Physiology of Finfish and Shellfish (2+1)

4.1.10. Effects of the Aquatic Habitat

All animals create ammonia which is mitogeous exectry pobduck during the catabolism of proteins. Ammonia is toxic and its accumulation in the body cannot not be tolerated. Mammals use energy to turn ammonia into less toxic urea in the liver. Urea is water soluble and mammals excrete it in the urine. In order to reduce water weight for flight, birds create another high energy nitrogenous waste product, uric acid, which is excreted with the feces. Since, fish have constant exchange with water at the gill, they can excrete ammonia directly, without conversion, as soon as it is created. Mammals and birds pay a caloric penalty for eating protein. When carbohydrate or fat is metabolized, about 5% of the assimilated energy is used in changing the structure of the food molecules into forms the body can use and the remaining 95% can be used for maintenance and growth. When mammals and birds metabolize protein, however, the penalty increases to about 15% because energy must be used to build urea or uric acid and these energy containing substances are excreted form the body and lost.

Metabolizable Energy (ME) = Feed Energy - Fecal Energy - Energy of Excreted.