Low-protein supplement increases protein retention and reduces the amountsof nitrogen and phosphorus wasted by rainbow trout fed on low-fat herring

The growth, food utilization and amounts of nitrogen and phosphorus lost in to the water by rainbow trout (initial size 485 g) fed on low-fat Baltic he rring and supplemented with a low-protein dry diet fed separately were stud ied in an 18-week net cage trial. Similarly to balancing protein and nonpro tein energy in full diets, giving nonprotein energy in different feeds seve ral hours apart improved the protein retention of low-fat herring to a leve l comparable to a commercially available dry diet control. The amounts of n itrogen and phosphorus lost into the water were also reduced to the same ex tent. The losses from the plain herring-feeding were twice as high. The imp roved protein retention and reduced nutrient losses were accompanied by 15- 25% reductions in growth of the fish compared with the control dry diet. Th e growth of the fish fed on plain herring was 14% lower than the growth on the dry diet control.