a single diet (FM or SM0 or SM1) in the same laboratory condition as Experiment 1. Averaged over all groups of Experiment 1, the preference tests demonstrated the capacity of European sea bass to discriminate between a diet with FM (P < 0.05) and a diet containing exogenous phytase SM1 (P < 0.05), their final choices being 15.8% SM0vs 84.2% FM; 26.7% SM1vs 73.3% FM; 9.8% SM0vs 90.2% SM1. In Experiment 2, the partial replacement of fish meal by soybean meal in diets (SM0 and SM1) resulted in reduced weight gain and lower specific growth rate (SGR), while, P and Ca retention increased significantly in fish fed diets SM1 and FM. It is believed that the metabolic consequences of the plant meal intake (SM0) negatively affected the feed acceptance in the studied species, and that exogenous phytase increased the acceptance of the plant diet. Finally, we showed that seabass defended an “intake target” by allowing them to select among different diets to satisfy their nutritional needs.