The number of grass carp required to control weed problems varies, depending on the degree of weed infestation, kind of weed, size of pond or lake, and size of fish stocked. A number of different approaches have been used to determine the appropriate number of carp to stock, and all are successful. The most precise method is to determine the weight of aquatic vegetation in the pond, and then, knowing the consumption rates of the fish, stock the appropriate number to consume that amount of vegetation in a specified period. This is seldom practical, however, and pond owners must use other means to determine how many carp to stock.

The general rule of thumb in farm ponds is to stock enough grass carp to control the weeds in one to two seasons, but not so many that they completely eradicate all vegetation in a short period of time. The best approach is to consider the carp as a weed "maintenance" tool, rather than an eradication tool. This usually results in less environmental disturbance of the pond, and the carp are not subject to starvation due to complete loss of vegetation. Additionally, recent research indicates that complete eradication of weeds from a pond may not be desirable, since vegetation harbors many tiny organisms that contribute to the productivity in the pond. Fishermen have long known that weed beds and other vegetative "structure" can be highly desirable, since they provide shade and cover for sport fish. The problem occurs when weed growth passes the "fine line" between desirable and undesirable amounts.

This rule of thumb does not apply to commercial aquaculture ponds, however. In these kinds of ponds, more rapid control of aquatic weeds is usually needed, and higher stocking rates are required. Long-term impacts of high densities of grass carp are not a concern, since fish farmers can remove excess carp when they harvest other fish in the pond. Commercial fish producers can often sell the carp for other uses after they have achieved the weed control needed in the aquaculture situation.

For most farm pond situations where weeds have already become a problem, 5 to 10 grass carp per surface acre will achieve desired weed control, without resulting in crowded conditions of the fish. In severely weed-choked cases, higher rates of 15 to 20 grass carp per acre may be necessary to attain control. In such cases, it is sometimes more effective to treat the pond with a herbicide first, and then stock moderate numbers of grass carp. You can get assistance in diagnosing the situation by contacting your county Extension agent, or a Mississippi Department of Wildlife Fisheries and Parks fisheries biologist.

In new ponds where grass carp are stocked as a weed preventive measure, three to five fish per acre usually does the job. In new ponds, fingerling grass carp may be successfully stocked anytime before the bass are stocked. Once bass are established, however, larger grass carp will be required to ensure that the bass do not eat them. In ponds where bass populations are present, a minimum carp size of 8 inches is required to ensure their safety from bass predation. Although this size fish cost more, it is a cost-effective stocking technique, since predation losses are minimized.

Grass carp can be purchased from many sources, instate and out of state. When possible, pond owners should stock certified triploid carp. Such fish have been tested individually to confirm that they possess an additional set of chromosomes and are, therefore, sterile. A list of hatcheries licensed to sell game fish and grass carp is available from your county Extension agent, the Mississippi Department of Agriculture, or Mississippi Department of Wildlife, Fisheries and Parks.