FORMULATION OF FISH DIETS WITH REDUCED PHOSPHORUS-CONTENT

The reduction of phosphorous waste from farmed rainbow trout (Oncorhyn chus mykiss) was attempted by feeding diets of reduced P content. The protein source was fishmeal in which the bone fraction had been remove d. This was achieved either by solubilizing the fish material (press c ake) by enzymatic treatment and subsequent filtering, or by removing t he fish bones by a skin/bone separator. Two fish species were used, sa nd eel (Ammodytes marinus) and sprat (Sprattus sprattus). Due to the s mall size of the fish, only bones were retained in the skin/bone separ ator. Traditional fishmeal from the same species were used for control experiments. The fishmeal samples were used together with other ingre dients in a traditional extruded dry feed formulation (Ecolife 19). Th e diets were used in full scale experiments in a fresh water fish farm in Denmark, as well as in small scale experiments in aquaria. Direct calculations on the feed conversion ratio and the specific growth rate on both experiments showed that the enzyme treated codes performed le ss well than the other codes. However, when compensating for the diffe rent energy content of the diets, the enzymatic treatment did not give rise to a higher feed conversion ratio, nor did it give rise to a low er average growth rate than the other codes. it was not possible to ma ke any final conclusions with respect to the effect of the enzymatic t reated fishmeal on the pollution risk from P. This was due to large va riation on the P determinations.