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Grass Carp for Pond Weed Control
by Scott Jones
One of the most cost-effective long-term aquatic vegetation control measures available to Arkansas pond owners is the grass carp. That said, grass carp do not control everything equally well and they rarely provide quick results on their own. Pond owners should understand these limitations so that realistic expectations can be formed.
WHAT ARE GRASS CARP
Grass carp are large elongated, minnow-like fish that eat aquatic plants. They are native to Asia, introduced nearly worldwide often for vegetation management. Most commercially-available grass carp are sold at around 8-10 inches in length. They can double in size within a year of stocking and often reach lengths of 36-48 inches in weedy ponds. Natural grass carp can live up to about 20 years. Triploid grass carp, the variety most often stocked in North America, appear to only live around 10-12 years in Arkansas.
WHAT CAN GRASS CARP CONTROL
Grass carp eat a variety of aquatic plants, but they do not control all species equally well. Common Arkansas aquatic weeds that grass carp will control effectively include chara, egeria, elodea, fanwort, hydrilla, southern naiad, sago pondweed, slender pondweed, and variable-leaf pondweed. Other common species that grass carp control moderately well include coontail, slender spike rush, nitella, and bladderwort. There are several other species that grass carp will eat, but do not control well. Examples include filamentous algae, duckweed, watermeal, and azolla. Grass carp teeth and throats are designed to grind up larger plants. These smaller plants often flush through the gills undamaged more than gets ingested. While the carp try to eat these plants, at realistic stocking rates, they cannot consume enough to achieve control. Many species are not vulnerable to grass carp at mature stages. Examples include American lotus, lilies, cattails, alligatorweed, water primrose, water willow and many others. In plants like these, the stems and leaves are too thick or tough for the grass carp to effectively consume. As a general guideline, grass carp usually cannot control emergent plants (those that have leaves or stems standing up out of the water). Most species that grass carp can control are fully submerged and easily ground-up between your forefinger and thumb.
WHEN ARE GRASS CARP APPROPRIATE
Grass carp should only be stocked into private impoundments for the purpose of vegetation control. Grass carp are best used as a vegetation prevention measure rather than a corrective one. New/renovated pond stocking often includes a low 3-5 grass carp per acre to ensure submerged weeds do not develop. In cases where moderate to severe infestations of species palatable to grass carp exist, stocking rates of 15-25 per acre can be appropriate. Often in these severe infestations, it is best to use a combination of aquatic herbicides, grass carp, and possibly aquatic dye for deeper impoundments. Herbicides work quickly (days to weeks) to knock the infestation back, but they often do not provide long-term (months to years) control. Dye’s help reduce light availability to weeds located deeper than about 3 feet, but they do not provide fast results and cannot affect shallow, emergent, or floating weeds. Grass carp can take a long time to control weeds on their own (months to years), but they also prevent submerged weeds from returning for the longest time (5-7 years) once control is achieved. Control is limited with a single approach, but integrating multiple approaches provides more complete and long-lasting control over aquatic weeds.
WHEN ARE GRASS CARP NOT APPROPRIATE
Grass carp cannot be stocked into public waters by private citizens; only state and federal agencies may stock them in public waters. Additionally, they should only be stocked into private waters with low probability of escape. Ponds and lakes with overflow spillways that continuously discharge water are likely to lose grass carp over the dam regularly. Grass carp tend to swim with current and are notorious for passing over dam spillways. Designs of spillway fish barriers can be found online, but these barriers must be inspected and cleared of debris more frequently than most pond owners are willing to carry out.
HOW DO I GET GRASS CARP
Grass carp are sold by several farms and pond stocking companies in Arkansas. Contact fisheries specialists for lists of these suppliers. Most states, including Arkansas, require that grass carp stocked within their borders be triploid. Triploidy is a condition intentionally induced at the farm to include an extra set of chromosomes in each cell that renders the fish sterile. This helps reduce the environmental impact of escaped grass carp on our natural waters, but also requires pond owners to restock them periodically to maintain vegetation control in their ponds. Some states require a permit from the state wildlife management agency to stock grass carp. Arkansas does not currently require such a permit, but encourages wise stocking strategies.
Grass carp are a valuable tool in the effort against nuisance aquatic weeds. But they, like all tools, are not the sole solution to every situation. Integrating grass carp with herbicides, mechanical removal, and changing the physical properties of water can help provide both rapid and lasting control for the informed pond owner.
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