Составители:
Рубрика:
present (also known as photoperiod) or a combination of the two. These types of
environmental stimule cause changes in the hormone activities within the aqua-
culture 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 scena-
rio 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 fertilized. 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) con-
structed in the gravel bottoms of lakes and streams. The newly hatched fish re-
main in the gravel for a considerable amount of time after hatching. In order to
control reproduction in the hatchery environment, the aquaculturist must under-
stand how the species of interest behaves under natural conditions. Those condi-
tions 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 oxy-
gen level and, in the 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 mechani-
cal 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 ma-
rine 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 oth-
ers may require several weeks or even months of incubation. Generally, warm wa-
ter species develop and hatch quickly, while coldwater species require considera-
ble time periods for development.
Word supplement
fall осень
to broadcast разбрасывать, распространять
to fertilize зд.: оплодотворять
to hatch выклевываться
nekton
нектон
80
present (also known as photoperiod) or a combination of the two. These types of
environmental stimule cause changes in the hormone activities within the aqua-
culture 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 scena-
rio 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 fertilized. 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) con-
structed in the gravel bottoms of lakes and streams. The newly hatched fish re-
main in the gravel for a considerable amount of time after hatching. In order to
control reproduction in the hatchery environment, the aquaculturist must under-
stand how the species of interest behaves under natural conditions. Those condi-
tions 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 oxy-
gen level and, in the 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 mechani-
cal 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 ma-
rine 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 oth-
ers may require several weeks or even months of incubation. Generally, warm wa-
ter species develop and hatch quickly, while coldwater species require considera-
ble time periods for development.
Word supplement
fall осень
to broadcast разбрасывать, распространять
to fertilize зд.: оплодотворять
to hatch выклевываться
nekton нектон
80
Страницы
- « первая
- ‹ предыдущая
- …
- 78
- 79
- 80
- 81
- 82
- …
- следующая ›
- последняя »
