ENERGY METABOLISM 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 expend energy to maintain a body temperature different from that of their environment; and
(b) the excretion of waste nitrogen requires less energy in fish than it does in homeothermic land animals.
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 nutritionist is to identify the needs of the animal and then to find feed materials which will most economically satisfy these requirements.

Energy Flow in Animals

The biological partition of energy in fish is shown in; Figure 1. It should be kept in mind that energy needs for maintenance and voluntary activity must be satisfied before energy is available for growth. Also during times of low food intake fat and protein are withdrawn from the animal body to provide the energy needs for maintenance and the animal loses weight.

Energy Loss

Energy is lost from the body of a fish in the faeces, urine, gill excretions and as heat. Also small amounts are lost from external body surface.
The energy lost as heat comes from three sources which are difficult to measure separately. These are:
(a) standard metabolism (SM), which is the energy required to keep the animal alive and is similar to basal metabolism measured in humans. Because of the difficulty of obtaining a "motionless" animal, the definition of basal metabolism is not applicable to fish. When a fish is restrained to a motionless condition it struggles to free itself and uses more energy than if allowed to swim freely in still water. SM is the minimum heat production of an undisturbed fish in the "post absorptive" state in still water;
(b) voluntary physical activity, which is the energy expended by a fish moving about, seeking food, maintaining position, etc.;
(c) heat of nutrient metabolism, also called heat increment or specific dynamic action (SDA), which is the heat released by the many chemical reactions associated with the processing of ingested feed. It includes the energy expended in digestion, absorption, transportation, and anabolic activities. It also includes the cost of excretion of waste products.