Originally published June 1997, revised 2007
Controlling aquatic vegetation with grass carp is one of the options available to pond owners with aquatic plant problems. In many situations, the use of grass carp is an economical, long lasting, and effective option. However, grass carp are not appropriate for every pond with abundant aquatic plants. Whether grass carp should be stocked in a pond or not, depends on the goals for the pond, the plant species causing problems, and the probability of grass carp escaping the pond.
Grass carp should not be stocked in a pond where attracting ducks is an important goal for the pond. Most ducks, especially surface-feeding (dabbling) ducks such as mallards, eat aquatic plants and are attracted to ponds with aquatic plants. Grass carp prefer many of the plant species that attract ducks.
Grass carp generally should not be stocked into a pond or wetland where maintaining a natural ecosystem is an important goal. Aquatic plants are a natural component of shallow ponds and wetlands. Grass carp are not native - they are introduced from eastern Asia. Aquatic plants are important components of habitat for many native animals.
When clear water is an important goal of a south central Oklahoma or a north central Texas pond, grass carp probably should not be stocked or should be stocked at relatively low densities. Ponds with aquatic plants generally have clearer water than ponds without aquatic plants. Aquatic plants stabilize a pond?s bottom and banks, which help prevent the pond from becoming muddy when wind or animals stir the water. Aquatic plants help flocculate suspended soil particles, maintaining clearer water by settling soil particles to the bottom or onto plants. Grass carp recycle the nutrients trapped in aquatic plants back into the water. When grass carp are stocked at rates high enough to control aquatic plants, the recycled nutrients often stimulate a phytoplankton bloom that generally reduces water clarity.
In a pond where largemouth bass-bream sport fishing is an important goal, it is desirable for aquatic plants to dominate 5-25% of the pond. Grass carp are probably not a good choice for a bass-bream sport fishing pond where plants cover less than 25% of the pond.
It can be difficult to obtain and sustain partial control of abundant aquatic plants with grass carp. A rate of 5-9 grass carp per acre frequently provides partial control. When using grass carp for aquatic plant control, a pond manager needs patience. The extent of control often does not become apparent until 18-24 months after stocking. It is easier to stock conservatively and add more fish later, if necessary, than it is to remove grass carp when too many are stocked.
Grass carp maintain control for a long period of time. Total or adequate aquatic plant control was maintained for more than 20 years in most Noble Research Institute ponds where grass carp were stocked with appropriate fish barriers on spillways.
Grass carp should be at least 8 inches long when stocked to avoid predation. Grass carp are usually stocked at rates ranging from 5 to 12 per acre. The 10 to 12 per acre stocking rate generally removes all submersed aquatic plants and most emersed aquatic plants within 2 years. This rate or even a higher stocking rate is appropriate for an irrigation pond or a fish culture pond because a pond manager in these situations generally prefers no submersed or emersed aquatic plants.
Stocked at appropriate rates, grass carp control most species of submersed aquatic plants and many species of emersed aquatic plants. Grass carp sometimes control large coarse stemmed aquatic plants such as cattail, bulrush, and American lotus, but in other situations they do not. Some emersed plants, such as water willow, are rarely or never controlled by grass carp.
Grass carp should never be stocked into a pond where there is a substantial risk of escape. Grass carp live in rivers in their natural environment so they actively search for moving water. They can escape through an unprotected spillway with as little as 3 inches of water flow. Grass carp can not reproduce in ponds but they can reproduce in some rivers. Grass carp can damage or destroy native wildlife and fish habitats by removing aquatic plants and indirectly causing increased water turbidity. For this reason, it is against the law to release grass carp into public waters in both Oklahoma and Texas. If grass carp escape, the investment in aquatic vegetation control washes downstream with the grass carp.
Grass carp should be stocked only in a pond with properly designed fish barriers on the spillways or in a pond with no overflow. If water will flow through both an overflow pipe and an emergency spillway, both need barriers. The best type of barrier is a parallel bar barrier. It is constructed of round metal rods welded horizontal with 1-inch gaps between the rods. Vertical supports should be no closer together than necessary to adequately support the barrier. A parallel-bar barrier clogs much less and lasts much longer than net wire barriers such as hardware cloth, poultry wire, fence wire, etc. Generally, a box-type, parallel- bar barrier works best over the intake of an overflow pipe. A larger box is less likely to clog than a smaller one. A panel- type barrier is generally placed at the entrance or crest of an emergency spillway. A barrier on an emergency spillway should have at least 2 feet of freeboard between the top of the barrier and the top of a dam.