Maryland Wages War on Invasive Walking Fish

Hillary Mayell
for National Geographic News
July 2, 2002
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An angler caught an air-breathing, land-crawling, voracious predator this past weekend in a pond in Crofton, Maryland.

The good news is that the fish, a northern snakehead that has been targeted by biologists for the last several weeks, was caught. The bad news is that it was 26 inches (66 centimeters) long; the fish caught in mid-May that alerted wildlife officers to the possibility of an invasion by an alien species was only 20 inches (51 centimeters) long.

"Either the fish grew six inches in a few weeks or we have more than one in the pond," said Bob Lunsford, a biologist with the Maryland Department of Natural Resources.

The knowledge, with the weekend catch, that there is more than one fish in the Crofton pond is what is keeping biologists awake at night.

"Our biggest fear is that there are more than one and they'll reproduce," said Lunsford.

A second fear, based on the fish's ability to breathe out of water and travel across land, is that the snakehead could leave the pond and travel the 75 feet (23 meters) or so to the Little Patuxent River, and from there invade the state's river system.

Battling Alien Species

The northern snakehead's arrival in Maryland is only the latest in a long list of invasions by alien species around the world.

Alien species—plants and animals that have become established outside of their natural range as the result of human activity—pose a huge threat to the biodiversity and health of an ecosystem. Once established, the alien species can eat the native species or compete with them for habitat, food, or both. Lacking natural predators in their new environment, the invaders can drive natives to extinction, drastically degrade ecosystems, and cost billions of dollars a year in eradication efforts.

Businesses also suffer. The U.S. government estimates the cotton boll weevil, an exotic insect, has cost the cotton industry $13 billion since its arrival, and that over a ten-year period, pipe-clogging masses of zebra mussels have cost the utility industry $3 billion.

Similar ecological disasters have occurred in other parts of the world too. An invasion of the Black Sea by the comb jelly led to the collapse of the anchovy fishery, estimated to be worth $250 million a year. [See sidebar.]

"Once an alien species establishes itself they're impossible to get rid of," said Paul Shafland, director of the Florida Fish and Wildlife Conservation's Non-Native Fish Lab.

Maryland's Snakehead Saga

An angler fishing in a pond in Crofton, Maryland, east of Washington, D.C., first caught the fish in mid-May. Unable to identify it, he took photographs before throwing it back in the water. Biologists at the state's Department of Natural Resources, working with other experts, identified the fish as a northern snakehead. They immediately began planning strategies for ridding the pond of the creature.

Thus far, sandbags, electroshock equipment, traps, and hordes of anglers have been recruited to capture the alien fish. Wanted posters alert anglers to cut and bleed the fish if they catch it, since it can live on land for several days at least.

"We want this fish dead," said Lunsford. "No question about it."

"The folks in Maryland are not overreacting one little bit," said Walter Courtenay, professor emeritus of zoology at Florida Atlantic University.

There are 28 species of snakeheads; three are indigenous to equatorial Africa, the other 25 to Asia. The species vary in size and aggressiveness, according to Courtenay, who has been preparing a risk assessment on the snakehead for the U.S. Geological Survey since September.

One species, the bullseye snakehead, has already established itself in the waters of southern Florida, although with relatively little impact so far.

"We have not seen, and we don't anticipate, that the presence of the snakehead in Florida waters will have a catastrophic impact," said Shafland. "But it's like throwing trash out the car window; it can't be good."

Courtenay disagrees with Shafland's assessment of the Florida snakehead's potential impact, but in regard to the species found in Maryland, the question is moot. The northern snakehead is nothing like the species down in Florida, according to Courtenay.

"Ninety percent of the northern snakeheads' diet consists of other fishes," he said. "Their temperature range is between zero to 30 degrees Celsius (32 to 86 degrees Fahrenheit), and it can live under waters that have been iced over. They can grow to almost a full meter in length (more than three feet), and the females lay more eggs per year—in the neighborhood of 100,000 annually—than other species."

The snakehead in Florida is a temperate species, unable to withstand water temperatures below 50 degrees Fahrenheit (10 degrees Celsius).

Some snakehead species are imported into the United States as part of the aquarium trade, although the northern snakehead is not one of them. Snakeheads are considered a food delicacy in Asia, and live fish can frequently be found in Chinese markets. Authorities suspect that the fish in the Crofton pond was purchased in Washington, D.C.'s Chinatown district.

"A snakehead species introduced in Uzbekistan in the early 1960s spread with such rapidity and with such devastating effect on native fish populations that fishermen were able to establish a commercial fishery, turning a negative into a positive," said Courtenay.

Courtenay's report, which will be submitted to the U.S. Fish and Wildlife Service later this summer, will recommend a total ban on importation of live snakeheads.

Educating the Public

Options for catching the fish in the Crofton pond are somewhat limited. Draining the pond would flush all the fish, including the snakehead, into the Little Patuxent River. In addition, the snakehead has the ability to bury itself in the mud for several months, so it could just hide. The lower end of the pond has been sandbagged to prevent the fish from crawling into the Little Patuxent. Electroshock hasn't worked, and can't be tried again until winter when the dense vegetation in the pond has died back, allowing larger pieces of equipment to be operated.

State and local authorities have about a dozen eel pots, some baited traps, and two floating D-traps in the pond, but even if they catch more snakeheads, there is no way of knowing that all of them have been caught.

Courtenay suggests the only option might be poisoning the pond using Rotenone, a plant-derived toxin. "None of the other methods guarantee that you'll get every fish in the pond," he said.

Although possessing a live snakehead is illegal in 13 states; Virginia, Maryland, and the District of Columbia are not among them. It is however, illegal to release non-native fish into Maryland waters, said Lunsford.

"Obviously we need to do more to educate the public about the serious ecological consequences that the illegal release of exotic species represents," said Shafland. "People need to understand that once exotic species are established they're impossible to eliminate and the consequences can be catastrophic. Releasing them into the wild is not humane and it's not smart."

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