WHAT IS ENERGY?

ENZYMES

The energy content of a diet is derived from fats, proteins and carbohydrates, and the amount of each of these nutrients in a food will determine its energy content. In nutrition, energy has traditionally been expressed in terms of kilocalories (kcal) where 1 kcal is defined as the quantity of energy (as heat) required to raise the temperature of 1 kg of water by 1° C. A more recent convention is to express energy in terms of the joule. The conversion between the two systems is such that 1 kcal is equivalent to about 4.2 kilojoules (kJ).

Like other animals, fish obtain energy by oxidising (burning) food but, unlike the burning process in a boiler or engine, the energy is released gradually by a series of complex chemical reactions, each regulated by an enzyme. Enzymes are special proteins that control the rate of chemical reactions and more importantly, enable these complex changes to occur in the relatively mild conditions within the body. Many enzymes require the presence of vitamins or minerals to function properly.

ENERGY DIGESTIBILITY

No animal is able to utilise all the energy from its food. Energy intake is therefore considered at three different levels: gross energy (GE), digestible energy (DE), and metabolisable energy (ME). Gross energy is the total energy released by the complete oxidation of the food, and is usually measured by burning it in an atmosphere of pure oxygen in an instrument (calorimeter) which accurately measures the heat released on combustion. Although a substance may have a high GE content, it is of no use unless the animal is able to digest and absorb it. The amount that is digested and absorbed is known as DE and equals GE minus energy lost in faeces. Some of the absorbed food may only be partially available to the tissues, the remainder being excreted via the urine, or across the gills (branchial excretion). The energy that is ultimately used by the tissues is known as ME, and is calculated as DE minus energy lost in branchial and urinary excretion. The DE and ME contents of foods depend on their composition and the species that eats them. For example, energy from a plant source is more digestible to the carp than the piranha!

MAINTENANCE ENERGY REQUIREMENTS OF FISH

Maintenance energy requirement is the amount of energy needed by fish to survive, neither gaining nor losing weight, in a particular environment. In general, the maintenance energy requirement of fish is less than 10% of the maintenance energy requirement of birds and mammals (Smith, 1989).

This is largely due to the fact that fish are cold-blooded (poikilothermic) and so their body temperature is dependent on the temperature of the surrounding water.

In contrast, warm-blooded animals have to expend considerable energy to maintain their body temperature.

The energy requirements of five popular ornamental fish species have been calculated from research conducted at the WALTHAM® Aquacentre (Pannevis and Earle, 1994), and are shown in Table 1. These fish had a high level of activity, thus maintenance energy requirements are higher than if the fish had been less active. Table 2 shows the maintenance energy level requirements of a goldfish compared to warm-blooded birds and mammals.

CHANGES IN ENERGY REQUIREMENTS WITH TEMPERATURE

Because fish are poikilothermic, an increase in water temperature leads to an increase in the body temperature of the fish. This increases the rate of the metabolic reactions in the body, and so the energy requirements also increase. However, increases in energy requirement do not increase with temperature in a simple manner; both the magnitude of the change and the range over which it occurs are important factors. Studies at the WALTHAM® Aquacentre have shown that the maintenance food requirement of goldfish (Carassius auratus) increases nearly three-fold as water temperature rises from 20° C to 24° C. For koi carp in garden ponds, the maintenance energy requirement doubles when the water temperature increases from 10° C to 20° C.

PONDFISH

Energy requirements also decrease with decreasing environmental temperature. This is an important consideration when planning a feeding strategy for pond fish, as they virtually stop feeding when water temperatures reach 5° C and lower. At these low temperatures, digestion and absorption are very slow, so metabolism is reduced to a quiescent level and the fish survive by drawing on their energy reserves until the water temperature rises again in Spring. Thus it is vital to prepare pond fish for winter torpor by feeding them a higher level of digestible energy in the late summer and autumn. In contrast, tropical fish are kept at constant temperatures all year round (24° C to 26° C), however they also have an energy requirement far lower than that of warm-blooded mammalian pets.

OTHER REASONS FOR LOW ENERGY REQUIREMENTS OF FISH

Fish are efficient in the excretion of the waste products of protein metabolism. Some protein from the diet may be broken down and used for energy production, and this, combined with the breakdown of old and damaged proteins from body cells and tissues, produces ammonia, a toxic waste product that must be removed from the body. In mammals, ammonia, is converted to less toxic urea before being temporarily stored in the bladder, however this process uses energy. Fish take advantage of their aquatic environment and release the toxic ammonia directly into the water where it is usually diluted to safe levels before removal by bacteria and plants. Thus fish do not have to expend energy by converting ammonia to urea.

EFFICIENT LOCOMOTION

Fish also save energy by living in an aqueous environment. Water has a much higher density than air, and so is better able to support the weight of the body. Thus fish do not have to expend energy maintaining their posture, and this together with the streamlined body-shape of most species means that locomotion through the water is also extremely efficient.

ENERGY SOURCES FOR FISH

Both protein and fat are highly available energy sources for fish, fat supplying approximately twice the energy of protein. The value of carbohydrate as an energy source is variable between species, depending on the normal diet of the animal. Protein and energy in the diet should be balanced. A diet too high in energy will eventually lead to obesity, whilst a diet too low in energy will result in too much protein being used for energy in place of fat and carbohydrate.

The associated ammonia excretion could result in unacceptably high water pollution. Studies at the WALTHAM®Aquacentre have defined the optimum protein to energy ratio for ornamental fish species to be between 19 to 28mg digestible protein/kJ DE, and this is incorporated into AQUARIAN diets. In summary, the energy requirements of fish differ markedly from the energy requirements of terrestrial animals. Fish are much more efficient in energy utilisation, not only because they are poikilothermic, but also because they are more efficient in the excretion of protein residues, and spend less energy maintaining their posture in water.

Finally, it is important to balance the levels of fat, protein and carbohydrate in fish diets to ensure that adequate energy is available to different species under differing environmental conditions.

REFERENCES

Pannevis, M.C. & Earle, K.E. (1994).

Maintenance energy requirements of five popular species of ornamental fish J Nutr.124: S2616-2618.

Smith, R.R. (1989).

Nutritional Energetics, In Fish nutrition, 2nd ed. Halver, J.E. Ed. Academic Press, New York, USA.