Grass carp (Ctenopharyngodon)
Grass carp are a large member of the minnow family and are related to common carp and goldfish. However, their appearance is quite unlike other carp, aside from their large scales. Grass carp are generally a dark color on the back that ranges from gray to green, dark olive, golden brown or bronze. The color fades to a light brown-yellow or silver on the side before becoming white on the belly. Fins are clear to dark in color.
They have a long, torpedo shaped body that resembles that of the creek chub or common shiner, only much larger. The body is covered with large scales, of which 40 to 42 make up the lateral line. The upper-side scales have a dark fringe and black spot that often give the grass carp a checkered look.
Grass carp have a broad, rounded head with a short snout. Compared with other carp, the large eyes are set relatively low on the head. The mouth is located at the end of the head and has thin, rounded lips. Sharp teeth are located in the back of the throat. Their mouths are clearly smaller than those of other carp. In addition, grass carp do not have the barbels found around the mouths of other carp.
Grass carp have a short dorsal fin composed of 7 to 10 rays. The anal fin, which is located closer to the tail than usual for a minnow, has 7 to 11 rays. The large tail is moderately forked.
Grass carp are almost exclusively vegetarian. As a result of this vegetarian nature, they almost never compete with popular game fish or commercial fish for food sources.
Their common name comes from their almost exclusive diet of aquatic grasses and weeds. They strongly prefer submerged grasses, such as hydrilla and coontail moss. When food supplies are low, they will seek floating vegetation. Occasionally they will eat small insects, invertebrates or decomposing organic matter. However, this is generally limited to fry until they reach about 3 to 6 inches.
The feeding habits of grass carp are influenced by water temperature. They are basically inactive feeders below 45 F. However, at temperatures above 68 F, they will feed almost continuously. Grass carp are capable of eating several times their own weight in vegetation each day. They ingest large amounts of food by shredding vegetation with tearing motions.
The native range of the grass carp was originally the larger rivers of China and eastern Siberia that drain to the Pacific Ocean. The common name, white amur, comes from the Amur River in Asia. As a result of introduction efforts, the species now resides in over 50 nations and at least 40 U.S. states. Because of their unique spawning habitat requirements, grass carp can only spawn naturally in few areas outside their native range. As a result, where they have been introduced, stocking programs are critical to maintaining their numbers.
They were first imported to the United States from Asia in the early 1960s in an effort to exploit their feeding habits for weed control purposes in selected waters. Because of their aggressive feeding habits, many of the states in which they are found have legal restrictions on grass carp. Generally, only the sterile triploid version is allowed. Their range in the United States encompasses an area generally running from Pennsylvania west to South Dakota and central Wyoming, south and west through Colorado to the four corners region, south and east through central Texas to Louisiana and along the Gulf and Atlantic coasts to New Jersey.
Grass carp are exclusively a freshwater species that inhabits primarily temperate lakes, ponds, and large river backwater areas. They prefer areas with slow current or standing water that is teeming with vegetation. They also congregate in deeper waters, usually around 15 feet or more depending on temperature and available depth, as long as vegetation is present.
They are, however, adaptable and can endure variations in temperature, high salinity in the environment or low oxygen levels. In relation to water temperature, they are known to be able to adapt to water ranging from 32 to 100 F.
Grass carp have become controversial in some places because they can alter the make-up of waters in which they are introduced. As aquatic vegetation is removed by aggressively feeding grass carp, it often turns clear water murky. However, it remains a popular species, especially in other parts of the world, for weed control in numerous bodies of water.
In most of the states in which grass carp reside, there are legal restrictions on their possession and release. In a number of states, it is illegal to introduce anything but the triploid version, which is sterile.
There is debate about how well they control aquatic vegetation because they often leave enough of a plant to re-grow and some aquatic plants are stimulated to grow more rapidly when aggressively consumed. They can also distribute plant life to other areas of the water. Many fisheries biologists will not recommend the stocking of grass carp to control floating vegetation; however, submerged vegetation will be consumed heavily by grass carp.