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From among the two types of energy, heat energy utilized for maintaining body temperature and the free energy available for biological activity and growth, the latter is more important for poikilothermal animals like fish. Free energy is needed for maintenance, growth and reproduction. Seaweeds and other plants can obtain it directly from the sun and water and synthesize the complex molecules that constitute its structural parts. Animal species have to depend on the oxidation of the complex molecules contained in the food that they eat for energy requirements. The complex molecules are broken down during digestive processes to simpler molecules and are absorbed into the body, where oxidation occurs and energy is released. The biological process of energy utilization is known as metabolism and the rate at which it is utilized is referred to as the metabolic rate.
Energy metabolism in cold-blooded animals such as fish is different from that in mammals and birds in that they do not expend energy to maintain a body temperature different from their environment as warm-blooded animals do, and the excretion of waste nitrogen requires less energy than in homeothermic land animals.
Fish are the most efficient converters of energy and protein among all farmed animals. While the energy and protein conversion efficiencies in farmed warm-blooded animals (sheep, cattle, pigs, chicken) are in the ranges of 1.7–17 per cent and 3–12 per cent respectively, the corresponding ranges for salmonids are much higher at 30–40 per cent and 20–25 per cent respectively (Rerat and Kaushik, 1995; De Silva, 1999).
The metabolic rate in fish, which is probably the most studied aquaculture animal group, is influenced by temperature, age or size, activity and seasonal and diurnal fluctuations of body function. It is also affected by oxygen or carbon dioxide concentration, pH and salinity of the water.The energy requirements necessary for all metabolic functions can be calculated for each species. For example, carp utilize 25 cal/dec2/h at 25°C. Approximately 70 per cent of this is used for maintenance and growth and the remaining 30 per cent is lost to the environment. As the body temperature of the fish is maintained at or near the environmental water temperature, the heat that is produced is lost to the environment. The biological partition of energy is shown in fig. 7.1. Energy is also lost in faeces, urine and gill excretions, besides the small amounts of heat lost from external body surface.
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