EFFECT OF DIETARY-LIPID ON GROWTH-PERFORMANCE AND BODY-COMPOSITION OFBROWN TROUT (SALMO-TRUTTA) REARED IN SEAWATER

An experiment was carried out with 12 groups of a fast-growing strain of brown trout reared in seawater. Each treatment was fed to triplicat e groups of 158 fish of 1.6 kg average body weight reared in 60-m3 flo ating cages. Four experimental diets corresponding to two levels (21 v ersus 29%) and two sources of added lipid (com, i.e. vegetable, versus cod liver, i.e. marine) were tested. Crude protein content was simila r (about 52%) in all diets. All fish were fed the same amount of calcu lated digestible energy. The level of fat had a slight but significant effect on growth rate and feed conversion; the higher dietary lipid l evel led to a faster daily growth index (+4.8%) and better feed conver sion (-12%). Protein utilization, estimated by both protein efficiency ratio and productive protein value, was also improved in the high-lip id compared to low-lipid treatments. The same factor also significantl y influenced fat and muscle water content which were higher and lower, respectively, in the high-lipid compared to low-lipid treatments. The source of added lipid did not influence growth rate, feed conversion, or protein efficiency. Lipid source had no effect on body composition except in the case of liver which contained more lipid and less water in the fish fed cod liver oil. The source of lipid had very pronounce d effects on the fatty acid (FA) profile of muscle and liver lipids bu t the variations mainly concerned saturated (in liver only), monounsat urated and 18:2n-6 FAs (in both tissues). On the other hand, n-3 polyu nsaturated FAs were significantly but only slightly modified by the so urce of lipid, corresponding for both tissues to that found in other s almonids. The level of 20:4n-6 remained constant. The elongation produ ct of 18:2n-6, namely 20-2n-6, was incorporated in muscle and liver li pids while the DELTA6 desaturation product, 20:3n-6, was only observed in liver.