The grass carp (Figure 1) is native to certain large rivers in China and Siberia. It spawns naturally only in relatively long, fast-flowing rivers with fluctuating water levels and is not capable of reproducing in ponds, lakes, or reservoirs. Grass carp are grown throughout Asia and parts of eastern Europe for food and for control of aquatic weeds. This species was first imported into the United States from Malaysia in 1963 by the U.S. Fish and Wildlife Service as a potential biological control agent for hydrilla and other aquatic vegetation. Its culture is restricted in the United States because of potential adverse impacts on native species.

Juvenile grass carp feed on plankton (microscopic plants and animals in the water) but change their diet to feed exclusively on vegetation when they reach a length of about 6 inches. These small fishes may feed effectively on filamentous algae, duckweed, and watermeal, but again their diets changes as they grow larger to feed almost exclusively on submersed aquatic macrophytes. Because they are totally herbivorous (unlike the common carp), they do not compete for food with desirable game fish and commercially valuable species. When water temperatures exceed 68deg.F, grass carp feed almost continuously. Under optimal conditions, they may eat several times their body weight in plant material each day. When food is abundant, grass carp grow rapidly, attain relatively large size (20- to 25- pound fish are normal; weights of over 40 pounds are not uncommon), and may live for 10 years.

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