Ammonia excretion rate indicates dietary protein adequacy for fish

Yearling rainbow trout Oncorhynchus mykiss were used to determine whether ammonia excretion rate could serve as an indicator of the dietary protein requirement of fish. Over 10-d periods, fish were fed a fixed quantity of feeds containing 35, 40, or 45% protein. Ammonia excretion rate was not significantly different between fish fed the 35% (deficient) and 40% (optimum) protein diets, but increased significantly between fish fed the 40% and the 45% (excessive) protein diets. Average gain in weight by fish increased significantly between the 35% and 40% protein diets but not between the 40% and 45% diets. Both ammonia excretion and weight gain consistently indicated that 40% crude protein was the optimum level. Ammonia excretion rate can serve as a good indicator of the optimum protein content in the diet, especially when combined with data on growth rate. This approach looks promising for determining protein requirements of fish through studies of relatively short duration.