The grass carp (Ctenopharyngodon idella) is a herbivorous, freshwater fish species of family Cyprinidae, and the only species of the genus Ctenopharyngodon. It is cultivated in China for food, but was introduced in Europe and the United States for aquatic weed control (see, e.g., Ponchatoula Creek). It is a large cyprind native to eastern Asia, with a native range from northern Vietnam to the Amur River on the Siberia-China border. It is a fish of large, turbid rivers and associated floodplain lakes, with a wide degree of temperature tolerance. Grass carp are usually thought to enter reproductive condition and spawn at temperatures of 20 to 30 °C (68 to 86 °F), but have been shown to sometimes spawn at temperatures as low as 15 °C (59 °F).
In the United States, the fish is also known as white amur, a name developed to avoid use of the name "carp", which has derogatory connotations in North America. The name derives from the Amur River, where the species is probably native, but has never been abundant. This is not to be confused with the white amur bream (Parabramis pekinensis), which is not a particularly close relative as Cyprinidae.
Appearance and anatomy
Grass carp have elongate, chubby, torpedo-shaped body forms. The terminal mouth is slightly oblique with non-fleshy, firm lips, and no barbels. The complete lateral line contains 40 to 42 scales. Broad, ridged, pharyngeal teeth are arranged in a 2, 4-4, 2 formula. The dorsal fin has 8 to 10 soft rays, and the anal fin is set closer to the tail than most cyprinids. Body color is dark olive, shading to brownish-yellow on the sides, with a white belly and large, slightly outlined scales.
The grass carp grows very rapidly. Young fish stocked in the spring at 20 centimetres (7.9 in) will reach over 45 centimetres (18 in) by fall, and adults often attain nearly 1.2 metres (3.9 ft) in length and over 18 kilograms (40 lb) in weight. According to one study, they live an average of five to 9 years, with the oldest surviving 11 years. They eat up to three times their own body weight daily. They thrive in small lakes and backwaters that provide an abundant supply of freshwater vegetation.
This species occurs in lakes, ponds, pools and backwaters of large rivers, preferring large, slow-flowing or standing water bodies with vegetation. In the wild, grass carp spawn in fast-moving rivers, and their eggs, which are slightly heavier than water, develop while drifting downstream, kept in suspension by turbulence. The eggs are thought to die if they sink to the bottom.
The species was deliberately introduced into the United States in 1963 for aquatic weed control. The grass carp is considered an invasive species in the United States; however, it is still stocked in many states as an effective biocontrol for undesirable aquatic vegetation, many species of which are themselves invasive. Grass carp require long rivers for the survival of the eggs and very young fish.
Grass carp have been introduced to many Northern Hemisphere countries including Taiwan, Japan, Philippines, USA, Mexico, India, Malaysia, Netherlands, Switzerland, Czechoslovakia, Yugoslavia, Denmark, Sweden, Romania, U.S.S.R. (western), Poland, Italy, West Germany, France, United Kingdom, and Venezuela. In the Southern Hemisphere they have been introduced to Fiji, New Zealand, Argentina, Australia, and South Africa. Grass carp are known to have spawned and established self-reproducing populations in only six of the many larger Northern Hemisphere rivers into which they have been stocked. Their failure to establish populations in other rivers suggests that they have quite specific reproductive requirements.
Use as weed control
The species was introduced in the Netherlands in 1973 for overabundant aquatic weed control. The release into national waters is controlled and regulated by the Dutch Ministry of Agriculture, Nature and Food Quality. Because grass carp mainly reproduce in water of 25 °C (77 °F), which is much higher than the water temperature reaches during the mating season in the Netherlands, it is necessary to maintain grass carp populations by artificial means, which is done by the person responsible for the water body in which the fish were introduced. Where grass carp populations are maintained through stocking as a biocontrol for noxious weeds, they should be returned to the water alive and unharmed.
Grass carp were also introduced into New Zealand in 1966 because of their potential to control the growth of aquatic plants. Unlike the other introduced fish brought to New Zealand, the potential value and impact of grass carp was investigated in secure facilities prior to their use in field trials. They are approved by the New Zealand Government as a biological control agent for aquatic weed control. Although the carp are unable to naturally reproduce, distribution is still carefully controlled by Conservation Agencies. The Government has also embarked on using grass carp in aquatic weed eradication projects, such as Lakes Tutira, Opouahi and Waikopiro for Hydrilla and in Lakes Swan, Heather, Kereta for Common Hornwort (Ceratophyllum).Weed eradication has been achieved in Lake Elands and Lake Parkinson. These fish at times are often mistaken for Koi Carp (Cyprinus carpio) which are an unwanted organism and noxious species in the country. 
When used for weed control, often the fish introduced to the pond or stream are sterile, triploid fish. The process for producing triploid fish involves shocking eggs with a rapid change in temperature or pressure. This process is not usually 100% effective, therefore, the young are usually tested for triploidy before being sold.
Fishing for grass carp
Grass carp grow large and are strong fighters on a rod and reel, but because of their vegetarian habits and their wariness, they can be difficult to catch. Chumming with corn adds to success. They will eat canned corn, cherry tomatoes, and, despite their primarily vegetarian habits, will also sometimes eat other vegetarians. Chumming with white bread, and a piece of bread pinched on a hook and floated on the surface works well, especially for pond grass carp. The fish are popular but wary quarry for bowfishers where bowfishing for grass carp is legal.
When searching for grass carp to fish, one may often spot fish cruising near the surface or very close to the shoreline. Often, an angler will spot a telltale swirl in the water near the shore without even glimpsing the fish. Grass carp often feed or rest near the shoreline, and are very wary in such places. They usually will dart away at the first sight of a person walking nearby. Stalking the fish to place a piece of bait nearby is sometimes successful. Casting bait on top of the fish usually results in spooking the fish.
Grass carp caught in ponds and lakes where they were stocked for weed control should be handled with care and released without harm.
- Mandrak and Cudmore. 2004. Biological Synopsis of Grass Carp (Ctenopharyngodon idella) http://sbisrvntweb.uqac.ca/archivage/24061712.pdf.
- Shireman, J.V. and C.R. Smith. 1983. Synopsis of biological data on the grass carp, Ctenopharyngodon idella (Cuvier and Valenciennes, 1844). Food and Aquaculture Organization Synopsis. 135: 86pp.
- Froese, Rainer and Pauly, Daniel, eds. (2007). "Ctenopharyngodon idella" in FishBase. May 2007 version.
- Kirk and Socha. J. Aquat. Plant Manage. 41:2003. http://www.apms.org/japm/vol41/v41p90.pdf
- Krykhtin, M.L., and E.I. Gorbach. 1981. Reproductive ecology of the grass carp, Ctenopharyngodon idella, and the silver carp, Hypophthalmichthys molitrix, in the Amur Basin. Journal of Ichthyology 21(2):109-123.
- Management and Control Plan for Bighead, Black, Grass, and Silver Carps in the United States http://www.anstaskforce.gov/Documents/Carps_Management_Plan.pdf
- Rowe, D. K., & Schipper, C. M. (1985). An assessment of the impact of grass carp (Ctenopharyngodon idella) in New Zealand waters. Rotorua [N.Z.: Fisheries Research Division, Ministry of Agriculture and Fisheries.
- Catching Grass Carp, Missouri Department of Conservation http://mdc4.mdc.mo.gov/Documents/18513.pdf