Water pollution with phosphorus of dietary origin by intensively fed rainbow trout ( Salmo gairdneri Rich.)

Abstract

Analysis of water entering and leaving nine commercial trout farms at three times during summer showed that concentrations of ortho-phosphate in the water may increase substantially during passage through the farm. Growth rates and feed conversion ratios were not impaired if P concentrations of semi-purified trout diets were reduced from about 10 to about 4 g per kg dry matter. It may thus be practical to limit P concentrations of trout feeds to less than 10 g/kg. Economically attractive performance and comparatively little pollution with phosphorus may result from a feed containing 30% low-ash fish meal, 20% hydrolysed feather meal, 37% hydrolysed wheat and 11% fat.