Hardy, greenish brown fish (Cyprinus carpio, family Cyprinidae) native to Asia but introduced into Europe, North America, and elsewhere. Large-scaled, with two barbels (fleshy, whiskerlike feelers) on each side of its upper jaw, the carp lives alone or in small schools in quiet, weedy, mud-bottomed ponds, lakes, and rivers. An omnivore, it often stirs up sediment while rooting about for food, adversely affecting many plants and animals. Carp grow to an average length of about 14 in. (35 cm); some grow to 40 in. (100 cm) and 49 lbs (22 kg). In captivity they may live more than 40 years.
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Carp, along with many of their cyprinid relatives, are popular ornamental aquarium and pond fish. The two most notable ornamental carps are goldfish and koi. Goldfish (Carassius auratus) were kept as ornamental fish in China for hundreds of years before being introduced to Japan in the 15th century, and to Europe in the late 17th century. The koi, a domesticated variety of common carp (Cyprinus carpio), also originated from China and spread widely in Japan. The koi are historically a prevalent symbol in Japanese culture of good luck. They are shown in competitive fish shows like those at the All-Japan exhibition. They are also popular in other parts of the world as pond fish. Goldfish and koi have advantages over most ornamental fishes, as they are tolerant of cold (they can survive in water temperatures as low as 4 degrees Celsius), and can survive at low oxygen levels.
Sir Isaac Walton said this about carp in his work The Compleat Angler "The Carp is the queen of rivers; a stately, a good, and a very subtil fish; that was not at first bred, nor hath been long in England, but is now naturalised."
However, in some countries, due to their habit of grubbing through bottom sediments for food and alteration of their environment, they destroy, uproot and disturb submerged vegetation causing serious damage to native duck and fish populations. In Australia there is enormous anecdotal and mounting scientific evidence that introduced carp are the cause of permanent turbidity and loss of submerged vegetation in the Murray-Darling river system, with severe consequences for river ecosystems, water quality and native fish species.