Aquaguide: Grass Carp Control
Weeds in Ponds and Lakes

Aquatic plants are a beneficial and necessary part of Missouri fishing lakes and ponds.They are a vital first link in the food chain leading to the fish and, eventually, to people.

Plants in ponds, lakes and streams convert solar energy into stored chemical energy and contribute dissolved oxygen to water.

Aquatic plants are an important food source for tiny animals, known as invertebrates, which are a major food source for small fish.

They also provide homes for aquatic insects, crayfish and small frogs, all important food items for fish.

Submerged aquatic plants serve as escape cover for fishes, and emergent vegetion provides important living and nesting habitat for bullfrogs, waterfowl and shorebirds.

Aquatic plants also help stabilize shorelines, preventing erosion of the banks.

In addition, many aquatic plants have aesthetic value, producing attractive leaf patterns and flowers.

Too Many Weeds

Although necessary for a diverse and thriving aquatic community, plants sometimes interfere with boating, swimming, fishing, irrigation, livestock watering or 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 amount of oxygen in the water to a level that can kill fish.

In some waters, an abundance of plants overprotects sunfish and other prey species, allowing them to overpopulate and outstrip food supplies. The result is inadequate 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, need to be reapplied frequently and are potentially hazardous, if misused.

Other methods of weed control are detailed in another Conservation Department Aquaguide, "Aquatic Plant Management in Missouri."

A Biological Alternative

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

The white amur (Ctenopharyngodon idella), commonly called grass carp, is a long, slender, silver-colored fish. This Asian minnow, which can grow up to 100 pounds, has a terminal mouth (not sucker-like, as does the common carp) and has large throat teeth that help it tear and shred plant material. Grass carp will not reproduce in ponds or lakes. 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. The larger they get, the more plant material they consume.

Weed decline is usually not apparent in a pond until the end of the second year, depending on the number of fish stocked.

          Common Name              Scientific Name
          Duckweed                 Lemna, Wolffia
          Pondweed                 Potemogeton, Najas
          Milfoil                  Myriophyllum
          Coontail                 Ceratophyllum
          Waterweed                Elodea
          Muskgrass                Chara

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

Stock With Care

When stocked at the conservative rates recommended by the Conservation Department, grass carp offer a trouble-free, ongoing method of weed control. Although not likely to reduce mature stands of cattails or water lilies, they may eat the new sprouts and prevent further expansion by these plants (see Table 1).

Grass carp will not control filamentous algae (moss) growth, except at stocking rates far beyond those suggested.

Grass carp may compete with other fish species for some food items when aquatic vegetation becomes scarce.

Stocking rates for grass carp have been investigated for several years, but no guidelines will fit all situations. Each pond or lake is different, having its own combination of fertility, water clarity, shallow water and chemical makeup. In addition, different pond owners desire different amounts of weed control.

Stocking rates may vary from as low as one to as many as 20 grass carp per acre, depending on the amount and types of vegetation.

The standard and recommended size of stocked grass carp is 8-12 inches. These fish are large enough to escape being eaten by bass.

          Percent of plant           Number of Carp
          coverage in lake           per acre of water

             10-20                   Mechanical or
                                     chemical spot
                                     treatment
             20-40                   2-5
             40-60                   5-10
             over 60                 10-20

        Table 2. Suggested grass carp stocking rates.

Table 2 suggests how many grass carp to stock per acre, based on the percent of weed coverage. These numbers can be modified based on the above-mentioned variables. If in doubt, contact your local Conservation Department office. Fisheries personnel will provide grass carp stocking information specific to your lake or pond.

Grass carp are available from commercial fish dealers. For the names of nearby grass carp sources, contact Fisheries Division, Fish Dealers List, Missouri Department of Conservation, P.O. Box 180, Jefferson City, MO 65102-0180, 314/751-4115.

Keep in mind that aquatic weed control with grass carp takes time. Making a simple map of the vegetation at the time of stocking will help you monitor progress.

If the desired results are not achieved after two growing seasons, consider increasing the number of grass carp in the pond, but do not exceed the next highest rate shown in Table 2.

Harvesting the Fish

When weed problems have been brought under control, particularly in ponds stocked to the maximum density (20 grass carp per acre), efforts should be made to reduce by about 50 percent the number of grass carp in your pond or lake.

Grass carp are difficult to catch with hook and line. Try chumming an area with whole kernel canned corn, then fishing the area with canned corn, worms, pieces of vegetation, such as lettuce or pea pods, or a dough bait with a vegetation base on your hooks.

Bowfishing can also be an effective method of harvesting grass carp.

Grass carp captured alive can be stocked into other waters with weed problems. The fish are also considered an excellent food fish.

Although grass carp are relatively long-lived, you will likely have to stock more in the future to maintain effective plant control. Observations of plant growth and reference to your vegetation map will help you decide when restocking is necessary.

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