The grass carp (Ctenopharyngodon idella) is a herbivorous, freshwater fish species of the 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. 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 will enter reproductive condition and spawn at temperatures of 20 to 30°C (68 to 86°F).
In the United States, the fish is also known as white amur, which is derived 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 elongated, 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 eight 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 cm (7.9 in) will reach over 45 cm (18 in) by fall. The average length is about 60–100 cm (24–39 in). The maximum length is 1.4 m (4.6 ft) and the maximum weight 40 kg (88 lb). 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.
Grass carp have been introduced to many countries around the world. In the Northern Hemisphere, countries of introduction include Taiwan, Japan, the Philippines, the USA, Mexico, India, Malaysia, the Netherlands, Switzerland, Czechoslovakia, Yugoslavia, Denmark, Sweden, Romania, Poland, Italy, West Germany, France, and the United Kingdom. In the Southern Hemisphere, they have been introduced to Argentina, Venezuela, Fiji, New Zealand, 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 they have quite specific reproductive requirements.
In the United States, the species was first imported in 1963 from Taiwan and Malaysia to aquaculture facilities in Alabama and Arkansas. The first release is believed to have been an accidental escape in 1966 from the US Fish and Wildlife Service's Fish Farming Experimental Station in Stuttgart, Arkansas, followed by planned introductions beginning in 1969. Subsequently there have been widespread authorized, illegal, and accidental introductions; by the 1970s the species had been introduced to 40 states, and it has since been reported in 45 of the country’s 50 states. In 2013 it was determined to be reproducing in the Great Lakes Basin. It is still stocked in many states as an effective biocontrol for undesirable aquatic vegetation, many species of which are themselves introduced.
Grass carp require long rivers for the survival of the eggs and very young fish.
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, grass carp populations must be maintained 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, so 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 herbivorous habits, will also sometimes eat other animals. 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 among bowfishers where bowfishing for them is legal.
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- Chapman, Duane C.; Davis, Jeremiah J.; Jenkins, Jill A.; Kocovsky, Patrick M.; Miner, Jeffrey G.; Farver, John; Jackson, P. Ryan (2013). "First evidence of grass carp recruitment in the Great Lakes Basin". Journal of Great Lakes Research 39 (4): 547–554. doi:10.1016/j.jglr.2013.09.019. ISSN 0380-1330.
- Catching Grass Carp, Missouri Department of Conservation http://mdc4.mdc.mo.gov/Documents/18513.pdf