Four high-energy diets, with protein contents of 42, 39, 37 and 35%, respectively, were compared in a seawater experiment on Atlantic salmon smolt (Salmo salar). The test facility consisted of twelve 32 m3 cages, 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 21.1% fat. The others were made by adding more fat to the basal diet, to produce fat contents of 25.5, 29.5 and 32.1%, and a proportional decrease in the other nutrients in the diets. The gross energy levels were 21.4, 22.2, 23.2 and 23.5 MJ/kg, respectively. The diets were administered isonitrogenously. To achieve this, the feed level was made inversely proportional to protein content.
The fish on the diet with the lowest protein level grew 27% better than those on the highest level. This means that they responded to the energy in the feed, despite the reduction of protein to 35% of the feed. The greatest response, however, was obtained with the 39% protein diet. The feed conversion ratio (feed/gain) was significantly lower for the 39 and 37% protein diets than for the 42% diet.
Slaughter quality, measured as dress-out percentage, dropped as the protein content of the diets decreased, but no significant differences were found between the diets containing 39% protein and those containing lower levels. Fillet fat content increased as dietary protein decreased, while fillet percentages remained unchanged.
Use of high-energy/low-protein diets seems to offer the fish-farming industry a means of reducing protein consumption.