Grass carp (Ctenopharyngodon idella) originated in Asia and were brought to the United States as a biological means to control nuisance aquatic weeds.
Also known as the white amur, they are a strictly herbivorous fish. Grass carp feed on a variety of aquatic plants, but prefer submersed vegetation such as pondweed, chara, and hydrilla. Grass carp will also consume duckweed and to a lesser extent, watermeal. Although grass carp do not prefer filamentous algae, they will often consume it in the absence of other forms of preferred vegetation. Grass carp can offer a long-term method of vegetation control at a relatively low cost; particularly if the problem weed is one of the fish's preferred plants. Grass carp should be stocked at a rate of 25 to 30 per vegetated acre to achieve good control. Grass carp should be stocked when they are about 8 to 12 inches in length to avoid predation by bass. They typically grow quickly and achieve very large sizes, often exceeding 50 pounds. They feed most efficiently when they are young and therefore should be restocked every few years. Grass carp will not reproduce in a pond environment. Even though grass carp do not reproduce in ponds and lakes, many states restrict the sale and use of diploid fish. Diploid (meaning 2 sets of chromosomes) grass carp are technically fertile and capable of reproducing, but require many miles of free-flowing water in order for their fertilized eggs to remain suspended. To avoid the potential for escaped fish to enter large river systems and possibly reproduce, many states require the use of sterile triploid (3 sets of chromosomes) grass carp.
Other things to consider
Grass carp will readily train to a pelletted food. Therefore, once they have removed the vegetation from your pond, they will begin to interfere with your bluegill or trout feeding program. If this happens, they can be removed by angling with a doughball type bait that resembles the food pellets. Also, since grass carp tend to cruise high in the water column, they can be easily targeted with a bow. Grass carp are notorious for finding their way out of a pond. If a rain event pushes water over your spillway, grass carp will follow the flow and quickly escape the confines of your pond. Therefore, a barrier should be placed across the emergency spillway to prevent the loss of the grass carp you stock.