Vocabulary

nourishment	питание
to feed on	питаться
food chain	пищевой симбиоз
macrophyte	макрофит
invertebrate (s)	беспозвоночные
to meet requirements	отвечать требования
crustacean	ракообразные
diet	пища
feed	корм
carbohydrate	углевод
poultry by product meal	пищевая добавка для домашней птицы
wheat	пшеница
corn meal	кукурузная мука
peanut meal	арахисовая мука
cottonseed meal	корм из хлопкового семени
strand	нить, волокно
starch	крахмал

Answer the questions

1. What are carps, oysters, mussels, clams fed on?

2. What are most fishes and invertebrates of aquaculture fed on?

3. What are usually prepared feeds composed of?

4. Which proteins do typical aquaculture diets contain?

Part 2

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 supplemental heat and extended exposure to high pressure to make pellets. Pressure pellet 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 expantion 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.

Advantages of floating pellets are that the aquaculturist can see that the fish eat feed. By watching the fish eat, the producer can control the amount of feed offered and thereby avoid overfeeding. This can save money. If the fish develop a disease, the aquaculturist may be able to identify the problem by observing changes in the behavior or appearance of the fish and can treat the problem before it progress very far. Floating feeds should not be used on all aquaculture species. Shrimp, for example, feed on the bottom and will not swim to the surface for pellets.

However, nutrients will be quickly lost from sinking pellets which may dissolve in a few minutes (floating pellets may take 24 hours or more to disintegrate), so valuable source of nutrition can be lost if the animals do not consume the feed quickly. Also, bacterial and fungal growth on feed particles that are not quickly consumed can lead to disease or toxicity problems.