NC State Fisheries and Pond Management Extension-Triploid Grass Carp

USE OF THE TRIPLOID GRASS CARP IN NORTH CAROLINA

Stocking of grass carp in North Carolina may require a permit. Policy changes instituted in 1991 in North Carolina allow stocking of a maximum of 150 triploid grass carp without a permit. A permit from the Wildlife Resources Commission (WRC) is necessary when more than 150 fish will be stocked in a single body of water. In these instances, a biologist from the WRC must visit the impoundment before issuing a permit to determine whether or not there is sufficient containment to prevent the fish from leaving the targeted body of water. In some cases, it may be necessary to install a containment structure, such as a spillway screen, to prevent the fish from escaping. This both protects your investment and keeps the fish out of areas in which they may cause damage. The WRC biologist determines whether or not a permit should be issued and what stocking rate is needed. Grass carp may be released in those bodies of water for which the permit was originally written and may not be moved to another area without permission of the WRC. A list of licensed distributors or an application for a permit to stock grass carp in North Carolina may be obtained from the Wildlife Resources Commission (telephone 919-733-3633).

Grass carp can control many kinds of aquatic vegetation effectively (Table 1). They prefer the softer-textured plants such as hydrilla and other submersed species and will consume these first. Grass carp occasionally feed on the newly-sprouted, tender shoots of some emergent and floating-leaved plants (for example, lotus, water lilies, reeds, rushes, and bulrushes), but rarely cause significant damage because these plants tend to become rather tough and woody soon after sprouting in the spring. Before stocking grass carp, make sure these fish will be effective in controlling the plants you with to eliminate. Your county Extension Service agent or a WRC district biologist can help you to identify the weed species in your pond.

Stocking rates may vary somewhat with the type and density of the vegetation to be controlled, but generally fall into the range of 10 to 15 fish per acre in small ponds and 10 to 20 per vegetated acre in larger impoundments. Large fish (8 to 10 inches long) are stocked to reduce losses from predation by largemouth bass and large wading birds. Grass carp should be stocked well before the end of the growing season to increase their chances of surviving over the winter. By the following spring, the grass carp should be large enough to prevent significant regrowth of many weeds. If first-year growth of the fish is slow, control may not be achieved until late in the second summer or the following spring. In cases where weed control is not attained, the cause usually is loss of fish (by escapes or predation), an inadequate stocking rate, or stocking for a weed that grass carp will not eat.

When a body of water is totally covered with dense vegetation, most of the oxygen produced during the daytime either is lost to the atmosphere or is used by the plants during the night. In these circumstances, there will not be enough oxygen left in the water for the grass carp. Herbicides or mechanical removal should be used before stocking to open small areas where the grass carp can get sufficient oxygen for survival.

Because grass carp fingerlings must be individually checked for triploidy and also must be raised to a fairly large size to prevent predation losses when stocked, they are relatively expensive (about $7 per fish). Even though an initial stocking costs about $100 per acre, the long-term cost is quite low, averaging about $10 per acre of weeds per year over the 10-year life span of the fish. This is a small price to pay in comparison with that of aquatic herbicides, which can cost from $100 to well over $300 per acre for a single treatment. The cost per fish also decreases when larger numbers are purchased.

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