HIGH-ENERGY LOW-PROTEIN DIETS FOR ATLANTIC SALMON - EFFECTS ON GROWTH, NUTRIENT RETENTION AND SLAUGHTER QUALITY

Four high-energy diets, with protein contents of 42, 39, 37 and 35%, r espectively, were compared in a seawater experiment on Atlantic salmon smolt (Salmo salar). The test facility consisted of twelve 32 M3 cage s, each containing 400 Atlantic salmon which grew on average from 100 to 600 g during the experimental period. All diets were based on high- quality, low-temperature fish meal (LT meal). The 42% protein diet was the basal diet, and contained 2 1.1% fat. The others were made by add ing more fat to the basal diet, to produce fat contents of 2 5.5, 29.5 and 32. 1 %, and a proportional decrease in the other nutrients in th e diets. The gross energy levels-were 21.4, 22.2, 23.2 and 23.5 MJ/kg, respectively. The diets were administered isonitrogenously. To achiev e this, the feed level was made inversely proportional to protein cont ent. The fish on the diet with the lowest protein level grew 27% bette r than those on the highest level. This means that they responded to t he energy in the feed, despite the reduction of protein to 35% of the feed. The greatest response, however, was obtained with the 39% protei n diet. The feed conversion ratio (feed/gain) was significantly lower for the 39 and 37% protein diets than for the 42% diet. Slaughter qual ity, measured as dress-out percentage, dropped as the protein content of the diets decreased, but no significant differences were found betw een the diets containing 39% protein and those containing lower levels . Fillet fat content increased as dietary protein decreased, while fil let percentages remained unchanged. Use of high-energy/low-protein die ts seems to offer the fish-farming industry a means of reducing protei n consumption.