Influence of level and type of dietary protein, and of level of feeding on feed utilization by rainbow trout.

In the first experiment, diets with 40%, 50% and 60% protein containing herring and soybean meals, and diets with 50% and 60% protein containing herring and soybean protein concentrates were fed at 100%, 85% and 70% of the ad libitum intake. Both increasing the protein level and using the concentrated protein sources resulted in lower gains, and in increases in the amounts of digestible protein and energy consumed for the deposition of protein and energy in the fish carcasses. This difference between the herring and soybean meals, and protein concentrates was not due to differences between their digestibilities. Restriction of feed intake reduced gain, but within these lower levels of gain, the 50% and 60% protein diets have higher final body weights than the 40% protein diet. Restriction of feed intake resulted in carcasses with higher protein and lower fat contents. In the second experiment, two series of diets containing either 40%, 35%, 30% or 25% protein from both herring and soybean meals, or 35%, 30% and 25% protein from herring meal were fed. Reducing the protein level below 40% resulted in lower final body weights, higher feed: gain ratios and carcasses with more protein and less fat. Comparisons of the isonitrogenous pairs of diets showed no differences in final body weights, feed: gain ratios and carcass composition. Reducing the protein level in the diet reduced the amount of protein consumed per unit protein level in the diet reduced the amount of protein consumed per unit protein deposition, but increased the amount of gross energy per unit energy deposited in the carcass.