68 JUNE 2014 • WORLD AQUACULTURE • WWW.WAS.ORG remain in the production line and may cause cross contamination. Alternatives must be developed for partial or complete replacement of all type of chemicals from aquaculture. Short shelf life of fish meat. Monitoring and controlling fillet quality is one of the main goals of the fish processing industry because fresh fish are among the most perishable food products. Fish shelf-life is influenced by a number of factors, including nutrition. The muscle tissue of fish undergoes faster spoilage than mammalian muscles. Fish flesh contains high water and free amino acid content and less connective tissue, which leads to rapid spoilage as compared to other meat products. Moreover, shellfish flesh, containing high carbohydrate and low nitrogen content, is more favorable for microbial growth. Low flesh quality of certain cultured fishes. Flesh quality is an important factor in determining consumer preference and price of the marketed fish. Farmed striped catfish Pangasianodon hypophthalmus from India is sometimes marketed at a lower price than the cost of production. Moreover, it is less preferred in export markets because of its poor flesh quality. Fish flesh can be tinged with yellow when it is high in lipids. Undesirable yellow color in fish also results from poor water quality and other environmental factors. High visceral and muscle lipid deposition is a result of high fat or high calorie feeding. Limited information on nutrient requirements. The data on nutritional requirements of fish is incomplete as a result of the diversity of species cultured. Also, the requirement of many nutrients will change with alterations in the environment. The role of micronutrients may change from nutrient to nutraceutical for fish according to its physiological status, namely reproductive season and a stressful environment. This may place restrictions on a precise nutrient requirement for a particular species. The use of purified and semi-purified diets also is not a feasible method for fish nutrient requirement studies because of the low acceptability of purified diets by farmed fish. Moreover, purified diets underestimate requirements by neglecting nutrient interactions and resultant reduced availability in practical diets. Challenges associated with larval and broodstock nutrition. Proper nutrition for larvae and broodstock, with the purpose of obtaining healthy and plentiful seed, is one of the greatest challenges faced by aquaculture nutritionists. Larval nutrition is a difficult area in fish nutrition because of the poor definition of nutritional requirements of larvae of many fishes. The study of nutrient requirements of larval fishes is difficult. Limited and variable energy reserves in the body of the larvae can lead to high mortality from starvation. Behavioral starvation is observed in larvae because it requires the right-size feed for the mouth size. The small size of larvae reduces the effective distance larvae can travel and the size of particle that can be consumed. The digestive capacities of larvae are low compared to larger animals. Considering all these vagaries, it is difficult to feed larvae with the aim of obtaining a high survival rate. Egg quality and larval survival are affected by broodstock nutrition, which is an explored but ever demanding area because of the introduction of new candidate species. Possible Strategies Novel and unconventional feed ingredients. The rapid expansion of aquaculture has accelerated the search for alternatives to fishmeal and fish oil. Some agricultural ingredients are widely used in aquafeed and considerable potential exists to increase their use. Plant breeding programs to reduce concentrations of anti-nutritients and increase essential nutrients, and processing to remove less digestible components such as fiber and carbohydrates, will improve the usefulness of plant ingredients. For animal byproducts, reduction of ash and saturated fat levels and improvement of processing methods will increase potential for use. The use of attractants, synthetic amino acids and enzymes offers the potential for formulation of aquafeed with reduced reliance on fishmeal. Development of methods for reduction of anti-nutritional factors. Although many traditional methods are available for reducing anti-nutritional factors, most are not user friendly. Electronbeam irradiation for reducing anti-nutritional factors is a novel approach that can reduce tannin, HCN and phytic acid to maximum levels of 60 percent in soybean, rubber seed cake and cotton seed cake. It is a fast, non-chemical method that offers a wide opportunity for use in the aquafeed industry. Exogenous enzymes, especially phytase, in aquaculture feeds are aimed at removing the anti-nutritional factor phytate, resulting in improved feed utilization, growth and production. The acidifiers/ organic acids and exogenous enzymes such as phytase result in enhanced nutrient utilization in fish. Methods for improvement of digestibility. Feed cost can be reduced by the use of cheaper ingredients and by using available ingredients more efficiently. For example, if digestibility of an ingredient is enhanced, the inclusion level of that or another ingredient can be reduced. High digestibility of an ingredient can reduce feed cost. Starch gelatinization is one method to enhance digestibility of carbohydrates and their utilization in fish. Removal of anti-nutritional factors that interfere with digestion and the use of exogenous enzymes may improve digestibility of feeds. Protein sparing and related nutritional research. As protein is the most expensive component of diet, it should be spared for growth, while meeting energy requirements from carbohydrates and lipids. Protein sparing by carbohydrates is preferable because it is the cheapest of the energy nutrients. The extent of protein sparing by carbohydrates and lipids varies widely among fishes. Although fishes have adequate metabolic machinery to regulate carbohydrate metabolism, carnivorous fishes are less able to use dietary carbohydrates and have a greater affinity for lipids than carbohydrates. It is important to study and identify the levels and forms of lipid and/or carbohydrate that can spare protein productively and economically. Nutraceuticals. Some additives, amino acids and their metabolites, and vitamins are important regulators of key metabolic pathways necessary for feed intake, nutrient utilization, maintenance, growth, immunity, behavior, larval metamorphosis, reproduction and resistance to environmental stressors and pathogenic organisms in various fishes. Nutraceuticals can act as buffering agents in biological systems by reducing the deleterious effects of stressors and by improving growth. High dietary protein supplementation has an ameliorating effects against different stressors (Manush et al. 2005). Dietary supplementation of different vitamins (e.g. vitamin C and vitamin E) can mitigate stress in shellfish and finfish. The role of tryptophan in growth, immunomodulation, disease resistance and stress mitigation has also been evaluated in fishes reared under thermal and salinity
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