It is the pit bull of the aquatic world, an air-breathing, predatory fish from Asia that likes to eat its prey whole and threatens to become a permanent pest in North American waters like the ubiquitous zebra mussel.
Since the first northern snakehead fish turned up in a pond in Maryland two years ago, fisheries officials across the United States have been attempting to stop the spread of a fish that is a favourite in some Asian restaurants and popular among some tropical fish owners.
"The people who keep snakeheads are the same kind of people who like to keep alligators or piranhas. It's a macho thing," said Kevin Farrell, owner of Critters, a pet shop in Bowie, Md.
Maryland fisheries officials, worried about the impact of the snakehead on the ecological balance of the state's waterways, have been trying to wipe out the fish since the first few turned up in Crofton Pond in 2002.
"Wanted posters" picturing the fish -- an ugly specimen with a narrow head, protruding jaw, large teeth and broad mouth -- have been circulated around the state, asking fishermen who come across the fish to show no mercy.
"Please do not release," the poster says. "Please kill this fish by cutting, bleeding or freezing." Anglers are then asked to report their catch to the authorities.
"They're a threat, but you won't know the magnitude of that threat until it happens," said Steve Early, inland fisheries program director at the Maryland Department of Natural Resources.
The worry is that if snakeheads get a firm foothold in U.S. waterways, they will upset the ecological balance. "They would eat bass. They would compete for habitat and they would eat the same food that bass would eat and they could bring in ailments and parasites," Mr. Early said in an interview.
How the snakeheads, which are native to China and Russia, made it into U.S. rivers and lakes is unclear, but it is believed some were bought from Asian fish markets and subsequently released.
Others may have been freed by tropical fish owners who may have tired of the fish, which can grow to one metre in length and are so powerful they can shatter the glass of their tanks.
Despite a determined response from Maryland officials, who poisoned the pond where the fish were first found, the snakehead is turning up in regional waterways. Seventeen snakeheads have been caught along a 20-kilometre stretch of the Potomac River this year.
Last month, two of the fish were found in a small lake near Philadelphia. Snakeheads have also been found in Florida, North Carolina, California and Massachusetts. None have yet turned up in Canada, where Ontario has banned the sale of snakehead along with several invasive species of carp and goby.
Last year, a House of Commons committee recommended banning the importation of all snakeheads and four kinds of invasive Asian carp but the Fisheries and Oceans Department is still conducting a risk assessment on the impact of a ban. Snakeheads are still being imported, primarily for food.
Importing snakehead into the United States is illegal, as is interstate transport of the fish, but Maryland wants to go one step further and ban ownership of the fish.
The ban has prompted a backlash from snakehead lovers, who say authorities are literally casting their net too wide. They say there are 29 kinds of snakeheads and most are tropical varieties that could never survive a Maryland winter and pose no threat to the ecology.
"They're real friendly. They're not dangerous," said Mark Hresko, who manages House of Tropicals in Glen Burnie, Md.
Snakeheads have an appeal similar to that of piranhas but aren't as bloodthirsty, he said.
While an unwary piranha owner could easily lose a fingertip if he got too close to one at mealtime, snakeheads turn up their noses at humans, preferring fish, frogs and the occasional mouse, usually swallowed whole.
Snakeheads have also developed a reputation for horror-film-like indestructibility. Stories abound about the fish being able to survive for hours, if not days, out of water or being able to cross roads on their extraordinary fins.
"The northern snakehead is not really adapted for that kind of overland movement," said Mr. Early, the Maryland fisheries official. "But it can stay alive out of water for several days if it's kept moist and out of the sun because it can breathe air. Its fin structure is not adapted to walking across land, but it can probably wriggle through mud."
Jim Karanikas vividly remembers the day he turned up for work at his tropical fish store in Gaithersburg, Md., and found several of his snakeheads waiting for him as he entered the shop.
"I had a dozen of them sitting there at the front door," he said, describing how the fish jumped out of their tanks and squirmed across 20 metres of flooring then died.
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