The protein sparing effects of high lipid levels in diets for rainbow trout (Oncorhynchus mykiss, W. 1792) with special reference to reduction of total nitrogen excretion

Abstract

This research sought to develop low-protein, high-energy (less-polluting) diets for rainbow trout to properly utilize protein and reduce total nitrogen excretion. Duplicate groups of rainbow trout were fed one of four experimental diets with a protein to energy (P:E) ratio of 25, 24, 22 or 19 mg/kJ and a lipid content of 12, 13, 17 or 26%, respectively. The diets were given to fish with a mean initial weight of 181 g for 70 days. At the conclusion of the trial, mean weights ranged from 353.17 g (94% weight gain) to 394.75 g (118% weight gain) with 100% survival in all treatments. Weight gain, feed efficiency and protein retention increased as the dietary lipid content increased and as the P:E ratio decreased. Fish fed the 22 mg protein/kJ energy diet (17% lipid, 44% protein, 20.34 kJ/g gross energy) performed best. This indicates that the protein content in practical trout feeds can be reduced from the currently used 47% to around 44% without reducing the growth rate and feed efficiency, if high quality protein is used and the gross energy is increased by lipid to about 20.34 kJ/g diet. The low-protein, high-energy diets reduced the total nitrogen excretion from the rainbow trout by 27%.