Английский язык: Сборник текстов и упражнений. Дьякова Н.П. - 57 стр.

reared during the days of the Pharoahs, and oyster culture was being practiced un-
der the Roman Empire.
    Most people think of acquaculture as the production of aquatic animals for
human consumption, and many of the aquaculture efforts around the world are be-
ing conducted for that purpose. There are, however, other purposes for which aq-
uatic organisms are grown. Examples include producing minnows for bait, rearing
tropical fishes and gold fish for the aquarium trade and producing ornamental aq-
uatic plants (water liles).
    In addition, the often large-scale hatchery programs that exist in the various
states and provinces of North America and throughout the world produce fishes
for release into streams, lakes, reservoirs and the marine environment to enhance
commercial and recreational fisheries as well as to repopulate water bodies with
endangered and threatened species. Aquatic plants are also produced for human
consumption. In the Orient, for example, seaweed production (e.g., red and brown
algae) involves the labour of several hundreds of thousands of people. The sea-
weeds may be consumed directly by man or extracts may be obtained that become
components of a variety of substances each of us uses every day. Ice cream,
toothpaste, cosmetics and a wide range of other house hold items contain extracts
from seaweeds. Some of those businesses are very large, and they are legitimate
aquaculture enterprises. Under natural conditions, as much as 100 pounds per acre
(kg/ha) of fish might be produced in a lake within a year. Aquaculture systems, by
contract, can produce several thousand to even a million pounds per acre (kg/ha)
in a year: The difference relates to the definition of aquaculture presented above.
The aquaculturist exerts control over the species being reared. That control may
include, but is not limited to the following:
    1. Design, construction and maintenance of the culture system being employed
Aquaculturists utilize ponds, cages, net-pens, race ways, tanks and other units.
They do not attempt to convert unaltered natural water bodies into culture system.
    2. Maintenance of suitable water quality. In order for an aquaculture venture
to be successful, the water must be of a quality suitable for the species being
reared. Of most importance are the levels of dissolved oxygen and ammonia, wa-
ter temperature and, in the case of marine species, salinity. Each of these and oth-
er water quality variables may or may not be controlled, depending on the nature
of the culture system.
    3. Control over reproduction. Unless the culture species can be reproduced in
captivity, there is no way to undertake genetic selection and the improvement of
the species with respect to its suitability for culture. Successful aquaculture of any
species ultimately depends on captive breeding and producing brookstock from
animals that are hatched in captivity.
    4. Provision of nutritionally complete feeds.
    Species being reared by aquaculturists are usually fed prepared feeds, similar
to the feeds used by livestock producers. Such feeds contain the nutrients neces-
sary to meet the daily requirements of the species under culture. In some in-
stances, natural foods are relied up (e.g., oyster and mussel culture), but for most
species, manufactured diets are employed.
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