The primary concern in fish culture is to increase fish production per unit of culture space. Supplementary feeding with artificial diets is an effective measure to increase fish production. However, to render the production economical, supplemental diets must be formulated in accordance with the basic nutritional requirements of the particular species, which include protein, lipid, carbohydrate and energy.
Knowledge of the optimum dietary protein level is a prerequisite to formulate a nutritionally balanced low-cost diet for feeding fish. As protein represents the most expensive component in fish feed, it is important to determine the optimal requirement level for growth and survival. A significant reduction in feed cost can be achieved if diets with low protein could be fed to fish without compromising growth and health. However, inadequate protein in the diet results in reduction or cessation of growth. On the other hand, if too much protein is supplied in the diet, only part of it will be used to make new proteins and the remainder will be converted to energy. Besides these, inclusion of dietary protein levels beyond the optimum level results in high level of ammonia production, which affects the voluntary feed intake and growth of fish. But the utilization of dietary protein by an organism depends on types of diet, digestibility of dietary protein, its amino acid profile, the ratio of energy to protein in diet and the amount of protein supplied. Other factors that affect protein utilization are animal size, sex, genotype and environmental conditions.
Carbohydrate serves as the least expensive source of dietary energy and helps in improving the pelleting quality of practical fish diets. Feed cost per unit of fish produced can be minimized by optimal use of low-cost energy carriers such as carbohydrate-rich ingredients, ensuring that the use of costly protein is kept as low as possible. Replacing dietary protein by carbohydrate or lipid energy may result in a higher production per unit spent of costly protein sources such as fish meal, and the effluent nitrogen can be reduced per unit of fish produced. Carbohydrates also served as precursors for various metabolic intermediates necessary for growth, i.e., dispensable amino acids and nucleic acids. In the absence of adequate dietary carbohydrates or lipids, fish have only protein available to meet their energy needs. Insufficient energy from non-protein sources demands more protein to be catabolized to provide energy leading to lower growth rate of fish as less protein is available for its growth. When other energy sources are available, the protein is utilized for growth instead of energy. Therefore, it is important to provide an adequate carbohydrate level in the diet in order to reduce catabolism of protein for energy and for synthesis of glucose, which reduce protein retention and increases the nitrogen release to the environment. The ability of fish to utilize carbohydrate appears to be related to their digestive and metabolic systems adapted to the different aquatic environment and dietary carbohydrate level and complexity. The carbohydrate utilization of the fish depends up on the feeding habit, structure and function of the digestive system. The capacity of fish to utilize carbohydrate varied by species and in response to variables such as digestibility and starch complexity, degree of gelatinization of starch, sources of carbohydrate, interaction of carbohydrate with other nutrients and ratio of gelatinized to non-gelatinized starch in the diet.
Lipid is considered as one of the important nutrient next to protein, which plays a major role for optimum utilization of dietary protein for growth. Lipids are almost completely digestible by fish and seem to be favoured over carbohydrate as an energy source. Special attention is now being given by researchers, feed manufactures and farmers to develop feeds, which maximize nutrient retention with minimum nutrient loss. Fish are known to utilize protein preferentially to lipid or carbohydrate as an energy source. Therefore, it is important from a nutritional, environmental and economical point of view to improve protein utilization for tissue synthesis rather than energy purposes. The optimization of dietary digestible protein/ energy ratio (DP/PE) has proven to have an important role on protein and energy utilization. The increase of DE content of fish diets, by lipid supplementation, has been shown to have a protein sparing effect, therefore reducing nitrogen to the environment.
The increase in dietary lipid levels must be carefully evaluated as it may affect the carcass composition due to an increase of lipid deposition. The localization and composition of lipid deposits also strongly influence the nutritional value, organoleptic properties, transformation yields and storage time of fish carcass. Excess lipid not only suppresses de novo fatty acid synthesis, but also reduces the ability of fish to digest and assimilate it, leading to reduced growth rate. Again, excess lipid in the diet may also result in the production of fatty fish ultimately having a deleterious effect on flavour, consistency and storage life of the finished product. Excessive amounts of lipid in diet also possess problem in feed manufacturing.
Successful fish culture depends up on the provision of diets containing adequate levels of energy and appropriate balance of nutrients to permit the most efficient growth and to maintain the health of the animal under given circumstances. Dietary energy level is also critical because protein in the feed is utilized as an energy source when feed deficient in energy is fed to fish; whereas, when feed excessive in energy is fed, feed consumption decreased and result in growth reduction due to lack of other necessary nutrients for normal growth. As protein constitutes the single most expensive item in fish diets, it is imperative to incorporate only the amount necessary for normal maintenance and growth. Any excess dietary protein is considered as biologically and economically wasteful. Incorporation of appropriate levels of non-protein energy sources in the diet determines the efficiency of protein utilization and hence the growth of fish. Carbohydrate and lipid are the major non-protein sources in fish diet. Compared to lipid, carbohydrate is much less expensive, available abundantly and a ready source of energy. Carbohydrate also improves the pelleting quality of the diet due their reasonably good binding properties. Therefore, it is suggested that the carbohydrate may be added in excess of the required amounts that can be efficiently utilized for energy by fish. Again, use of high level of lipid as dietary energy source may create problem in pelleting and keeping quality of feed in addition to adversely affecting the fish whole body composition.
Not only the assessment of dietary protein (P) and energy (E) are necessary but also understanding the relationship between these two requirements has become increasingly important. The diets with optimum PIE ratio produce best growth, feed utilization and protein retention. Improper dietary protein, energy levels and/or their ratio will lead to an increase offish production cost and deterioration of water quality resulting from waste feed and fish excreta; thus, they are important in formulating commercial feed.
Protein sparing effect of carbohydrate
The amount of protein to be included in a fish diet is influenced by protein to energy ratio (PIE), protein digestibility and amount of non-protein energy in the diet. When insufficient non-protein energy is available in the feed, dietary protein is deaminated in the body to supply energy rather than being used for tissue growth and protein synthesis. Excess protein results in high level of ammonia production, which might affect voluntary feed intake and growth of fish. It is reported that adequate levels of non-protein energy sources, like lipid and carbohydrate in the diet, could minimize the use of protein as a source of energy, of which carbohydrate is the most economic. In omnivorous and warm water fishes such as carps, carbohydrate utilization is high compared to other species. It is important to provide adequate carbohydrate in the diet in order to reduce the catabolism of protein for energy and for synthesis of glucose, which reduces protein retention and increases nitrogen release to the environment. Knowledge of optimal level of protein and the protein-sparing effect of carbohydrate may be useful to reduce fish feed cost.
Fish feed ingredients and their digestibility
In order to meet the nutrient and energy requirement of fish through practical feeding and to replace one ingredient with the other for economic reasons, knowledge of digestibility coefficients for ingredients is essential. A feed stuff may appear from its chemical composition to be an excellent source of nutrients but will be of little actual value unless it can be digested and absorbed in the target species. Together with chemical analysis, digestibility determination may allow a more thorough estimation of nutritive value of a particular protein source in a complete diet for fish. Protein is useful only if it can be digested and the various degradation products obtained absorbed efficiently by the fish. Therefore, the determination of apparent nutrient and energy digestibility coefficients of different fish feed ingredients is very much essential before formulating the practical diets.