Replacement of protein by fat and carbohydrate in diets for Atlantic salmon (Salmo salar) at the end of the freshwater stage

The optimal dietary protein level in diets for Atlantic salmon (Salmo salar:) at the end of the freshwater stage (start weight, 80g) was investigated in a 9 week comparative slaughter experiment. At each of two main fat levels (24-30% (HF) and 16-17% (LF), dry matter basis), four diets were formulated in which protein (41-64%) was replaced by carbohydrate (9-22%). A minimum of 55% protein in the diet, in combination with a digestible energy content of 22.7 MJ kg(-1), resulted in the most optimal growth (131 +/- 2g) and feed efficiency ratio (FER) (1.57 +/- 0.01g wet gain per gram dry feed). Nitrogen retention was improved by increasing the carbohydrate content from 11 to 18% in the HF diets and from 9 to 22% in the LF diets. This occurred, however, with decreases in growth and FER. Protein sparing effects of fat were also found, but tended to result in reduced gain. Fat content in whole fish was positively related to fat level in the diet and varied from 7.4 to 11.6% of body weight. Carbohydrate digestibility was inversely related to carbohydrate inclusion level and was significantly lower in the high-fat diets than in the low-fat diets, at each carbohydrate level. The diet giving the best gain and FER at the lowest possible dietary protein level resulted in greater metabolic nitrogen loss per kg gain but lower fecal DM loss and body fat content compared with diets containing lower protein levels. This is a compromise which needs to be continually evaluated depending on the importance of the economical, environmental and product quality factors. (C) 1997 Elsevier Science B.V.