Relationship between ration size and dietary protein level varying at the expense of carbohydrate and lipid in triploid brown trout fry, Salmo trutta

Abstract

A feeding trial was conducted with brown trout fry (initial body weight: 2 g) in order to estimate the best combination of feeding level and protein content. The factorial design consisted of feeding four dietary protein levels (47, 53, 58 and 63%) at four ration sizes (70, 85, 100 and 115% of a previous empirical reference). Fish were weighed at 21 and 42 days and at the end of the experiment that lasted 57 days. Two replicates were used for each protein×ration level combination. Diets had approximately the same energy level (about 18.5 MJ DE) and contained the same protein source with varying carbohydrate: fat ratio. Protein efficiency, estimated by productive protein value, was improved by low dietary protein level and feed restriction. Body composition was slightly but significantly affected by diet composition, with body lipid content decreasing with high dietary protein. The lowest ration (70%), even with the highest protein content, as well as the lowest protein content (47%), even with the highest feeding level, did not allow maximal growth which was nevertheless achieved with several protein content×ration size combinations. Combinations which yielded equivalent growth rates did not give the same feed efficiency, this criterion being best with 58 or 63% protein diets fed at 85% of the previous reference. Protein requirement was found to depend on feeding level; absolute requirement was estimated to be between 10 and 13 g balanced protein/kg body mass/day, while calculated absolute energy requirement was between 330 and 450 kJ DE/kg body mass/day. In these conditions, specific growth rate was of 2.7%. When compared to other fishes growing at the same rate, the absolute requirement of brown trout for protein appears to be rather low.