Energy is defined as the capacity to do work, and is derived by animals through the catabolism of dietary carbohydrates, lipid and protein within the body. Although many forms of energy exist in nature (ie. radiant, chemical, mechanical, heat, and electrical energy), all have the capacity to do chemical, electrical and mechanical work. Energy is therefore essential for the maintenance of life processes such as cellular metabolism, growth, reproduction, and physical activity. In particular, life on earth is dependent on radiant solar energy and its subsequent fixation and conversion by green plants during photosynthesis into stored chemical energy (ie. carbohydrates) for use as an energy source by plants themselves or for animals that consume them through respiration. For example, when an animal during respiration releases the potential energy of glucose, approximately two-thirds of it is converted into mechanical energy to be used for work (activity and growth), and one-third is given off as heat.
All forms of energy are inter-convertible and obey the laws of thermodynamics. The first law of thermodynamics states that energy may be transformed from one form into another, but can never be created or destroyed. For example, solar energy can be transformed into heat energy or into plant-food energy (chemical energy). During this transformation no energy is lost or destroyed. The second law of thermodynamics states that no transformation of energy will occur unless energy is degraded from a concentrated form to a less concentrated or more dispersed form, and further that no transformation is 100% efficient. All biological systems follow this law during the conversion of solar energy (a high energy form) into chemical energy during photosynthesis; a part of the energy transformed from solar to chemical energy being dissipated as heat energy into the surrounding environment.
Energy is usually expressed in terms of heat units, since all forms of energy are convertible into heat energy. The basic heat unit normally used is the calorie. One calorie is defined as the amount of heat required to raise the temperature of one gram of water by one degree centigrade. Since for many purposes the calorie (cal) is too small a unit of measurement, the kilocalorie (kcal) is often used; 1 kcal = 1000 cal. In many scientific studies the calorie is now being replaced by the joule (J) as the unit of energy; 4.184 J = 1 cal.
It follows from the above introduction that the major food nutrients (ie. carbohydrates, proteins and lipids) are required by animals not only as essential materials for the construction of living tissues, but also as sources of stored chemical energy to fuel these processes as work. The ability of a food to supply energy is therefore of great importance in determining its nutritional value to animals.
The chemical energy of food can be measured in heat units by means of a bomb-calorimeter. Here a known quantity of food is placed in an insulated metal container and oxidised by combustion to carbon dioxide, water, ash, nitrogen, and other gases, and the liberated heat energy measured; the bomb-calorimeter first being calibrated using a known weight of a thermochemical standard such as benzoic acid (benzoic acid has a total heat energy value of 26.45 kJ/g). The quantity of heat resulting from this oxidation is known as the heat of combustion, calorific value, or gross energy value of the food or substance. Using this technique the mean gross energy value for carbohydrate, lipid and protein has been estimated to be 4.1 kcal/g (17.2 kJ/g), 9.5 kcal/g (39.8 kJ/g), and 5.6 kcal/g (23.4 kJ/g) respectively (Cho, Slinger and Bayley, 1982). In view of the much higher energy value of dietary lipid, it follows that the total gross energy value of individual feed sources will depend on the relative proportions of the major food nutrients and water present. For example, Table 14 shows the gross energy value of some selected feed ingredients commonly used for fish and shrimp.
|Feed ingredient||Gross energy value (kJ/g)|
|Meat and bone meal||17.6|
|Poultry by-product meal||19.6|
|Fish meal, herring||20.7|
|Full-fat soybean meal||22.3|
1 Source: Cho, Slinger and Bayley (1982)
Energy metabolism is concerned with the catabolism and oxidation of carbohydrates, lipid and protein within the animal body, and the consequent release and use of the liberated energy as work for the maintenance of the life process. The sequence of biochemical events which lead to the release and utilization of food energy is well known and common to all farm animals. For detailed information on this aspect readers should refer to any standard text book on cell biochemistry. It is sufficient to mention here that the free energy liberated from the catabolism and oxidation of the major food nutrients is not utilized directly by the animal, but rather is trapped in the chemical form of the energy-rich phosphorus bond of adenosine triphosphate (ATP). It is ATP which is the principal driving force in the energy-requiring biochemical processes of life. For example, energy for muscle contraction is provided by the cleavage of ATP to adenosine diphosphate (ADP) and inorganic phosphate. The central role of ATP in cellular energetics is shown in Figure 5.
Energy metabolism in fish and shrimp is different from that of terrestrial farm animals in two important respects. Firstly, in contrast to warm-blooded animals, fish and shrimp are aquatic ectotherms and so do not have to expend energy in maintaining a body temperature well above ambient at 37°C. Fish and shrimp therefore have much lower maintenance energy requirements than terrestrial farm animals (Cho and Kaushik, 1985). For example, the maintenance energy requirement per unit body weight of the chick is reported to be about five times greater than that of common carp growing at 23°C (Nijkamp, van Es, and Huisman, 1974). Secondly, 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). For example, in salmonids 85% and 15% of the branchial and urinary nitrogen loss is reported to be in the form ammonia and urea respectively (Luquet, 1982). The excretion of waste nitrogen therefore requires less energy in these aquatic animals; a diagrammatic representation of nitrogen balance in fish is shown in Figure 6.
Figure 5. Central role of ATP in cellular energetics
Figure 6. Generalized view of nitrogen balance in fish (after Luquet, 1982)
A schematic representation of the fate of dietary energy in salmonid fish is shown in Figure 7. It is apparent from the energy balance presented that only a portion of the total chemical energy contained within the food ingested is available to the animal (ie. net energy) for maintenance, activity and growth; 45% of the ingested food energy being lost as undigested food (faeces), metabolic excretion and as heat. Although the energy losses will vary depending on the composition and digestibility of the feed ingredients used, feeding regime, water temperature, fish size and the physiological status of the animal, since all biological systems obey the laws of thermodynamics the energy balance equation can be represented as follows:
Figure 7. Utilization of dietary energy in salmonids (after Luquet, 1982)
C = P + R + U + F
where C (for consumption) is the gross energy content of the food ingested, P the energy utilized in growth materials (production), R the net loss of energy as heat (R standing for respiration), U (urinary loss) the energy lost in nitrogenous excretory products, and F the energy lost in the faeces (Brafield, 1985). Using this approach Brett and Groves (1979) have produced the following generalised energy budgets for young carnivorous and herbivorous fish fed on natural foods;
Carnivores: 100C = 29P + 44R + 7U + 20F
Herbivores: 100C = 20P + 37R + 2U + 41F
where the figures are expressed as a percentage of the ingested food energy. For a more detailed account of energy partitioning in fish readers should refer to the reviews of Cho and Kaushik (1985), Brafield (1985), Soofiani and Hawkins (1985), and Knights (1985).
It is evident from the above that the energy needs for maintenance and voluntary swimming activity must be satisfied before energy can be made available for growth. Clearly, since fish like other animals, eat primarily to satisfy their energy requirements (Cho and Kaushik, 1985), it is essential therefore that they receive either unrestricted access to food, or a palatable ration of sufficient energy density to meet all their energy requirements. Providing the optimum energy level in diets for shrimp or fish is important because an excess or deficiency of useful energy can result in reduced growth rates (NRC, 1983). For example, excess dietary energy may result in decreased nutrient intake by the fish or excessive fat deposition in the fish, whereas a low dietary energy density will result in the animal utilizing nutrients for energy provision rather than for tissue synthesis as growth (Robinson and Wilson, 1985).
At present there is very little useful information on the practical dietary energy requirements of fish or shrimp; this has been due primarily to difficulties encountered with the quantitative measurement of the energy losses within the energy budget equation (Brafield, 1985). However, factors known to influence the energy requirements of fish and shrimp, include 1) water temperature (metabolic rate, and consequently maintenance energy requirements, increasing with temperature; Brett and Groves, 1979), 2) animal size (metabolic rate, and consequently maintenance energy requirements, decreasing with increasing animal size; Brett and Groves, 1979), 3) physiological status (energy requirements increasing during periods of gonad production and reproductive activity such as spawning migration; Wooton, 1985), 4) water flow (energy requirements for maintaining station in water increasing with increasing water flow; Brett and Groves, 1979; Knights, 1985), 5) light exposure (energy requirements for voluntary activity being less during night-time ‘rest’ periods), and 6) water quality and stress (pollutants, increased salinity, low dissolved oxygen concentration, and excessive crowding increasing the maintenance energy requirements; Talbot, 1985; Knights, 1985).