1. ENERGY METABOLISM IN FISH
2. ENERGY SOURCES
3. ENERGY REQUIREMENTS OF FISH
R. R. Smith
Tunison Laboratory of Fish Nutrition
1.1 Energy Flow in Animals
1.2 Energy Loss
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.
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 Also during times of low food intake fat and protein are withdrawn from the animal body to provide the energy needs for maintenance and the animal loses weight.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 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.
Fig. 1. Partition of Feed Energy (Adapted from: Harris)
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 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 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.
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.
3.1 Energy Distribution in Relation to Feeding Level
3.2 Maintenance Energy
3.3 Energy Cost of Growth
3.4 Factors that Alter Energy Needs
As discussed earlier the energy needs for maintenance and activity must be satisfied before any growth can occur. Feeding levels must be high enough to supply maintenance needs and still have energy remaining for growth. Digestion efficiency in fish decreases as feeding level is increased. The problem becomes one of finding the feeding level at which the increased efficiency of energy utilization at a high feeding rate is balanced by the lower efficiency of digestion at the higher feeding rate.
Fig. 2 illustrates the distribution of dietary energy intake in relation to feeding level in fish. Basal or standard metabolism in fish is relatively constant under constant environmental conditions. It can change with changes in temperature and fish size among other factors. The energy expended on voluntary activity usually increases somewhat with increasing feeding level. Starved fish are less active than well-fed fish but there is always some expenditure of energy for activity. The heat of nutrient metabolism is proportional to the level of feeding. The energy excreted in urine and gill excretions is also a function of feeding level. The reduced efficiency at high levels of feeding is shown in Fig. 2 by the proportionally large area representing faeces at high levels of feeding. The amount remaining for growth is zero at maintenance feeding and becomes proportionately greater as feeding level is increased, until it is balanced by the decreased efficiency, of digestion. Fig. 2 is not intended to show the relative magnitude of the fractions but only their relationship.
All of the energy lost due to standard metabolism, heat of nutrient metabolism and physical activity appears as heat. The maintenance requirement can be determined by measuring the heat produced. The heat production can be measured directly in a calorimeter or it can be estimated by measuring oxygen and applying the appropriate heat equivalent. The factor most commonly used is 3.42 kcal/mg O2. This factor is largely an extrapolation of data for mammals and has not been directly measured in fish. The heat equivalent of oxygen also varies with the type of substrate being oxidized. Maintenance energy can also be estimated by measuring energy loss during starvation.
It has been shown in mammals that the cost of growth is fairly constant after maintenance energy is subtracted from ME fed. This probably holds true for fish, but it has not been experimentally determined. More research is needed in this area.
There are several factors which can alter the energy requirements of fish. Feeding rates should be adjusted to compensate for these factors to avoid overfeeding, but still providing sufficient energy for optimum growth.
(a) Temperature. As environmental temperature declines homeotherms must increase their metabolic rate to compensate for the additional heat loss if they are to maintain a constant body temperature. Most freshwater fish do not attempt to maintain a body temperature which is different from the environment. As water temperature declines, body temperature of the fish declines and metabolic rate is reduced. The low metabolic rate at low temperatures enables fish to survive for long periods under ice where little food is available. There is considerable species difference in metabolic adaptation to environmental temperature changes. Each species seems to have a preferred temperature at which it functions most efficiently. If temperature gradients exist, the fish will seek the most favourable temperature. Usually this is the temperature at which the difference between maintenance requirement and voluntary food intake is greatest and at which optimum efficiency of growth occurs.
(b) Water Flow. Energy which is used for physical activity is not available for growth. Fish which are forced to swim against a strong current are expending energy which would otherwise be used for growth. However, still water allows stratification and the accumulation of waste products. Fish rearing facilities should be designed to obtain maximum use of water without undue stress on the fish.
(c) Body Size. Small animals produce more heat per unit weight than do large animals. Small fish should be fed a higher percentage of body weight than large fish. In mammals the metabolic rate is proportional to the three-fourths power of body weight (W0.75). The exponent applicable to fish has been reported from 0.34 to 1.0. The factor W0.8 usually used. Obviously more work is needed in this area. Work at the Tunison Laboratory of Fish Nutrition (Idaho, U.S.A.) has indicated that rainbow trout from 1.0 to 4.0 g in weight have a metabolic rate proportional to W1.0. Fish from 4.0 to 50.0 g in weight have metabolic rates proportional to W0.63.
(d) Level of Feeding. The level of feeding also has an effect on the energy expenditure of fish. This becomes important in design of fish rearing facilities. Dissolved oxygen is usually the first limiting factor in fish rearing. The oxygen consumption increases shortly after feeding due to the physical activity of feeding and the heat of nutrient metabolism. Facilities must be designed with adequate safety margins. The oxygen required per unit weight of feed also varies with feeding level, being higher at maintenance level when all the food is oxidized than at higher feeding levels when much of the energy is stored as growth.
(e) Other Factors. Several other factors can contribute to high energy requirements. Anything which makes the fish uncomfortable increases physical activity and reduces growth. Crowding, low oxygen and waste accumulation are some of these factors.
Fig. 2. Distribution of dietary energy intake in a growing fish at various levels of feeding. (DE - digestible energy, ME - metabolizable energy, NEp - net energy for production, NEm - net energy for maintenance, Hp - heat production)
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