The silver carp (Hypophthalmichthys molitrix) is a species of freshwater cyprinid fish, a variety of Asian carp native to north and northeast Asia. It is cultivated in China for food but was introduced to North America in the 1970s to control algae growth in aquaculture and municipal wastewater treatment facilities. They escaped from captivity soon after their importation. They are considered a highly invasive species and by 2003 had spread into the Mississippi, Illinois, Ohio and Missouri rivers and many of their tributaries in the US. They are now (April 2007) abundant in the Mississippi River watershed from Louisiana to South Dakota and Illinois. Navigation dams on the Mississippi River seem to have slowed their advance up the Mississippi River, and few silver carp have been captured north of central Iowa on the Mississippi. Dams that do not have navigation locks are complete barriers to natural movement of silver carp, and it is important that fishermen do not assist this movement by the unintentional use of silver carp as bait.
The silver carp is also called the flying carp for its tendency to leap from the water when startled. They can grow to over 40 lb (18 kg), and can leap 10 ft (3 m) in the air. Many boaters traveling in uncovered high-speed watercraft have been injured by running into the fish while at speed. In 2003 a woman jet-skiing broke her nose and a vertebra colliding with a silver carp and nearly drowned.
Pound for pound, more silver carp are produced worldwide in aquaculture than any other species. Silver carp are usually farmed in polyculture with other Asian major carps, or sometimes Indian major carps or other species. It has been introduced to, or spread into via connected waterways, at least 88 countries around the world. The most common reason for importation was for use in aquaculture, but enhancement of wild fisheries and water quality control were also important reasons for importation..
The silver carp is a filter feeder, and possesses a remarkably specialized filtration apparatus capable of filtering particles as small as 4 µm. The gill rakers are fused into a spongelike filter, and an epibranchial organ secretes mucous which assists in trapping small particles. A strong buccal pump forces water through this filter. Silver carp, like all Hypophthalmichthys species, have no stomachs; they are thought to feed more or less constantly. Silver carp are thought to feed largely on phytoplankton; they also consume zooplankton and detritus. Because of their plankton-feeding habits, there is concern that they will compete with native planktivorous fishes, which include paddlefish Polyodon spathula, gizzard shad Dorosoma petenense and young fish of almost all species.
Because they feed on plankton, they are sometimes successfully used as methods for controlling water quality, especially in the control of noxious bluegreen algae. However, these efforts are sometimes not successful. Certain species of bluegreen algae, notably the often toxic Mycrocystis, can pass through the gut of silver carp unharmed, and pick up nutrients while in the gut. Thus, in some cases bluegreen algae blooms have been exacerbated by silver carp. Also, Mycrocystis has been shown to produce more toxins in the presence of silver carp. Silver carp, which have natural defenses to the toxins produced by bluegreen algae, sometimes can contain enough algal toxins in their systems that they become hazardous to eat.
Silver carp are filter-feeders, and thus are difficult to catch on typical hook and line gear. Special methods have been developed for these fish, the most important being the "suspension method" usually consisting of a large doughball that disintegrates slowly, surrounded by a nest of tiny hooks that are not embedded in the bait. The entire apparatus is suspended below a large bobber. The fish feed on the small particles that are released from the doughball and will bump against the doughball, with the intention of breaking off more small particles that can be filtered from the water, eventually becoming hooked on the tiny hooks.
In some areas, it is also legal to use "snagging gear" in which large, weighted treble hooks are jerked through the water, to snag the fish. In the United States, silver carp are also popular targets for bowfishermen; they are shot both from the water and from the air. In the latter case, boats are used to scare the fish and entice them to jump, and the fish are shot from the air when the fish jump.
There are two other species in the Hypophthalmichthys genus, the bighead carp (H. nobilis) and the largescale silver carp (H. harmandi). The genus Aristichthys is also sometimes used for bighead carp, but this genus was based on fallacious theory and should no longer be used . The bighead carp differs from the silver carp in its behavior (it does not leap from the water when startled) and also in its diet. Bighead carp are also filter feeders, but they filter larger particles than silver carp and in general consume a greater proportion of zooplankton in the diet than silver carp, which consume more phytoplankton. In at least some parts of the United States, bighead and silver carp hybridize in the wild and produce fertile offspring.
The largescale silver carp is very closely related to the silver carp, but its native range is to the south of that of the silver carp, mostly within the country of Vietnam. Unlike bighead and silver carp, largescale silver carp have not been widely introduced around the world for use in aquaculture, although there was at least one introduction to some waters of the Soviet Union, where they hybridized with the also introduced silver carp.