As, the energy requirements of fish and other cultured organisms are generally supplied by carbohydrates, proteins and fats. Most of the available information on nutrient requirements of aquaculture species is based on researches on a small number of these (trouts, salmon, channel catfish, common carp, grass carp, eel, plaice, gilt-head bream, red sea-bream and yellowtail). Work on penaeid shrimps and the giant fresh-water prawn has shown considerable similarities with the fish species studied, although there are some differences. Very little work has been done on the nutritional needs of molluscs, as culture has been based on filter feeding of naturally occurring phytoplankton and similar organisms. However, efforts are presently underway to develop encapsulated or fine-particulate feeds, which should lead to a better understanding of their feed requirements.
Energy requirements have in most cases been derived primarily from experimentation, in which fish were fed rations varying in calorific value. The ration yielding the best growth was assumed to be the most satisfactory calorific value for the species concerned.
Carbohydrates are the most abundant and relatively least expensive source of energy in animal aquaculture. These may range from easily digested sugars to complex cellulose which is difficult to digest. Based on results of research on carnivorous species, doubts have been expressed on the value of carbohydrates in fish feeds, but practical experience in fish culture shows that digestible carbohydrate can be an energy source if kept in proper balance with other nutrients. The ability to assimilate starches depends on enzymatic activity (production of amylase). In herbivores, amylase occurs through the entire digestive tract. Up to levels of 25 per cent in the diet, it can be as effective an energy source as fat for several species of fish, such as channel catfish, rainbow trout and plaice (Cowey and Sargent, 1972). Stickney and Shumway (1974) have shown the presence of cellulase activity associated with cellulolytic microflora in several species of brackish-water fish and freshwater catfish. Metabolizable energy values of carbohydrates may range up to 3.8 kcal/g for easily digestible sugars, whereas for indigestible cellulose it may be near zero.
Values for raw starch range from 1.2–2.0 kcal/g. When processed in high moist temperatures, for making pelleted feed, starch gelatinizes and its digestibility therefore improves. When digested, the products of hydrolysis are assimilated into the blood-stream, where their known function is to provide energy. Therefore they have a protein sparing action and any excess is partially stored in the liver as glycogen and partially converted into visceral and muscular fat.
Successful fish feeds contain a certain amount of carbohydrates, as for example 20 per cent for cold-water fish feeds and 30 per cent for warm-water fish feeds. Besides providing energy, they have the physical function of texturizing manufactured feeds and acting as a binder in the formulation of pellets. Cereal grain products are generally used as ‘fillers’ to complete feed formulae. Formulae for expanded pellets often contain up to 50 per cent of whole cereal grains, to achieve the floating properties.
Fish and shrimp vary in their ability to digest carbohydrate (New, 1989). The utilization of dietary carbohydrate has also been found to vary with the complexity of the carbohydrate source used.
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