RELATIONSHIP BETWEEN RATION SIZE AND DIETARY-PROTEIN LEVEL VARYING ATTHE EXPENSE OF CARBOHYDRATE AND LIPID IN TRIPLOID BROWN TROUT FRY, SALMO-TRUTTA

A feeding trial was conducted with brown trout fry (initial body weigh t: 2 g) in order to estimate the best combination of feeding level and protein content. The factorial design consisted of feeding four dieta ry protein levels (47, 53, 58 and 63%) at four ration sizes (70, 85, 1 00 and 115% of a previous empirical reference). Fish were weighed at 2 1 and 42 days and at the end of the experiment that lasted 57 days. Tw o replicates were used for each protein X ration level combination. Di ets had approximately the same energy level (:about 18.5 MJ DE) and co ntained the same protein source with varying carbohydrate: fat ratio. Protein efficiency, estimated by productive protein value, was improve d 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 p rotein content (47%), even with the highest feeding level, did not all ow maximal growth which was nevertheless achieved with several protein content X ration size combinations. Combinations which yielded equiva lent growth rates did not give the same feed efficiency, this criterio n being best with 58 or 63% protein diets fed at 85% of the previous r eference. Protein requirement was found to depend on feeding level; ab solute requirement was estimated to be between 10 and 13 g balanced pr otein/kg body mass/day, while calculated absolute energy requirement w as between 330 and 450 kJ DE/kg body mass/day. In these conditions, sp ecific 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;. (C) 1998 Published by Elsevier Science B.V.