FEATURE
Bioenergetics - application in aquaculture nutrition by Ingrid Lupatsch, Centre for Sustainable Aquaculture, Swansea University, United Kingdom
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ioenergetics describe the flow of energy and nutrients within a biological system in our example a fish or shrimp. It describes the biological process of utilisation and transformation of absorbed nutrients for energy, for own body synthesis. The feed, that is consumed, is transformed in the body, complex chemical compounds are broken down into simpler components protein into amino acids, carbohydrates into glucose, lipids into fatty acids and with this process energy is released - which is used for maintenance, for renewing worn out tissue and building new tissue - for growth. The major organic compounds in feeds such as lipid, protein and carbohydrates are the sources of energy but they also supply the building material for growth. There are different types of energy, chemical energy, electrical energy, mechanical energy and heat. These different forms of energy can be transformed into each other but only at a cost, the transformation is not 100 percent efficient. What is lost is mostly in the form of heat. Heat is also the only form of energy, into which all the others can be transformed and measured. The chemical energy stored in feed and animal tissue is measured using a bomb calorimeter. The amount of heat produced by complete oxidation of feed or tissue is known as the heat of combustion or gross energy (GE). Heat energy is usually expressed in kilocalories (kcal) or kilojoule (kJ). One kcal equals the energy needed to raise the temperature of one kg of water by one degree Celsius (째C). One kcal equals 4.184 kJ.
For the bio-energetic model, the two laws of thermodynamics can be applied 1. Energy cannot be created or destroyed within a system but may be changed into different forms (what goes in must go out) 2. In a system where energy is transformed (from feed to flesh) there is a degradation and loss of energy in the form of heat (nothing is 100 percent efficient)
The flow of energy from feed to growth in an animal is illustrated in Figure 1. Not all the energy from the feed is digested, substances such as fibre and cellulose from plant ingredients pass through the digestive system without being available to the fish. The consumed GE minus faecal energy losses (FE) is called the digestible energy (DE) which is then available for the metabolic processes of an animal. The next major losses occur, when energy containing compounds (on DE basis) are transformed by the fish, broken down to smaller units and then used to build its own energy reserves or to deposit protein as growth. As mentioned above, this process of transformation is never 100 percent, there are always losses and they are mostly in the form of heat. In poikilotherms 18 | International AquaFeed | March-April 2013
such as fish this heat is lost to the surrounding water, in homeotherms it is partly used to keep the body temperature constant. Only the net energy (NE) is now available for maintenance and for growth. Maintenance requirement represents energy needed for movements, osmo-regulation, blood circulation, first this energy has to be supplied before the remainder can be channeled into growth - the main product in fish culture.
Quantification of energy demand in fish By quantifying the energy budget - the energy input on one hand and the various energy losses on the other hand, valuable information can be gained in order to optimise feeds and guarantee optimal fish growth. By defining demands for maintenance and growth (Figure 1) and anticipating certain losses beforehand, feeds can be formulated and feeding tables established.
Figure 1: Schematic presentation of the energy flow through a fish