EFFECT OF DIETARY-LIPID ON GROWTH-PERFORMANCE AND BODY-COMPOSITION OF BROWN 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 triplicate groups of 158 fish of 1.6 kg average body weight reared in 60-m3 floating cages. Four experimental diets corresponding to two levels (21 versus 29%) and two sources of added lipid (com, i.e. vegetable, versus cod liver, i.e. marine) were tested. Crude protein content was similar (about 52%) in all diets. All fish were fed the same amount of calculated digestible energy. The level of fat had a slight but significant effect on growth rate and feed conversion; the higher dietary lipid level led to a faster daily growth index (+4.8%) and better feed conversion (-12%). Protein utilization, estimated by both protein efficiency ratio and productive protein value, was also improved in the high-lipid compared to low-lipid treatments. The same factor also significantly 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 pronounced effects on the fatty acid (FA) profile of muscle and liver lipids but the variations mainly concerned saturated (in liver only), monounsaturated and 18:2n-6 FAs (in both tissues). On the other hand, n-3 polyunsaturated FAs were significantly but only slightly modified by the source of lipid, corresponding for both tissues to that found in other salmonids. The level of 20:4n-6 remained constant. The elongation product of 18:2n-6, namely 20-2n-6, was incorporated in muscle and liver lipids while the DELTA6 desaturation product, 20:3n-6, was only observed in liver.