Partial Replacement of Herring Meal with Soybean Meal and Other Changes in a Diet for Rainbow Trout (Salmo gairdneri)

Growth rate, feed efficiency, and mortality were not adversely affected by reducing the level of herring meal in an open formula dry-pellet diet fed to rainbow trout (Salmo gairdneri) in aquaria through which 80–90% of the effluent water was recirculated. The herring meal was reduced from 35 to 18%, and the level of soybean meal increased from 10 to 39%. The fish gained 5.5 and 5.4 kg/100 fish from 8 to 40 wk of age on the high and low herring meal diets, respectively. Deletion of brewers' yeast, corn fermentation extractives, and whey powder from the diet with the higher level of herring meal increased weight gain to 7.0 kg/100 fish. When the fermentation by-products were removed from the diet with the lower level of fish meal the gain was reduced to 4.8 kg/100 fish. All the diets contained 4% of soybean or rapeseed oil or a marine oil; the type of oil did not influence the gains, but the conversion of feed to gain was most efficient for the diet containing the marine oil. The apparent digestibilities of the proximate components of three of the diets suggested that the fermentation by-products were not as digestible to the fish as the other components of the diet.