ARTICLE FEED & ADDITIVE MAGAZINE February 2024 69 is zero at maintenance feeding and becomes proportionately greater as the feeding level is increased, until it is balanced by the decreased efficiency, of digestion. FACTORS THAT ALTER FEEDING LEVELS TO MEET ENERGY NEEDS Feeding rates should be adjusted in order to compensate for these factors to avoid overfeeding, while still providing sufficient energy for optimum growth and these factors are: (a) Temperature. As environmental temperature declines homeotherms increase their metabolic rate to compensate for the additional heat loss and maintain a constant body temperature. However, most freshwater fish do not attempt to maintain a body temperature. As water temperature declines, body temperature consequently declines and metabolic rate gets 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 as each species seems to have a preferred temperature at which it functions most efficiently. Temperature fluctuations are rare in the wild but occur in aquaculture setups. Therefore, this factor is quite significant. (b) Water Flow. Fish which are forced to swim against a strong current expend energy which would otherwise be used for growth, while on the other hand if the water is stagnant, it causes stratification and the accumulation of waste products. Therefore, fish-rearing facilities must be designed such that maximum water is utilised wisely without undue stress on the fish. (c) Body Size. Small animals produce more heat per unit weight than large animals. Small fish should be fed a higher percentage of body weight than large fish. Energy demand of a piece of tissue depends on the size of the animal of which it is a part. This is the scaling effect. Y= aXᵇ Where, Y - any physiological variable, here metabolic rate a - Proportionality constant X - body mass b - Describes the effect of size on Y In fishes, the metabolic rate is proportional to the three-fourths power of body weight (W0.8) (commonly used). The exponent applicable is reported to range from 0.34 to 1.0. (d) Level of Feeding and Oxygen requirement. The oxygen required per unit weight of feed varies with feeding level, being higher at the 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, such as those Figure 3. Energy Flow in Fishes
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