Description of Method

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 (seen on the left) 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: Potamogeton crispus (curly leaf pondweed) = P. pectinatus (sago pondweed) > P. zosteriformes (flat-stemmed pondweed) > Chara sp.(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) > Nyphaea sp. (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.

Here are some facts about grass carp:  

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 much longer in Washington waters.
  
• Will grow rapidly and reach at least ten pounds. They have been known to reach 40 pounds in the southern United States.
  
• Feed only on plants at the age they are stocked into Washington waters.
  
• Will not eat fish eggs, young fish or invertebrates, although baby grass carp are omnivorous.
  
• 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 will eat the organic material out of the sediments.
  
• Have definite taste preferences.  Plants like Eurasian milfoil and coontail are not preferred. American waterweed and thin leaved pondweeds are preferred. Water lilies are rarely consumed in Washington waters. 
  
• 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 recapture if a waterbody has been overstocked. 
  
• 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 milfoil, may take over the lake.
  
• Stocking grass carp may lead to algae 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

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. 

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 $10 to $20 per fish with shipping.

Vendors

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 Kathy Hamel at kham461@ecy.wa.gov.

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: http://wdfw.wa.gov/hab/sepa/grasscarpchecklist.pdf

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.

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Contact Kathy Hamel by e-mail at kham461@ecy.wa.gov