Grass Carp for Weed Control

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Image of grass carp
Joseph R. Tomelleri. Used with permission.

Learn more about grass carp

Aquatic plants add dissolved oxygen to the water and are a vital first link in the food chain. These plants are the main food source for the tiny invertebrates that small fish eat. Submerged, floating, and emergent aquatic plants provide important habitat structures for a variety of aquatic wildlife.

Rooted aquatic plants stabilize shorelines, preventing bank erosion by wave action. In addition, they enhance water clarity by reducing the amount of suspended material introduced to the system. Plants also provide visual appeal with attractive flowers, colors, and leaf patterns.

Although aquatic plants can be beneficial, they can easily overpopulate and become a nuisance for the landowner.

Too Many Weeds

Although necessary for a diverse and thriving aquatic community, uncontrolled plant growth interferes with boating, swimming, fishing, irrigation, livestock watering, and most other uses of lakes or ponds. Plants can also harm the fishing potential of a body of water. An excess of decaying plants can lower the water’s oxygen levels and can kill fish. In some waters, an abundance of plants overprotects sunfish and other prey species, allowing them to overpopulate and outstrip food supplies which can result in poor growth, or stunting, of the fish.

When aquatic plants hurt, rather than help, a lake, they technically become weeds and some form of control becomes necessary. Aquatic herbicides are commercially available to control weeds, but they can be expensive, time consuming with reapplication, and potentially hazardous, if misused.

A Biological Alternative

A non-chemical method of aquatic weed control is possible, thanks to a weed-eating member of the minnow family.

Grass carp, is a long, slender, silver-colored fish. This Asian minnow has a terminal mouth (not sucker-like, as does the common carp) and large throat teeth that help it tear and shred plant material.

Grass carp feed almost exclusively on aquatic plants. They can eat 2-3 times their weight each day and may gain 5-10 pounds in a single year. Grass carp can grow up to 100 pounds. The larger they get, the more plant material they consume.

Since grass carp cannot reproduce in ponds and lakes, they are an excellent biological control agent. They can only affect the impoundment during their individual life span. They are usually most effective after their first growing season until around age eight. Due to this growth period, weed decline is usually not apparent in a pond until the end of the second year, depending on the number of fish stocked.

Although grass carp will probably not reduce mature stands of aquatic flora, they may eat the new sprouts and prevent further expansion by these plants.

Table 1: Some common aquatic plants eaten by grass carp.

Common Name Scientific Name
Pondweed Potemogeton, Najas
Milfoil Myriophyllum
Coontail Ceratophyllum
Waterweed Elodea
Muskgrass Chara
Cattail Typha spp.
Water lilies Nymphaea odorata

The Department recommends combining control agents if an overabundance of aquatic plant growth restricts impoundment use. To gain immediate control, spot treat the nuisance plants with a herbicide and stocking of grass carp at a rate appropriate to the estimated reduced percentage of plant coverage in the lake to prohibit the return of the offensive growth.

Stock With Care

Where to stock

As effective as grass carp can be, remember that they are an exotic species. Since they cannot reproduce in impoundments, they have a small long-term effect. However, when released into the flowing waters of a large stream or river, the grass carp can reproduce at an astounding speed. Due to this, precautions should always be taken to restrict the carp from escaping the impoundment. This can be as simple as placing a mesh screen across the spillway.

When to stock

Grass carp should never be stocked as a preventative measure. Introducing grass carp into a new impoundment before aquatic flora is established can lead to an impoundment denuded of all vegetation, a problem equally as severe as vegetation overgrowth.

Grass carp will not control filamentous algae (moss) growth, except at stocking rates far beyond those suggested, which leads to other complications. Learn about methods for reducing algae on the algae control page.

How much to stock

There are no guidelines for grass carp stocking density rates that will fit all situations. Each pond or lake has its own combination of fertility, water clarity, shallow water, and chemical makeup. Each of these variables affect the number of grass carp required to achieve the level of plant control desired. In addition, different pond owners often desire different amounts of weed control.

Table 2 suggests an approximate number of grass carp to stock per acre, based upon percentage of weed coverage. These numbers may be modified based upon the variables mentioned above. If in doubt, contact your local fisheries personnel for stocking information specific to your pond or lake.

Table 2: Suggested grass carp stocking rates.

Percent of plant coverage in lake Number of carp per acre of water
10-20 percent mechanical or chemical spot treatment
20-40 percent 2-5 carp
40-60 percent 5-10 carp
over 60 percent 10-20 carp

What to stock

The standard recommended size of stocked grass carp is 8-12 inches. Fish at this size are large enough to escape being eaten by bass.

Grass carp may be obtained from commercial fish producers throughout the state. For the names of nearby grass carp sources, you may contact either your local MDC office or download the Missouri Fish Producers list.

Keep in mind that aquatic weed control with grass carp takes time. Monitor their by making a simple map of the vegetation when the grass carp are stocked. After three growing seasons, refer to the map. If the desired results have not been achieved, you may want to increase the number of grass carp in the pond, but do not exceed the next highest rate shown in Table 2.

Harvesting the Fish

There are generally two circumstances that lead to harvesting grass carp from a pond. The first occurs when the nuisance plant growth becomes controlled, particularly in ponds stocked to the maximum density (20 grass carp per acre). Reduce the grass carp by about 50 percent. This prevents over grazing and eventual denuding of the pond bottom.

The second circumstance is determined by the growth rate of the fish. Since grass carp feeding habits decline around age eight and they cannot reproduce in ponds, older fish need to be removed and restocked with younger fish. Observations of plant growth and reference to your vegetation map will help determine when restocking is necessary. Before restocking, it is very important that a similar number of fish be removed from the pond. There are several methods of attempting this.

Grass carp can be very difficult to catch with a pole and line. They are a very cautious and reclusive fish, preferring to feed unobserved. Their capture makes the effort gratifying due to grass carp being spectacular fighters on a line and very difficult to land. If you decide to attempt pole and line fishing for grass carp, try first chumming the area to be fished with whole kernel canned corn. Then, fish the area using canned corn, worms, a dough bait with a vegetation base, or pieces of vegetation like lettuce, pea pods, or cherry tomatoes as bait. Remember, grass carp are primarily herbivores, so vegetable baits are the most effective.

Bow fishing is often an effective method of harvesting grass carp, especially in smaller ponds. If you do not bowhunt, contact a local bowhunter organization for volunteers to remove the excess grass carp. Many bow hunters are eager for the challenge of stalking this wary prey.

Grass carp can be difficult to capture alive because they escape seines by leaping. One of the most effective methods of seining grass carp is to repeatedly bait a small cove. The landowner block off the cove while the fish are eating, giving them a much smaller area to escape to if they avoid the seine. If they can be captured alive, grass carp may be stocked into other waters with weed problems. This method allows landowners to avoid waiting for the fish to reach a large enough size to affect the vegetation.

Grass carp killed during their capture may be prepared for the table. They are considered an excellent food fish worldwide, often praised for both their flavor and texture. Learn how to cook your carp on the Department’s recipe page.

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