Yearling rainbow trout Oncorhynchus mykiss were used to determine whet
her 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% (defici
ent) and 40% (optimum) protein diets, but increased significantly betw
een fish fed the 40% and the 45% (excessive) protein diets. Average ga
in in weight by fish increased significantly between the 35% and 40% p
rotein diets but not between the 40% and 45% diets. Both ammonia excre
tion and weight gain consistently indicated that 40% crude protein was
the optimum level. Ammonia excretion rate can serve as a good indicat
or of the optimum protein content in the diet, especially when combine
d with data on growth rate. This approach looks promising for determin
ing protein requirements of fish through studies of relatively short d
uration.