The bioenergetics of grass carp, Ctenopharyngodon idella (Val.): the influence of body weight, ration and dietary composition on nitrogenous excretion

Nitrogenous excretion by grass carp, Ctenopharyngodon idella (Val.), was measured in the form of ammonia and urea. Endogenous nitrogen excretion (ENE) was estimated as the daily rate of excretion by grass carp which had been starved for 2 days. ENE was scaled allometrically with body weight with weight exponents of 0.75 for ammonia, 0.63 for total nitrogen and 0.63 for the energy lost. The proportion of nitrogen attributable to urea was smaller than that attributable to ammonia and decreased from 25 to 12% as fish weight increased from 2 to over 10 g.
Linear relationships were found between daily rates of ammonia, total nitrogen and energy loss and daily rates of food intake. High carbohydrate and high lipid diets were not shown to have a protein-sparing action compared to a high protein diet. Differences in the amount of nitrogen excreted were explained by the differing nitrogen contents of the diets. Nitrogen budgets were erected and their implications discussed.