Aquatic Plant Management

Description of Method - Triploid Grass Carp

The grass carp, also known as the white amur, is a vegetarian fish native to the Amur River in Asia. Because this fish feeds on aquatic plants, it can be used as a biological tool to control nuisance aquatic plant growth.

In some situations, sterile grass carp may be permitted for introduction into Washington waters. Permits are most readily obtained if the lake or pond is privately owned, has no inlet or outlet, and is fairly small. 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 in Washington, grass carp often fail to control the plants or all the submersed plants are eliminated from the waterbody.

The Washington Department of Fish and Wildlife determines the appropriate stocking rate for each waterbody when they issue the grass carp stocking permit. Stocking rates for Washington lakes generally range from 9 up-to 25 eight- to eleven-inch fish per vegetated acre. This number will depend on the amount and type of plants in the lake as well as spring and summer water temperatures. However, Fish and Wildlife generally err on the side of stocking the least amount of grass carp as possible. 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.

The fish screens that were recently installed (2002) in the outlets for Campbell Lake and Erie Lake cost $22,000 to design, fabricate, and install. About $8,000 of this was for the engineering and design of the structures. The fish screen at Campbell Lake was designed to allow the passage of sea run cutthroat trout. A tube at the bottom of the screen allows adult trout to swim upstream. The tube contains a baffle that allows fish to move in only one direction. The juvenile trout are small enough to move through the grate.

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 in Washington has been variable. Sometimes the same stocking rate results in no control, control, or even complete elimination of all underwater plants. Bonar et. al. found that only 18 percent of 98 Washington lakes stocked with grass carp at a median level of 24 fish per vegetated acre had aquatic plants controlled to an intermediate level. In 39 percent of the lakes, all submersed plant species were eradicated. It has become the consensus among researchers and aquatic plant managers around the country that grass carp are an all or nothing control option. They should be stocked only in waterbodies where complete elimination of all submersed plant species can be tolerated.

Grass carp exhibit definite food preferences and some aquatic plant species will be consumed more readily than others. Pauley and Bonar performed experiments to evaluate the importance of 20 Pacific Northwest aquatic plant species as food items for grass carp. Grass carp did not remove plants in a preferred species-by-species sequence in multi-species plant communities. Instead they grazed simultaneously on palatable plants of similar preference before gradually switching to less preferred groups of plants. The relative preference of many plants was dependent upon what other plants were associated with them. The relative preference rank for the 20 aquatic plants tested was as follows: (listed in order of decreasing preference)

Potamogeton crispus(curly leaf pondweed), P. pectinatus (sago pondweed)
P. zosteriformes (flat-stemmed pondweed)
Chara spp.(muskgrasses), Elodea canadensis (American waterweed), Thin-leaved pondweeds Potamogton spp.
Egeria densa (Brazilian elodea) (large fish only)
P. praelongus (white-stemmed pondweed), Vallisneria americana (water celery)
Myriophyllum spicatum (Eurasian watermilfoil)
Ceratophyllum demersum (coontail)
Utricularia vulgaris(bladderwort)
Polygonium amphibium (water smartweed)
P. natans (floating leaved pondweed)
P. amplifolius (big leaf pondweed)
Brasenia schreberi (watershield), Juncus sp.(rush)
Egeria densa(Brazilian elodea) (fingerling fish only)
Nymphaeasp. (fragrant waterlily)
Typha sp. (cattail)
Nuphar sp. (spatterdock).

Generally in Washington, grass carp do not consume emergent wetland vegetation or water lilies even when the waterbody is heavily stocked or over stocked. A heavy stocking rate of triploid grass carp in Chambers Lake, Thurston County resulted in the loss of most submersed species, whereas the fragrant water lilies, bog bean, and spatterdock remained at pre-stocking levels. A stocking of 83,000 triploid grass carp into Silver Lake Washington resulted in the total eradication of all submersed species, including Eurasian watermilfoil, Brazilian elodea, and swollen bladderwort. However, the extensive wetlands surrounding Silver Lake have generally remained intact. In southern states, grass carp have been shown to consume some emergent vegetation.

Grass carp stocked into Washington lakes must be certified disease free and sterile. Sterile fish, called triploids because they have an extra chromosome, are created when the fish eggs are subjected to a temperature or pressure shock. Fish are verified sterile by collecting and testing a blood sample. Triploid fish have slightly larger blood cells and can be differentiated from diploid (fertile) fish by this characteristic.

Grass carp imported into Washington must be tested to ensure that they are sterile. Because Washington does not allow fertile fish within the state, all grass carp are imported into Washington from out of state locations. 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 Thurston County.

Grass carp:




For Washington residents, a private fish stocking permit must be obtained from the Washington Department of Fish and Wildlife. Check with your Fish and Wildlife regional office to obtain a permit application. Also, if inlets or outlets need to be screened, an Hydraulic Project Approval application must be completed for the screening project. Grass carp may not be permitted to be stocked in some states.


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 $10 to $20 per fish with shipping.


The following list of vendors is provided for your information. It is not our intention to endorse or promote specific vendors or products and this list may not be comprehensive. Vendors who wish to be added to this list should contact us.

The Washington Department of Fish and Wildlife supplies a list of fish farmers when they issue a grass carp stocking permit. Vendors must be on this approved list. See this form for more information: Grass Carp Checklist.

Recently the Nisqually Trout Farm located in Lacey, Washington has begun selling triploid grass carp to people with approved grass carp permits. Their phone number is: (360) 491-7440. Each 10-12 inch fish is $12.00 with the price negotiable with larger orders.

Opaline in Melba Idaho also sells grass carp. They can be reached at 888-495-3474 or 208-495-2654 FAX: 208-495-2946

Vendor.pdf - triploid grass carp suppliers in the Florida area. - Indiana grass carp supplier list. - brochure about grass carp including a list of suppliers.

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