Английский язык: Сборник текстов и упражнений. Дьякова Н.П. - 76 стр.

countries, ground raw fish is often used to feed aquacultured animals (the fish
may be fixed with small) amounts of dry ingredients).
    Oysters, mussels and clams are among the various shellfish that feed by algae
and other organic nutrients from the water. The culture of those animals requires
the presence of large algae concentrations.
    Most fishes and invertebrates of aquaculture interest are fed prepared feeds.
Such feeds are composed of various ingredients in proper combinations so that the
final product will meet the nutritional requirements of the species being fed. Diets
vary considerably from one type of aquaculture animal to the next because of dif-
ferences in nutritional requirements. For example, many crustaceans are unable to
synthesize cholesterol, so that chemical must be provided in the feed. Fishes, on
the other hand, do not require dictory cholesterol. Determining the nutritional re-
quirements of an aquaculture species can require many years of research. Diets
are prepared in which various ingredients are varied with respect to quality and
quantity. Then the feeds are presented to the aquaculture species over a period of
several weeks to months and the growth response is evaluated. Experimental diets
may be prepared to examine the responses of the animals to variations in dietary
protein, fat, carbohydrate, minerals, vitamins or energy. Typical aquaculture diets
are relatively simple. They usually contain some type of animal protein (fish
meal, poultry by product meal, meat and bone meal) and other proteins supplied
by plants (soy bean meal, wheat, corn meal, peanut meal and cottonseed meal).
The plant products also supply high levels of carbohydrates (sugars and starches).
Some species, such as channel catfish, can tolerate levels of 40% carbohydrate in the
diet, whereas others, such as trout, tolerate only low carbohydrate levels. Fat is sup-
plied by the various ingredients mentioned, but supplemental fat is offen added in the
form of corn oil, fish oil or a variety of others. A mixture of required vitamins and
minerals is also usually added. In some instances, wet, ground fish is used in the
United States as a dietary ingredient. This is particularly true in the Pacific North-
west, where waste products from fish processing plants are readily available.
    Once a diet has been formulated and the ingredients have been mixed together
in the proper proportions, the material is usually made into a pellet. Pellets are
made by exposing the material to high pressure in a pellet mill or extruder. Pellet
mills may use steam to help bind the ingredients together. Extruders use supple-
mental heat and extended exposure to high pressure to make pellets. Pressure pel-
let mills and extruders pass the feed mixture through a small aperture which leads
to a product which, is not cut to short lengths, would be much like spaghetti
strands. The diameter of the pellets varies, but is typically 0,2 to 0,24 inches (5 to
6 mm). A knife cuts off the strands as they exit the pelleting equipment. Feeds
produced by pressure pelleting are more dense than water; thus, they sink. During
the extrusion process, on the other hand, the high heat used causes changes within
the ingredients so that starches expand when the pellets leave the machine and
come into contact with air. This rapid expansion of the material traps air within
the pellets, which float when placed in water. Because of the higher temperatures
and other factors, extruded pellets are more expensive than pressure pellets. Ad-
vantages of floating pellets are that the aquaculturist can see that the fish eat feed.
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