Snakeheads Invade the United States
Technical Support Specialist, Live Materials
The import and sale of live, nonnative organisms supports the food, bait, aquaculture, sportfishing, and horticulture industries in the United States. Nonnative organisms are species found living outside of their native range. Sometimes a small number of these organisms escape or are intentionally released from captivity, survive, and establish breeding populations.
In recent years, populations of nonnative species have become more common, and some are cause for ecological concern. Controlling and repairing the damage they cause can cost billions of dollars each year in the United States alone. This is due to lost crops, livestock, timber, fisheries, and other resources, as well as spreading and introducing diseases and causing damage to property. The northern snakehead (Channa argus) is a nonnative species of concern for US fisheries biologists, ecologists, and anglers alike.
Under the Lacey Act, importation and transport of species determined to be injurious to human health, agriculture, horticulture or forestry, and wildlife resources is prohibited. The US Fish and Wildlife Service oversees and updates the list of injurious species. Over 2 dozen varieties of snakehead, including the northern, were added to the prohibited list in 2002, effectively stopping import and sale of these fish in the United States. Species identified by the Lacey Act may not be imported or transported between states without a permit issued by the US Fish and Wildlife Service.
Snakeheads (family Channidae) are a group of air-breathing freshwater fishes represented by 2 genera. The genus Channa includes species native to Asia, while the genus Parachanna includes species native to Africa. The 2 genera differ mainly in the morphology of the air-breathing (suprabranchial) organ. The northern snakehead discussed here originates in the rivers and estuaries of China, Russia, and Korea.
The name snakehead describes the anatomical features common to fishes in the family Channidae. These features include elongated, cylindrical bodies and flattened heads. Many species also display large scales on their heads, akin to the large scales also present on the heads of snakes.
Northern snakeheads are identified by long dorsal and anal fins; pelvic fins located beneath the pectorals; and a large mouth reaching beyond the eyes with large, sharp teeth on the upper and lower jaws. The coloration of these fish varies based on the age of specimen, but adults appear golden tan to pale brown with dark, irregular patches on the sides and a saddle-like brown patch along the back. The maximum length is approximately 85 cm.
Northern snakeheads reach sexual maturity at 2 to 3 years of age and can produce young 1 to 5 times per year, releasing 20,000 to 50,000 eggs at a time. Parents guard the young until they have absorbed their yolk sacs. Juvenile fish eat small aquatic invertebrates and zooplankton. Adults typically feed on fishes, frogs, crustaceans, and aquatic insects. Snakeheads are ambush predators, meaning they hide and attack prey by surprise.
Temperature is the most important factor that determines the northern snakehead’s ability to expand in range. In general, snakeheads are a tropical fish, but the northern can survive in waters ranging in temperatures from 0 to 30° C, which would allow it to survive conditions present in any body of water in the United States.
The northern snakehead prefers stagnant water with mud or aquatic vegetation. Specimens are often found in slow, muddy streams. Like other freshwater fish, snakeheads have limited tolerance for salt water concentrations in excess of 10 parts per thousand, with juveniles showing more tolerance than adults at colder temperatures.
Introduction to the United States
In many cultures, consuming snakehead is believed to have healing properties due to the fish’s ability to survive out of water. For this reason, snakeheads are cultivated or wild caught and sold as a food item in many Southeast Asian and African countries. In those countries, the fish are kept alive until they are prepared for consumption.
Until the amendment of the Lacey Act in 2002, which banned the sale or possession of live snakehead, snakehead could be purchased live at fish markets in any major US city. Fisheries biologists speculate that wild populations of northern snakeheads were originally brought into the country to be sold in fish markets, and were then deliberately or accidently released into local waterways.
Contain and control campaigns
Northern snakehead have been recorded in open waters of the United States. Reports of live capture have been submitted from California, Florida, Arkansas, Massachusetts, North Carolina, and Maryland. Breeding populations have been documented in Virginia, Maryland, Pennsylvania, New York, and Arkansas.
The first breeding population was discovered in Maryland in 2002. It was traced back to a single introduction of 2 fish purchased at a fish market and released into a series of stormwater retention ponds in Crofton, Maryland. Mitochondrial DNA studies of the specimens taken from the site revealed that the fish were offspring of a single breeding pair.
In September of 2002, the Maryland Department of Natural Resources applied rotenone, a chemical used in fisheries management to eradicate unwanted species from a body of water, to the 3 ponds in which snakeheads were captured. In spite of the eradication effort, there is evidence of a well-established population in the Potomac River and several of its tributaries in Virginia and Maryland. In addition to the Potomac River population, breeding populations have also been established in the Rappahannock River and several of its tributaries in central Virginia.
Some snakehead species are able to travel overland for short distances using wriggling motions. This behavior, however, has not been observed in adult northern snakeheads. There is little discussion in the scientific literature at this time regarding the specific impacts of the snakehead introduction on the ecology of the rivers of Virginia, Maryland, and other states where they have been captured.
The most pressing concern for fisheries biologists is the impact that the snakehead’s predatory nature will have on endangered and threatened species within the ecosystem. Because this fish preys upon a wide range of species, some biologists are concerned that there is significant dietary overlap between it and the largemouth bass, an important sport and game fish for much of the Potomac. However, further research into the magnitude of this impact is needed before any management decisions are finalized.
Ecologists are concerned with the potential transmission of parasitic infections and diseases from the snakehead that could impact native fish populations. One disease that has received a large amount of attention is epizootic ulcerative syndrome (EUS), which causes high mortality in snakeheads and other fishes it infects. Snakeheads have also been implicated as an intermediate host of the parasitic nematode that causes gnathostomiasis. This disease is transmitted to humans through consumption of raw or incompletely cooked infected flesh.