Physiology of Finfish and Shellfish (2+1)

4.3.2.3 Metabolism of energy in fish

Energy is needed for the maintenance of all living organisms. Most plants obtain their energy directly from the sun and use that energy to synthesize the complex molecules which make up the structural and storage parts of the plant. Animals cannot utilize/radiant energy from the sun. They get their needed energy from oxidation of the complex molecules which are eaten by the animal. The energy in feed is not available until the complex molecules are broken down to simpler molecules by digestion. The products of digestion are then absorbed into the body of the animal where oxidation processes occur which release the energy.

Energy metabolism in fish is similar to that in mammals and birds with two notable exceptions. These exceptions are:

(a) Fish do not expand energy to maintain a body temperature different from that of their environment. Fish and shrimp therefore have much lower maintenance energy requirements than terrestrial farm animals.

(b) The excretion of waste nitrogen requires less energy in fish than it does in homeothermic land animals. Fish and shrimp are able to obtain 10–20% more energy from the catabolism of proteins than terrestrial farm animals, as they do not have to convert ammonia (the end product of protein catabolism) into less toxic substances (ie. urea or uric acid) prior to excretion (Brett and Groves, 1979).

There are large differences in the ability of different species of fish to digest feed materials. Fish species range all the way from strict herbivores through omnivores to carnivores. The food requirements of different species of fish vary greatly. The job of the nutritionalist is to identify the needs of the animal and then to find feed materials which will most economically satisfy these requirements.