ENERGY SOURCES

Energy is stored in the chemical structure of the complex molecules of feed materials. When oxidation occurs, energy is released and is available to do work. This released energy is trapped by biochemical reactions and is used to drive the energy requiring reactions necessary to sustain life.
The energy needs of fish are supplied by fats, carbohydrates, and proteins.

Fats

Fats are the principal form of energy storage in plants and in animals. Fat contains more energy per unit weight than any other biological product. The inclusion of fat usually increases the palatability of a feed. Generally fats are well digested and utilized by fish. There is little hard data on the ability of fish to digest fats of different melting points. It is usually estimated that fat provides 8.5 kcal metabolizable energy (ME) per gramme. The fatty acid products of digestion are well utilized by most fish. There is some evidence that high levels of short chain fatty acids can depress growth. This is seldom a problem in practical diets.
Natural diets may contain as much as 50 percent fat. High levels of fat can also be used in manufactured feeds if other nutrients are adequate. Full fat oilseed meals may be the most practical way to add fats.

Carbohydrates

Carbohydrates are the cheapest and most abundant source of energy for animals. Most of plant material is carbohydrate. Carbohydrates in feed material range from easily digested sugars to complex cellulose molecules which cannot be digested by animals. It is only through their symbiotic relationship with bacteria that ruminant animals can utilize large amounts of cellulose. There is controversy as to the value of carbohydrate in fish feeds. It appears, however, that digestible carbohydrate can be well utilized as an energy source if it is kept in proper balance with other nutrients.
The ME values of carbohydrates for fish range from near zero for cellulose to about 3.8 kcal/g for easily digested sugars. Raw starch ranges from 1.2 to 2.0 kcal ME/g. Cooking of starch can increase the ME to about 3.2 kcal/g. Heat and moisture associated with the pelleting process improves the digestibility of starchy feed materials. The value of carbohydrate in fish diets depends on the source and type of carbohydrate and the processing to which it has been subjected.

Protein

In nature, carnivorous fish consume diets which are about 50 percent protein. Fish have a very efficient system for excretion of waste nitrogen from protein which is catabolized for energy and therefore high protein diets are not harmful. Protein is often the most expensive source of energy in manufactured diets and should be kept to a minimum, consistent with good growth and feed conversion. Protein has a ME value of about 4.5 kcal/g for fish, which is higher than that for mammals and birds. The low energy cost of excreting waste nitrogen in fish is primarily responsible for this.
In general, proteins from animal sources are more digestible than those from plant sources. Processing methods can also influence protein quality. Heating increases the digestibility of some proteins and reduces that of others. Protein is used very efficiently by fish as a source of energy but for economic reasons should be kept to a minimum, consistent with good growth, and cheaper carbohydrate and fat should be used to supply most of the energy.