Influence of Dietary Protein and Lipid on Nitrogen and Energy Losses in Lake Trout,Salvelinus namaycush

Abstract

Nitrogen and energy loss in fecal and nonfecal excretions by lake trout, Salvelinus namaycush (171–268 g), varied directly with dietary protein (40 and 50% dry weight) and inversely with lipid (10 and 20%) among four test diets. Diets high in lipid increased dry matter digestibility resulting in reduced nitrogen and energy loss in feces. NH3-N constituted 74.0–93.0% of total N excreted in fasted (4–12 d) and fed trout, while urea-N constituted 4.6–17.8% of total N. Excretion of urea-N was high in fish fed the high-lipid diet, particularly when dietary protein was also high. Total N and NH3-N excretion rose to peak values between 4 and 12 h after feeding and thereafter declined to prefeeding levels by 24 h. Urea-N excretion rates varied significantly following ingestion of all but one (50% protein, 20% lipid) test diet, reaching peak values between 4 and 12 h. In fasted fish, total N and NH3-N excretions were significantly low and failed to show a daily pattern; the urea-N excretion rate was significantly high compared with fish fed diets 1, 2, and 3 and followed a daily pattern of change. On average, a larger proportion (13–25%) of food energy is lost through nitrogenous excretion than in the feces (8–22%).