Energy Requirements of Fish1

Commercial diets for fish contain higher percentages of protein than commercial diets for livestock and poultry, although the dietary protein requirements for protein gain are nearly equal for fish and monogastric farm animals. The reason for this dietary disparity is that fish have lower energy requirements due to lower expenditures for maintenance and heat increment. Fasting heat production by subadult rainbow trout was measured to be 8.85 BW.82 kcal per fish per day, compared to Brody’s values of 70 BW.73 for mammals and 83 BW .75 for birds. Heat increment is lower for fish than for land animals, primarily because fish excrete most of their waste nitrogen as ammonia through the gills and do not have to expend energy for synthesis of urea or uric acid. The optimum dietary ratio of digestible energy (kilocalories) to digestible protein (grams) for fish growth is 9 to 11, compared to 15 to 19 for livestock and poultry.