Physiology of Finfish and Shellfish (2+1)

4.1.11. Nitrogenous Compounds

Since fish do not have to expend energy to change ammonia to urea or uric acid, they do not have to pay the energy expense to use protein for food as birds or mammals do. Fish pay about a 5% penalty whether protein is part of the meal or not. Since fish do not pay a penalty for eating protein, they can direct more of the energy in food towards production.