Английский язык: Сборник текстов и упражнений для студентов специальностей "Промышленное рыболовство". Дьякова Н.П. - 63 стр.

12. Капсулы, погружающиеся на дно могут быстро терять питательные веще-
ства, так как они растворяются в течение нескольких минут.
4. Do written translation into Russian.

                         GENETICS AND REPRODUCTION

If an aquaculturist wants to undertake selective breeding in order to improve the per-
formance of the species under culture, it is necessary to have all components of the
life cycle under direct control. For some species it has been necessary to obtain young
animals for stocking by collecting them in the wild since reliable means of producing
young in a hatchery have not been developed. In other cases, wild broodstock are ob-
tained because we do not have the knowledge required to grow fish to adult size in
captivity or, more often, the cost of growing and maintaining brood fish may be too
high. However, for most succeful aquaculture species, the like cycle from egg
through adult is controlled by the aquaculturist. Each aquaculture species requires
certain conditions for reproduction. Those conditions vary widely. For example, some
species spawn in fall or winter, some in spring, and some spawn almost continuously.
The key to inducing spawning may be changing temperature (such as falling tempera-
tures late in the year or rising temperatures in the spring), increasing or decreasing the
amount of daylight present (also known as photoperiod) or a combination of the two.
These types of environmental stimule cause changes in the hormone activities within
the aquaculture species and lead to the development of eggs and spermand, finally,
induce the behavioral activity that accompanies the actual spawning act.
Many aquatic species broadcast their eggs and sperm into water. The fertilized eggs
will become members of the plankton community. Upon hatching, the larval animals
may continue to swim about in the plankton until they grow sufficiently large to enter
the benthos or nekton community. This type of reproductive scenario is typical of
shrimp, crabs, lobsters, oysters and certain types of fishes (red drum and striped bass).
More specialized reproductive modes are used by many of the species of aquaculture
interest. Channel catfish lay eggs in adhesive masses. (about 30.000 eggs). Tilapia
males construct shallow nests in pond bottoms into which eggs are deposited and fer-
tilized. After fertilization, the female picks up the eggs in her mouth and retains them
until after the fry hatch and are able to survive on their own. Trout and salmon lay
their eggs in shallow nests (known as redds) constructed in the gravel bottoms of
lakes and streams. The newly hatched fish remain in the gravel for a considerable
amount of time after hatching. In order to control reproduction in the hatchery envi-
ronment, the aquaculturist must understand how the species of interest behaves under
natural conditions. Those conditions can be replicated then. In some instances, how-
ever, culture environments have been set up to duplicate natural conditions, but the
brood animals refuse to spawn. Under those circumstances, hormone injections can
be used to induce spawning. Once fertilized eggs are obtained, they need to be incu-
bated under the proper conditions (temperature and light may be important, as are
dissolved oxygen level and, in the case of marine fishes, salinity, among others).

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