Reproduction And Genetics

Species such as carp, salmon, trout, channel catfish, and tilapia have been bred for many generations in captivity, though they usually differ little in appearance or genetically from their wild counterparts. A few exceptions exist, such as the leather carp, a common carp strain selectively bred to produce only one row of scales, and the Donaldson trout, a strain of rainbow trout developed over numerous generations to grow more rapidly to larger size and with a stouter body than its wild cousins.

Selective breeding has been long practiced as a means of improving aquaculture stocks. In some instances it has not been possible or is at least quite difficult and expensive to produce broodstock and spawn them in captivity, so cul-turists continue to rear animals obtained from nature. Most of the species that are being reared in significant quantities around the world are produced in hatcheries using either captured or cultured broodstock. Milkfish is a notable exception. That species has been spawned in captivity, but most of the fish reared in confinement are collected as juveniles in seines and sold to fish culturists. Wild shrimp postlarvae continue to be used to stock ponds in some parts of the world, though hatcheries may also be available in the event sufficient numbers of wild postlarvae are unavailable in a given year. In the United States, where shrimp culture involves the use of exotic species, all the animals reared come from hatcheries, both in the United States and in Latin America.

Spawning techniques vary widely from one species to another. Tilapia and catfish are typically allowed to spawn in ponds. Artificial nests are provided for catfish while tilapia dig their own nests in the pond bottom. Fertilized eggs can be collected from the mouths of female tilapia, but it is common practice to collect schools of fry after they are released from the mother's mouth to forage on their own. Catfish lay eggs in adhesive masses. Spawning chambers such as milk cans and grease cans are placed in ponds and may be examined every few days for the presence of egg masses. Some catfish farmers allow the eggs to hatch in the pond, though most farmers collect eggs and incubate them in a hatchery.

Adult Pacific salmon die after spawning. Females are usually sacrificed by cutting open the abdomen to release the eggs. Milt is obtained by squeezing the belly of males. Trout and Atlantic salmon can be reconditioned to spawn annually. Eggs are usually obtained from those species in the same fashion as from male Pacific salmon. Banks of egg-hatching trays, called Heath trays, through which water is flowed are typically used to hatch trout and salmon eggs (Fig. 9).

Unlike catfish, tilapia, trout, and salmon that produce from several hundred to several thousand relatively large eggs per female, many marine species produce large numbers of very small eggs. Hundreds of thousands to millions of eggs are produced by such species as halibut, flounder, red drum, striped bass, and shrimp. Catfish, salmon, and trout spawn once a year, while tilapia and some marine species spawn repeatedly at intervals of a few days to a few weeks for as long as several months if the proper environmental conditions are maintained (2).

Fish breeders have worked with varying degrees of success to improve growth and disease resistance in several species. As genetic engineering techniques are adapted to aquatic animals, dramatic and rapid changes in the genetic makeup of aquaculture species may be possible. However, since it is virtually impossible to prevent the escape of animals into the natural environment from aquaculture facilities, potential negative impacts of such organisms on wild populations cannot be ignored. Maintaining genetic diversity like that of the wild population in cultured stocks

Figure 9. A bank of Heath trays in a salmon hatchery in Washington State.

makes good sense, particularly in conjunction with enhancement and when escape from confinement systems would produce a high likelihood of escapees intermingling with wild counterparts.

For some species, one sex may grow more rapidly than the other. A prime example is tilapia, which mature at an early age (often within six months of hatching). At maturity, submarketable females divert large amounts of food energy to egg production. Also, since they are mouth brooders (holding the eggs and fry within their mouths for about two weeks) and repeat spawners (spawning about once a month if the water temperature is suitable), the females grow very slowly once they mature. Males, on the other hand, continue to grow rapidly and can become marketable within a few months after reaching maturity. All-male, or predominantly male, populations of tilapia can be produced by feeding androgens to fry, which are undifferentiated sexually. Various forms of testosterone have been used effectively in sex reversing tilapia and other fishes.

In species such as flatfish, females may grow more rapidly than males and ultimately reach much larger sizes. For them, producing all-female populations for growout might be beneficial.

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