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The Northern Snakehead: An Invasive Fish Species
(Released August 2002)

  by Robert Hilton  


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During the summer of 2002, several individuals of an exotic fish species called the Northern Snakehead were found in a pond in Crofton, Maryland, about 20 miles north east of Washington, D.C. The potential impact of this introduced species was considered so damaging that the event made national headlines. Officials posted signs encouraging anglers to kill any snakeheads that they caught.1

Adult Snakehead
Adult Snakehead
Source: Maryland Department of Natural Resources

Why so much press just for a fish? The Northern Snakehead, Channa argus, is no ordinary fish, biologists explain. It is a voracious top-level predator, meaning that it has no natural enemies, and could decimate populations of native fish. About 90% of its diet consists of other fish, though it also eats crustaceans, insects, and plants. In its native range it can live in water with temperatures ranging from 0 to 30 degrees C; it is found in muddy or vegetated ponds, swamps, and slow-moving streams. Snakeheads can breathe air and survive for up to four days out of water, and can survive for longer periods of time when burrowed in the mud. They are capable of traveling over land to new bodies of water by wriggling their bodies over the ground. These features are adaptations to the seasonal drying of shallow bodies of water in the snakeheads native habitat in China and allow it to disperse widely should local conditions become unfavorable. It is capable of surviving in much of North America should it become established. An established population of snakeheads in Maryland could have long-term disastrous consequences for the ecology of the region.

Juvenile Snakehead
Juvenile Snakehead
Source: Maryland Department of Natural Resources

The Crofton discovery demonstrated that the Northern Snakehead is capable of overwintering and breeding in the Maryland climate. The snakeheads are thought to have entered the Maryland ecosystem more than two years ago. A local man ordered a pair of live snakeheads from a market in New York's Chinatown so that he could prepare a traditional soup remedy for his ill sister (see Washington Post news story). However, by the time the snakeheads arrived, the sister had recovered, so he kept them in an aquarium for a while, then released them into a local pond.2 The man has since confessed his actions to local authorities, but cannot be charged with release of an exotic species because the two-year statute of limitations has expired.

Officials first found out about the snakeheads in May of 2002, when a fisherman visited a government office in Annapolis, Maryland, with a photograph of an unfamiliar fish he had caught and then re-released in Crofton. Biologists soon identified the fish as a Northern Snakehead. One of the many people who converged on the pond after the initial news reports in late June captured a second adult. On two different days in early July, an angler and a fisheries biologist each pulled about six young snakeheads out of the pond; some were only two inches long, indicating that these were not simply individuals that had been released, but offspring from a breeding population. In a survey in late July, a freshwater fisheries manager captured more than 100 juvenile snakeheads estimated to be about five months old.3

Scientists are concerned that the snakeheads could spread beyond the Crofton pond. The Little Patuxent River is only about 75 feet from the pond, and a major flood could move fish into the river system where they could quickly spread. Experience with escaped and introduced species shows that it is essentially impossible to eradicate an exotic species that becomes established: witness the European Rabbit (Oryctolagus cuniculus) in Australia and Kudzu (Pueraria lobata) in the U.S. Exotic species are usually harmful because they can alter the energy flow of an ecosystem, introduce parasites and diseases, cause genetic pollution by interbreeding with related species, and compete with native species for resources, sometimes driving them to extinction.4

On August 18, Fisheries Service personnel began a two-step process to eliminate the snakeheads. The herbicides Diquat Dibromide and Glyphosate (also known as Rodeo) were applied, causing oxygen levels in the pond to drop, and making likely a subsequent fish kill (see Washington Post news story). Two weeks after the initial application, the piscicide Rotenone was used to kill the remaining fish. Dead fish were removed daily, and the site was closely monitored. Within a few weeks, water quality levels returned to normal. Waters of the nearby Little Patuxent River were sampled to make sure that the snakeheads did not escape from the pond and spread further in Maryland. The threat from Crofton had been eliminated. 5

Snakeheads in the U.S.

Snakeheads are sold in the U.S. both as food in Asian markets and as pets, being prized for their hardiness and aggressive habits. A six-inch snakehead that costs $7 will eventually eat up to $8 of goldfish a day. Many snakeheads that appear in U.S. waters are thought to be former pets that were released when their owners no longer wanted to feed them.

Reproducing populations of snakeheads have now been discovered in Maryland, California, and Florida. Individual fish have also been caught in Maine, Massachusetts, Rhode Island and Hawaii.

It has been illegal to possess snakeheads in 13 states, but not in Maryland, Virginia, or the District of Columbia. However, it is illegal to release exotic species into Maryland waters. In late August 2002, news accounts reported the states of Maryland and Virginia were considering bans on possession of any snakehead within their borders (see Washington Post news story).

In October, the U.S. Fish and Wildlife Service added snakeheads to the list of injurious species. This listing essentially stopped the importation and interstate transport of all 28 snakehead species in the U.S., a move that has probably prevented thousands of shipments of snakeheads from entering the country. This move was under consideration before the discovery of the Maryland fish.

Snakeheads in the World

Snakeheads are Old World fishes belonging to the order Perciformes, family Channidae. They have protruding lower jaws, long narrow torpedo-shaped bodies that taper in depth toward the tail, long dorsal and anal fins without spines, and long toothed mouths. A snake-like rosette of large scales on the head gives the group their English name. As they mature, the fish change color. For instance, younger fish may be gold-tinted brown to pale gray in color, while older fish are generally dark brown with large black blotches. Some snakeheads have a distinctive eye-spot or ocellus, a black spot rimmed with orange, near the base of the tail fin. Most species occur in flowing waters with a few found in swamps, ponds, and ditches. All species are air breathers and can tolerate waters with low oxygen concentrations. Two larger species, Channa marulius and C. maruloides, bear a superficial resemblance to the North American Bowfin (Amia calva), but the position of the pelvic fins is further forward in the snakeheads, their anal fins are elongated, and their heads have snake-like scales.

Within the family Channidae, 25 of the 28 species are classified in the genus Channa, and are found in southern and eastern Asia. The three other species are African and are in the genus Parachanna.6 The genus name Ophicephalus (or Ophiocephalus) is sometimes used for snakeheads other than Channa striata.

The Northern Snakehead spawns in June and July in its native territory; tropical species are capable of spawning throughout the year. Snakeheads exhibit parental behavior; some species guard the young while two species brood the eggs in the mouth. The young initially feed on zooplankton, but switch to larger food as they mature, such as crustaceans and insects. Adults prefer to feed on fish, though they also eat frogs, crustaceans, and even small reptiles, birds, and mammals. Plants and phytoplankton seem to be ingested incidentally.

Snakeheads are used in rice-fish farming both as food sources for farmers and as predators of rice pests.7 Interestingly, other species used in rice-fish farming systems in Southeast Asia are the catfish Clarias macrocephalus and C. batrachus (the so-called walking catfish that has spread over the state of Florida), which also are capable of lying buried in mud for extended periods of time and of moving overland when conditions become unfavorable. The species of snakehead known to disperse overland live in tropical regions where there are seasonal wet and dry (monsoon) conditions.

Several species have been found in the United States and other regions outside the deep tropics (English names are from FishBase and may differ from those used in other sources):

- C. argus, the Northern Snakehead, can grow up to 1.2 m (47 inches) and 6.8 kg (15 pounds). Specimens have been found in California (in 1997), Florida (2000), and Massachusetts (2001). The subspecies C. argus argus is common in China and Korea. It can remain out of water for up to four days at air temperatures of 10 to 15 degrees Celsius (50 to 59 degrees Fahrenheit) and from latitudes 25 to 54 degrees North. They can live in waters that are covered in ice, so they are capable of surviving winters in Maryland. The species matures at the age of 2 years and at a length of 30 cm. In its native range it spawns in June and July and is extremely fertile; females can lay as many as 100,000 eggs a year. Northern Snakeheads hunt mainly in early and late daylight hours, lying in wait for prey on the bottom of a pond or stream. In 1923 it was introduced to Japan, where it has established a self-reproducing population causing ecological effects. The related subspecies, the Amur Snakehead, C. argus warpachowski, is found to the north in the Amur River of Russia and China. In the 1960s it was introduced into the Aral Sea basin and has become widespread in that basin's rivers.8

- C. micropeltes, the Giant Snakehead of mainland Southeast Asia and Indonesia, can reach 130 cm (51 inches) and 20 kg (44 pounds) in size. There are three verified occurrences from New England, in Maine (1976), Massachusetts (1990), and Rhode Island (1968). These are thought to have been aquarium releases. It prefers temperatures from 25 to 28 degrees C so it is not likely to survive in the U.S., and it frequents deep, slow-flowing waters such as large streams and canals. It is known as Red Snakehead in the pet trade because the juveniles are considered to have an attractive red color.9

- C. striata, the Snakehead Murrel (also known as Chevron Snakhead), is a native of tropical Asia that was introduced in Oahu, Hawaii before 1900. This introduction is thought to be a deliberate release of live fish to provide a food source for immigrants. FishBase lists introductions in Madagascar, Mauritius, Papua New Guinea, Indonesia, Malaysia, and the Philippines, and indicates "probably yes" for ecological impact in Madagascar, with no information from other countries. The species can reach 1 m in length; a 57 cm fish had a mass of 3 kg. It breeds in sluggish waters such as rice fields. It can survive the dry season, as long as its skin and breathing mechanisms remain moist, by burrowing in mud. It prepares a nest by biting vegetation from a circular area in shallow depths near shore. It is normally sold live at fish markets and can remain alive, half-butchered, for extended periods of time.10

- C. asiatica, the Small Snakehead, has been proposed as an addition to the Noxious Aquatic Species List of the state of Victoria, Australia. This species is a native of southern Asia and has a preferred temperature range of 22 to 28 degrees C. Fish can breed after nine months, when they are only 21 cm (8 inches) long. Incubation lasts just three days.11

- C. marulius, the Great Snakehead, is another tropical species that ranges through South and Southeast Asia from India to China, and is now established in Florida. Its large size, up to 183 cm and 30 kg, permits it to eat aquatic birds and snakes.12

A Native Fish Resembling Snakeheads: the Bowfin

Snakeheads, especially the two large species Channa marulius and C. maruloides (a southeast Asian species not yet found in North America) bear a superficial resemblence to the native North American Bowfin or mudfish, Amia calva.

Channa argus argus
Channa argus argus
Source: Dr. Ik-Soo Kim, FishBase

Amia calva
Amia calva
Source: Native Fish Conservancy

Bowfins have evenly-matched jaws, a fairly tubular body shape, long dorsal but small anal fins, and reach a maximum length of about 90 cm (36 inches). They are dark green above, lighter on the sides with dark net-like markings (reticulated pattern), and creamy to greenish on the belly (venter). The head is brown or yellow with darker markings, and the lower fins are quite green. At the base of the tail males have a black spot with an obvious yellow or orange border. The Bowfin is a hardy species that lives in swampy, vegetated, and stagnant waters and, like the snakehead, can breathe oxygen directly from the air. It feeds voraciously on fish, crayfish, vegetation, and insects. Through evolutionary convergence these unrelated fish resemble each other and occupy a similar niche. The Bowfin occurs in much of the Mississippi River drainage as well as Atlantic coast drainages north through the Chesapeake Bay basin. In addition, it is found in parts of the Great Lakes basin.

The Future

The presence of Northern Snakehead in North America is of great concern. It is a hardy, predaceous fish with few predators, can disperse widely, and is adapted to a wide range of temperatures and environmental conditions. Once established, the species would be virtually impossible to eradicate from the continent's fresh waters, and could adversely impact indigenous aquatic species. If the Crofton fishes are all killed, and snakehead importation and transport to and inside the USA is curtailed, a significant problem could be averted.

Update: Return of the Snakehead

On April 28, 2004, less than two years after the eradication of the snakehead infestation in Crofton pond, a single snakehead was found in Pine Lake in Marylands Wheaton Regional Park. Like Crofton pond, this was a relatively small, isolated body of water, and it was quickly drained, with no additional snakeheads found. This is not the end of the story, however. On May 7, an angler in Little Hunting Creek, a Virginia tributary of the Potomac, found an immature female snakehead; this was followed, a week later, by a similar catch in the Potomac proper. As of July 9th, a total of fourteen have been caught in the Potomac, with lengths of up to 18 inches, including a sexually mature female ready to lay eggs. For a complete inventory, see: It seems likely that a breeding population has been established, but if it has it will be virtually impossible to eliminate. Explains Florida fisheries scientist Kelly Gestring, once a snakehead population gets established, "neither man nor nature can get rid of them. We have to deal with that fact and develop management strategies."13

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