Triploid grass carp are plant-eating fish from the Amur River Basin and lowland rivers in China and Russia. They are used as biological control for overabundant aquatic plants in some Washington waterbodies. Only sterile fish (triploids) are allowed to be stocked into Washington waters. Click here for more information about grass carp.
Grass carp are generally not recommended for milfoil control because milfoil is not a highly preferred food. Some research has indicated that grass carp have food preferences and will consume more palatable plant species, such as pondweeds and waterweed, before they will eat milfoil. As a result, the concern is that they can enhance milfoil growth by removing competition from native plants and opening up more area for milfoil to colonize. In Washington state, grass carp can be used for milfoil eradiation/control only in waterbodies where the eradication of ALL submersed aquatic plants can be tolerated. Sites where grass carp may be suitable for milfoil control are rare in Washington. They include very urban lakes like Green Lake in Seattle, privately-owned artificial lakes, or small lakes with a virtual monoculture of milfoil.
The Washington Department of Fish and Wildlife (WDFW) requires that all inlets and outlets to the lake be screened to keep grass carp from leaving the system. Therefore, grass carp are generally not allowed in waterbodies with salmon or steelhead since these fish need to pass freely between the lake and salt water. WDFW requires a lake-wide plan before allowing grass carp to be stocked into public lakes and they must issue a grass carp stocking permit before fish are allowed to be stocked.
The Department of Fish and Wildlife determines the applicability of stocking grass carp into a waterbody and provides a grass carp stocking rate. Generally no more than 9 fish per vegetated acre are allowed. This number of fish may not be enough for milfoil control projects. To achieve milfoil eradication, a higher stocking rate of fish per vegetated acre must be used. Since milfoil is not a preferred food, grass carp will eat the more palatable plants first. If too low a stocking rate is used, grass carp may actually enhance milfoil growth by removing competition from native plants and opening up more area for milfoil to colonize. In the few Washington lakes where grass carp have eradicated milfoil, all the other submersed plants in the lake have also been eliminated (e.g.Silver Lake, Cowlitz County; Surfside Lakes, Pacific County). In Washington, grass carp do not appear to eat floating leaved plants like water lilies or emergent vegetation such as cattails and bulrush.
Once grass carp stocking has been approved, Fish and Wildlife will issue a permit and provide a list of fish farmers to the project sponsor. Most grass carp farms are located in the southern US because fish grow faster in warm southern waters. Also fertile fish are not allowed in Washington so they can’t be raised here. The fish farmers generally sell ten to twelve inch fish. This size of fish is considered to be large enough to avoid bass predation. It is sometimes possible to purchase larger fish, but the costs per fish increase. Depending on the number of fish, grass carp are either transported to the site in special trucks or air freighted. One concern is that the fish farmers certify that the water that the grass carp are transported in is free from exotic organisms such as zebra mussels or the spiny water flea. The fish must also be certified as being triploid (sterile) and disease-free. The grass carp are released into the lake immediately upon their arrival. Most fish survive the trip from the fish farm, but some mortality from shipment stress is expected.
Many people prefer to stock their lakes in the spring to avoid winter stress. Once the fish are stocked, they are at risk from predation from birds of prey and otters. With abundant food and warm waters, the fish generally grow rapidly during their first summer and soon become too large for most birds to capture. Once the fish are stocked, observers may occasionally see them basking near the surface or moving in schools through the water. Their back fins often emerge from the water causing them to look like little sharks. If the correct numbers of fish have been stocked and mortality has been low, the amount of plants should slowly decline in the lake over two-three years with the palatable species disappearing before the milfoil plants. Once all submersed plants are eaten, grass carp have been known to consume detritus and organic material from the sediments (Gibbons, 1997).
As the stocked fish age, their feeding rate declines. Each year some mortality occurs and these sterile fish will eventually die out. As their population declines, native plants that have seeds or long-lived reproductive structures in the sediment may return. It is hoped that when this happens, milfoil will not reoccur in the waterbody.
There can be significant impacts to the waterbody following grass carp stocking. Since native plants provide habitat, sediment stabilization, and many other important functions, removal of all submersed plants can have a severe impact on the waterbody. Most of the impacts due to grass carp stocking are attributed to the removal of the plants rather than direct impacts of the fish.
WDFW investigated the effects of grass carp on the water quality of 98 Washington lakes and ponds (Bonar, et. al, 1996). The average turbidity of sites where all submersed aquatic plants were eradicated was higher (11 nephelometric turbidity units [NTU’s]) than sites where aquatic plants were controlled to intermediate levels (4 NTU’s) or at sites where the vegetation was not affected by grass carp grazing (5 NTU’s. In Silver Lake, NTU’s of 50 were observed after all submersed plants were removed (Gibbons, 1997). Although there have been some reports that grass carp stocking can increase algal blooms, this does not appear to be the case in Washington. The increase in turbidity was all abiotic (probably suspended sediments). In other words, once the submersed species are removed or partially removed the lake becomes more turbid or muddy. Never the less, the satisfaction rate of the pond owners or lake residents with the results from stocking grass carp was high.
Frodge et. al (1995) observed positive water quality changes in Bull Lake, Washington and Keevies Lake, Washington after they were stocked with grass carp . Grass carp stocking and the resultant plant removal reduced some of the deleterious problems caused by excessive plant growth, such as low dissolved oxygen and high pH. The lake bottom in Silver Lake went from being anoxic and devoid of bottom dwelling invertebrates to oxidized and supportive of benthic organisms after grass carp had removed all submersed vegetation (Gibbons, 1997).
Pauley et. al (1995) studied fish communities for a six year period in three lakes before and after grass carp stocking. They concluded that while changes in fish populations did occur in the lakes, no consistent trend occurred after the introduction of grass carp. It should be noted that in two of the lakes, aquatic plants were not totally eliminated.
Waterfowl that feed on submersed plants are affected when these plants disappear. A report from Silver Lake (Gibbons, 1997) showed that although there were no clear indications that the number of waterfowl in the lake had declined after grass carp introduction in May 1992, there was a sharp decrease in American coots in 1994, 1995, and 1996. These data suggest that the loss of submersed plants from the lake resulted in fewer birds that depended on these plants for food from Silver Lake.
Lake groups are strongly advised to monitor plant species and area of coverage, before and for several years after stocking grass carp. If the plants have not reduced in area or biomass after three years, more grass carp should be added. Since Fish and Wildlife issues the permit for extra fish, having monitoring data will provide them with the information to evaluate the request for extra fish.
Bonar, S.A., Bolding, B., and Divens, M. 1996. Management of aquatic plants in Washington State using grass carp: effects on aquatic plants, water quality, and public satisfaction 1990-1995. Washington Department of Fish and Wildlife Research Report No. 1F96-05.
Frodge, J. D., Thomas, G.L., and Pauley, G. B. 1995. Chapter 6 - Water quality effects of stocking sterile triploid grass carp in Keevies Lake and Bull Lake. In: The Biology Management and Stocking Rates of Triploid Grass Carp Ctenophyaryngodon idella, and Their Effects on the Plant Community, Fish Assemblage, and Water Quality of Several Pacific Northwest Lakes. Final Report to the Washington Department of Ecology.
Gibbons, H.L. 1997. Silver Lake Phase II Study, 1996 Annual Report prepared for Cowlitz County by KCM, Inc.
Pauley, G.B., Marino, D.A., Thiesfeld, S.L., Vecht, S.A., Thomas, G.L., Beauchamp, D.A., and Bonar, S.A. 1995. Chapter 9 - Impacts of triploid grass carp grazing on the game fish assemblages of Pacific Northwest Lakes. In: The Biology Management and Stocking Rates of Triploid Grass Carp Ctenophyaryngodon idella, and Their Effects on the Plant Community, Fish Assemblage, and Water Quality of Several Pacific Northwest Lakes. Final Report to the Washington Department of Ecology.
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