Effects of dietary protein and lipid on nitrogen losses in rainbow trout, Salmo gairdneri

Abstract

Nitrogen excretion (total N, NH 3 and urea-N) across the gills ( Z N) and in the urine ( U N) as well as in the feces ( F N) was measured for individual rainbow trout (250–550 g) in relation to dietary protein and lipid. Total N excreted across the gills represented over 96% of the combined Z N and U N. The proportion of N excreted as NH 3N was much higher across the gills than in the urine. For the four diets of 35 or 49% protein and 10 or 20% lipid fed at 0.5 and 1.0% body weight d −1, total N excretion ( Z N + U N) was proportional to N intake raised to the power of 0.383. N excretion was slightly but not significantly reduced by high dietary lipid levels. The proportion of NH 3N excreted ( Z N + U N) relative to ingested N increased with dietary protein concentration at both meal sizes. Urea-N relative to N intake did not differ with dietary protein or lipid concentration. Fecal-N relative to intake varied inversely with dietary N and was independent of dietary lipid at both meal sizes. Digestibility varied directly with dietary concentration of protein and lipid and inversely with starch. Recovered N ( R N) was approximately one-half of N intake for all diets at both meal sizes.