Grass Carp (Ctenopharyngodon idella)

A large member of the minnow family and an aquaculture species of worldwide importance, the grass carp is used for weed control because of its aggressive and herbivorous feeding habits. In the United States, where it was introduced in the early 1960s, it has become an extremely controversial species because of the biological damage it inflicts in the process of eliminating vegetation. This species is called the grass carp by critics, whereas supporters often refer to it as the white amur to avoid the negative connotations associated in North America with the name “carp.”

Identification

The grass carp has an elongate and fairly compressed body, a wide and blunt head, a very short snout without the barbels found on common carp, a short dorsal fin, and a moderately forked tail. The terminal and nonprotractile mouth has thin lips and sharp pharyngeal (throat) teeth especially suited to its feeding habits.

The grass carp is covered with large scales; the ones on the upper sides of the body have a dark border and a black spot at the base and give the fish a cross-hatched appearance. It is colored gray or green on the back, shading to white or yellow on the belly, and has clear to dark fins.

Size/Age

The grass carp grows quickly and to large sizes; some have been reported at 100 pounds in native waters. It can add 3 to 5 pounds a year to its weight under favorable conditions. The largest fish taken by rod and reel was a 68-pound, 12-ounce Arkansas specimen.

Life history/Behavior

Spawning takes place once a year over gravel bottoms in rivers, between April and September, according to temperature; adults will migrate upstream to find acceptable spawning sites. The round eggs of the grass carp are semibuoyant and amber colored, hatching in 24 to 30 hours without the protection of the parents.

After they absorb the nutrients in their yolk sacs in the first 2 to 4 days of their lives, the larvae feed on microplankton in quiet waters. The young hide in deep holes in riverbeds during the winter.

Food and feeding habits

Primarily vegetarians, grass carp have earned their name by eating aquatic plants and submerged grasses, adding the occasional insect or invertebrate. With the help of teeth on the pharynx, they tear off vegetation with jerking motions. Unlike common carp (see: Carp, Common), grass carp do not muddy the water with their browsing, but their aggressive feeding habits cause other problems. Grass carp tend to break off the upper portions of grasses, leaving the roots to grow, so they are not as useful in eradicating vegetation as they are supposed to be.

Also, grass carp cannot digest all the plant matter they take in, so instead of eliminating a vegetation problem, they make it worse by excreting plant material and distributing it to new areas. In addition, they contribute to increased water turbidity and to eutrophication. Finally, heavy browsing may stimulate faster than normal growth in certain kinds of plants.

Triploid grass carp A technique that consists of exposing fertilized eggs to heat shock was invented by researchers in 1981 to produce sterile grass carp. This method creates nonreproducing fish of both genders. They are called triploid grass carp because they have three sets of chromosomes, instead of the usual two sets (those fish are called diploid).

They are as hardy as the ordinary variety of grass carp, but they have the benefit of not being able to overpopulate their habitats. They look like large creek chub, flourish in warm water, and may reach weights of 25 pounds or more. Triploid grass carp are useful in controlling unwanted aquatic plants, but the water clarity may deteriorate due to the substantial passing of plant material as fecal matter.

Other Names

white amur, amur, carp; French: carpe amour, carpe herbivore, amour blanc; German: graskarpfen; Japanese: sogyo.

Distribution

Found originally in China and eastern Siberia, specifically in the Amur River basin from which it gets its name, the grass carp has been widely introduced to more than 20 countries. Only those in certain areas have been able or allowed to reproduce naturally; these places include the Danube River in central Europe, the Mississippi River in North America, and Russia and southern Africa.

In the United States, the grass carp was first stocked in Arkansas waters in 1963 and intentionally released in 35 states, although it has subsequently spread to other bodies of water where it was unwanted. In fact, many states have made it illegal to stock grass carp within their borders, unless a permit issued by the appropriate fisheries management agency has been obtained.

Habitat

Occurring in freshwater, grass carp inhabit lakes, ponds, pools, and backwaters of large rivers, with a preference for slow-flowing or standing bodies of water with vegetation. They are able to withstand temperature variation, extreme salinity, and low oxygen concentrations.

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