Triploid Grass Carp Stocking For Aquatic Vegetation Control

General History

The white amur or grass carp is a rapid growing, plant-eating fish native to the large rivers of eastern China and Siberia. They are one of the largest members of the minnow family and fish as large as 110 pounds have been collected from the Yangtze River in China. A more typical size for Virginia waters would be 20 lbs. Life span typically ranges from 5 to 11 years, but fish over 20 years old have been collected in China.

Grass carp were first brought into the U.S. in 1963 for aquatic plant control research. Many researchers viewed the fish as being a natural weed control agent. However, fears of reproducing populations and reports of environmental damage caused most states to prohibit their use. Major spawning areas are large turbulent rivers. Reproduction outside its native range is rare, but has been documented in the U.S. (lower Mississippi River) and other countries. Therefore most research has been directed towards production of sterile fish. In 1984 a major breakthrough occurred with the production of sterile “triploid” grass carp. These genetic derivatives have 72 chromosomes instead of the normal 48. This is caused by shocking fertilized eggs during incubation either by heat, cold, or pressure to stimulate retention of chromosomes normally expelled during cell division. Because the method used to produce sterile fish is not 100% effective, individual fish must be examined to confirm sterility. The U.S. Fish and Wildlife Service does this testing before the fish are imported into Virginia.

Under good conditions a five-pound fish will eat about five pounds of aquatic plants a day! As fish become larger consumption decreases, and a 20-pound fish may eat only four pounds of plants a day. Feeding rates are temperature dependent and slow down drastically below 60°F. Therefore grass carp are not recommended for trout ponds.

Management Objectives

Nuisance aquatic plants are pond owner’s greatest threat to a productive and enjoyable impoundment. When vegetation becomes over abundant in a pond or lake, it can have adverse impacts. Too much vegetation reduces nesting sites for fish; provides too many hiding areas for small fish, allowing them to overpopulate; hampers navigation of motor boats; restricts swimming areas; and interferes with fishing. If controlled, aquatic vegetation is both desirable and beneficial. Generally, 10-30% plant coverage will result in good fishing. This level provides areas for nesting, feeding, and protection of forage fish and insects. Vegetation produces oxygen, helps balance mineral and nutrient levels, and helps stabilize and prevent sedimentation problems. If your pond or lake is used exclusively for swimming and boating, total plant eradication may be appropriate. However, if fishing is a primary concern, controlling vegetation is more important than eradication.

Controlling plants in your swimming area or boat ramp may require immediate results, and mechanical harvesting or chemicals would be more effective. Carp will NOT work immediately, it can take up to one year post stocking for the desired result. To reduce heavy plant infestations to > 30% of the surface area, a combination of chemicals for spot treatments, and stocking triploid grass carp for long term control may be appropriate. Getting the desired results is no accident. Planning your treatment and following your plan achieves results!