Vegetation Management – Chemicals Or Grass Carp?

Devising a vegetation management plan poses two common options. Utilize grass carp for a biological method or apply chemicals. Which is best? Both groups have valid points. The plant to be controlled will determine the treatment method.

Our friend Michael P. Masser with the Texas Cooperative Extension at Texas A&M published an informative article on grass carp for the Southern Regional Aquaculture Center. Opening remarks describe grass carp as native to large river systems of eastern Asia. Because of their association with the Amur River, they sometimes are called White Amur. They have been introduced among more than 50 countries for foodfish culture and aquatic vegetation management. The U.S. Fish and Wildlife Service, in cooperation with Auburn University, brought them to the U.S. in 1963, to investigate usefulness in controlling aquatic vegetation. There are no native species available for this purpose.

Diploid (normal or non-sterile) carp have escaped into U.S. river systems and appear to have established reproducing populations among Mississippi, Missouri, and Trinity River drainages. Fear that grass carp might devastate beneficial, native vegetation in public waters prompted many states to ban their stocking. Although naturalized species do not appear to have established large, destructive populations, they are a controversial topic among natural resource managers. They are legal in most states. Some states allow only sterile fish and require landowners obtain a permit to stock them. Texas requires a permit. Grass carp have pharyngeal teeth (in the throat) that enable them to cut/shred vegetation it consumes. Their flesh is white, firm, and not oily, but muscle mass contains Y bones. They are considered a delicacy by many seafood enthusiasts.

Triploids, or sterile carp, are produced in hatcheries by physically shocking eggs immediately after fertilization with temperature or pressure. Resulting fish are triploids because each cell has an extra set of chromosomes. This increased handling and testing is the reason triploids cost two or three times more than diploids. There appears to be no difference in vegetation preferred by either species. However, triploids consume on 90 percent of the amount diploids partake. Triploids live 10 to 12-years compared to 20+-years for diploids.

Research has shown size and type of vegetation are factors that influence consumption rates. Generally, grass carp prefer soft, tender tips of young growing plants and submerged vegetation. It should be emphasized that even though grass carp will eat a particular plant, they may not control that plant unless stocked in sufficient numbers and preferable plants are not available. They are fed fish food pellets as juveniles and don’t forget them as a feeding opportunity. Growth rates are rapid under favorable conditions. They forage intermittently at temperatures as low as 37-degreesF and steadily at 50 to 60-degrees. Optimal consumption occurs between 70 to 86-degrees. Fish weighing 2 to 2.5-pounds can consume more than their body weight daily (in some cases 300-percent).

Stocking rates should be based on carp vegetation preferences and biomass of plants. Often, managers stock too few fish to control the problem. While grass carp live many years, effectiveness decreases significantly after 5 to 7-years. Usually, ponds should be restocked every 5 to 7-years. In new or renovated ponds, 3 to 5 carp per surface acre can be stocked to prevent submerged vegetation from becoming established.

Because carp are natural inhabitants of rivers, they readily escape overflowing ponds. Flow of only a couple inches allows even large sizes to escape. Consequently, many states recommend, or mandate, barriers be placed across emergency spillways before allowing stocking. Once stocked, it’s difficult to remove carp. They jump seines. Other methods include angling, bow-fishing, and rotenone baits.

Carefully research state laws regarding grass carp before stocking. Accurately identify problematic plants and confirm they are a variety grass carp can manage efficiently. They cannot process certain types. In some cases, chemical treatment may be the only resolution.