Vocabulary

pellet	гранула
pellet mill	пресс для гранулирования
extruder	пресс для выдавливания
steam	пар
extruded pellets	экструдированные гранулы
pressure pellets	прессованные гранулы
floating pellets	плавающие гранулы
sinking pellets	тонущие гранулы
shrimp	креветка
to float	держаться на поверхности, плавать
fungus	гриб, плесень

Topic for discussion

1. Speak about the difference between pressure pellets, extruded pellets and floating pellets.

14. Genetics and reproduction Part 1

If an aquaculturist wants to undertake selective breeding in order to improve the performance 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. On other cases wild brood stocks are obtained 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, the most successful aquaculture species like cycle from egg through adult is controlled by the aquaculturist. Each aquaculture species requires certain conditions for reproduction. These 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 temperatures 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 stimulus cause changes in the hormone activities within aquaculture species and lead to the development of eggs and sperm and, 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 to shrimp, crabs, lobsters, oysters and certain types of fishes (red drum and striped bass). More specializes reproductive modes are used by many of the species of aquaculture interest.

Vocabulary

wild brood stocks	косяк (стая) рыб в реке
captivity	неволя
brood fish	ручьевая рыба
spawn	нереститься, выметывать икру
fall	осень
to broadcast	разбрасывать
to fertilize	оплодотворять
to hatch	выклевываться
benthos	бентос – донные живые организмы (личинки насекомых и др.)
nekton	нектон
red drum	красный горбыль
striped bass	полосатый окунь

Topic for discussion

1. Speak about conditions required for reproduction.

Part 2

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 fertilized. After fertilization their 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 environment, the aquaculturist must understand how to species of interest behaves under natural conditions. These conditions can be replicated then. In some instances, however, 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 incubated under the proper conditions (temperature and light may be important, as are dissolved oxygen level and, in case of marine fishes, salinity among others).

Eggs of trout, salmon and catfish are relatively large and resilient, so high water flow rates are tolerated. The eggs of many species, however, tend to be susceptible to mechanical damage, so care must be taken to provide a calm environment to prevent the eggs from bumping into each other or into the walls of culture tanks. Various marine fishes fall into the latter category, as do many invertebrates. At the same time, some exchange of water is required to dilute waste products like ammonia which are produced by developing eggs and larvae.

Some eggs hatch into larvae within 24-48 hours after fertilization, while others may require several weeks or even months of incubation. Generally, warm water species develop and hatch quickly, while cold water species require considerable time period for development.