Grass Carp

Grass carp/delivering grass carp

The Western Aquatic Plant Management Society

Description of Method

The grass carp, also known as the white amur, is a vegetarian fish native to China and the Soviet Union. Because this fish feeds on aquatic plants, it can be used as a biological tool to control nuisance aquatic plant growth. The objective of using grass carp to control aquatic plant growth is to end up with a lake that has about 20 to 40 percent plant cover, not a lake devoid of plants. In practice, grass carp often fail to control the plants or all the submersed plants are eliminated from the waterbody.

Stocking rates generally range from 9 to 25 - eight- to eleven-inch fish per vegetated acre. The number of fish will depend on the amount, density, and type of plants in the lake as well as spring and summer water temperatures. To prevent stocked grass carp from migrating out of the lake and into streams and rivers, all inlets and outlets to the pond or lake must be screened. For this reason, residents on waterbodies that support a salmon or steelhead run are rarely allowed to stock grass carp into these systems.

Once grass carp are stocked in a lake, it may take from two to five years for them to control nuisance plants. Survival rates of the fish will vary depending on factors like presence of otters, birds of prey, or fish disease. A lake will probably need restocking about every ten years. Success with grass carp has been variable. Sometimes the same stocking rate results in no control, control, or even complete elimination of all underwater plants. It has become the consensus among researchers and aquatic plant managers around the United States that grass carp should be considered an all or nothing control option. They should be stocked only in waterbodies where complete elimination of all submersed plant species can be tolerated.

Generally only disease free, sterile fish are allowed to be stocked. Sterile fish (called triploids because they have extra chromosomes) are created when the fish eggs are subjected to temperature or pressure shock. All fish are verified sterile by collecting and Grass carp stocking in Chambers Lake testing a blood sample. Triploid fish have slightly larger blood cells and can be differentiated from diploid (fertile) fish by this characteristic.

Most grass carp farms are located in the southern United States where warmer weather allows for fast fish growth rates. Large shipments are transported in special trucks and small shipments arrive via air. The photograph shows the truck used to transport grass carp for Chambers Lake in Washington.

Here are some facts about grass carp:

Are only distantly related to the undesirable European carp, and share few of its habits.
Live for at least ten years and probably longer (if not eaten by otters or eagles).
Will grow rapidly and reach at least ten pounds. They have been known to reach 40 pounds in the southern United States.
Feed on plants.
Although baby grass carp are omnivorous, older fish do not prefer to eat fish eggs, young fish, or invertebrates, .
Feed from the top of the plant down so that mud is not stirred up. However, in ponds and lakes where grass carp have eliminated all submersed vegetation, the water becomes turbid. Hungry fish eat the organic material out of the sediments and can stir up the sediments. Also loss of vegetation allows wind mixing to disturb the sediments.
Have definite taste preferences. Plants like water lilies and Eurasian milfoil are not preferred. Brazilian elodea and American waterweed are readily eaten.
Are dormant during the winter. Intensive feeding starts when water temperatures reach 68^o F.
Are a river fish and have the desire to move from still waters into flowing waters.
Are difficult to catch. Once grass carp are stocked, it is extremely difficult to remove them from a waterbody.
They may not feed in swimming areas, docks, boating areas, or other sites where there is heavy human activity.

Advantages

Grass carp are inexpensive compared to some other control methods and offer long-term control, but fish need to be restocked at intervals.
Grass carp offer a biological alternative to aquatic plant control.

Disadvantages

Depending on plant densities and types, it may take several years to achieve plant control using grass carp and in many cases control may not occur or all submersed plants may be eliminated.
The type of plants grass carp prefer may also be those most important for habitat and for waterfowl food.
If the waterbody is overstocked, all submersed aquatic plants may be eliminated. Removing excess fish is difficult and expensive.
If not enough fish are stocked, less-favored plants, such as Eurasian watermilfoil, may take over the lake.
Stocking grass carp may lead to algal blooms.
All inlets and outlets to the lake or pond must be screened to prevent grass carp from escaping into streams, rivers, or other lakes.

Permits

Some western states do not allow grass carp to be planted in their waterbodies, other states allow some limited stocking with permits. Check with your state and local jurisdictions to determine if stocking grass carp is legal for your state.

Costs

In quantities of 10,000 or more, 8 to 12 inch sterile grass carp can be purchased for about $5.00 each for truck delivery. The cost of small air freighted orders will vary and is estimated at $8 to $10 per fish.