The scene is a sheriff’s office near a mountain lake, where a hunter and his dog have been found dead. The sheriff sets a bright orange hunting vest on his desk in front of an anxious woman. She nods, identifying it as her husband’s. “He loved that dog,” she says, crying.

“Listen, Norma,” the sheriff says. “If there’s anything at all that I can do, you tell me.”

“You can find the animal that did this and send it straight to hell. You can do that.”

The culprit in the Sci Fi Channel’s made-for-TV movie Snakehead Terror turns out to be a lakeful of monster fish. This star turn is fitting for the toothy “Frankenfish” that has generated many hair-raising newspaper and television news stories—the northern snakehead.

In addition to inspiring filmmakers, the snakehead’s appearance in North American waters in the past few years has worried wildlife biologists and commercial and sport fishermen. They fear that it will invade new rivers, multiply rampantly and edge out other species.

The northern snakehead is native to Asia and is one of 29 snakehead species. It made its national news debut in 2002, after an angler at a pond behind a strip mall in Crofton, Maryland, caught a long, skinny fish, about 18 inches from end to end, that neither he nor his fishing buddy recognized. They photographed the fish before throwing it back; a month later, one of them took the picture to the Maryland Department of Natural Resources (DNR). An agency biologist e-mailed the picture to fish experts, who told Maryland it had a snakehead on its hands.

It was after another angler caught a snakehead in the same pond and netted some babies that all hell broke loose. National newspaper and TV news reports described snakeheads as vicious predators that would eat every fish in a pond, then waddle across land to another body of water and clean it out. A reporter from the Baltimore Sun called it “a companion for the Creature from the Black Lagoon.” The scariest reports, fortunately, turned out to be mistaken. While some species of snakeheads can indeed wriggle long distances across the ground, the northern snakehead—the only species found in the Crofton pond—appears not to be one of them. But northern snakeheads do like to eat other fish, and a heavy rain could conceivably wash one or more from the pond into a nearby river that runs through a National Wildlife Refuge and into the Chesapeake Bay, the largest estuary in North America. To eliminate the snakehead menace, Maryland wildlife officials dumped the pesticide rotenone into the Crofton pond, killing all of its fish. Six adult snakeheads went belly up—as did more than 1,000 juveniles. Problem solved. Or so it appeared.

Two years later, northern snakeheads fulfilled biologists’ worst fear and showed up in the Potomac River. Experts worried that snakeheads in the Potomac, by eating other fish or out-competing them for food, could drive down numbers of more desirable species, such as shad or largemouth bass. You can dump poison in a little, enclosed pond, but you can’t poison the Potomac. It’s a wide, shallow river that originates in West Virginia and runs 380 miles before emptying into the Chesapeake. The bay fuels the region’s economy through recreation and fishing. Snakeheads couldn’t survive in the mildly salty water of the bay, but they could scarf down shad, fish that spawn in the Potomac and other freshwater tributaries. Millions of dollars have already been spent on fish stocking, dam modifications and other projects to help the shad, which used to be plentiful enough to support a commercial fishery in the bay.

Besides Crofton and the Potomac, the fish have popped up in several other places in the United States. In 1997, one was caught in a Southern California lake. A couple more appeared in Florida waters in 2000. In Massachusetts, one was caught in 2001 and a second in 2004. And in July 2004, an angler caught two in a lake in a Philadelphia park. Like the Crofton fish, the Philadelphia ones had settled in and started reproducing. But unlike the Crofton fish, they had access to a river—the Schuylkill, which feeds into the Delaware. Moreover, tidal gates that normally keep fish in the park had been stuck open for two years. Philadelphia fisheries managers decided that poisoning or draining the park’s interconnected ponds would cause more harm to resident fish than the snakeheads would, and have resigned themselves to snakeheads becoming a new member of the park’s ecosystem. The most recent surprise appearance was this past October when a northern snakehead was pulled out of Lake Michigan. The catch has raised fears that the voracious predator might take over the Great Lakes.

The northern snakehead, which is native to parts of China, far eastern Russia and the Korean peninsula, may seem plug-ugly to the undiscerning eye—it has big, pointy teeth and, given its particularly heavy mucus covering, a slime problem. It can grow up to five feet long. Like its reptilian namesake, it’s long and slender and can sport blotchy snakelike patterns on its skin. Unlike most fish, the northern snakehead has little sacs above its gills that function almost like lungs; the fish can surface and suck air into the sacs, then draw oxygen from the stored air as it swims. The air sacs are handy for surviving in waters that are low in oxygen, and even allow the fish to survive out of water for a couple of days, as long as it doesn’t dry out. A female lays thousands of eggs at a time, and both parents guard their offspring in a large nest they make in a clearing of aquatic plants.

Northern snakeheads are a popular food in their native range; they’re said to be good eating, particularly in watercress soup, if a bit bony. They’re fished commercially and raised in fish farms in Asia. They’ve also been sold live in markets in the United States. The Crofton snakeheads were eventually traced to a Maryland man who’d bought two of the fish in New York City for his sister to eat. When she demurred, he kept them in his aquarium and later released them. The U.S. Fish and Wildlife Service soon banned the importation and interstate transport of snakeheads, a plan that had already been in the works precisely because of fears that some snakehead species could thrive in parks, rivers and lakes if they got loose. The ban made it illegal to import all live snakehead species, including the colorful tropical species that populate the odd aquarium. Virginia has outlawed the possession of all snakeheads.

But the bans haven’t stopped everyone. A Los Angeles grocer was arrested this past May for allegedly smuggling live northern snakeheads into the country from Korea and selling them in his store; he pleaded guilty to importing an injurious species. U.S. fans of snakehead soup and other delicacies, however, may still legally obtain killed, frozen snakeheads, which are available in many of the Asian markets that once sold them live.

One day this past April, an angler caught a feisty northern snakehead in Pine Lake, in Wheaton, Maryland, outside Washington, D.C. Local officials drained the lake but found no more snakeheads. Then, like an ecological game of Whac-a-Mole, another northern snakehead reared its toothy head the very next week when a professional bass fisherman pulled a 12 1/2-incher from Little Hunting Creek, a Potomac tributary in Virginia about 15 miles south of the nation’s capital. Biologists tried using nets to capture snakeheads in the river, but eventually decided that a better way would be to let anglers go at the fish with plain old hooks and lines—which led to one of the odder fishing tournaments in recent memory.

On an overcast Friday morning in July, I joined a few dozen anglers at Columbia Island Marina in Arlington, Virginia, across a narrow channel from the Pentagon. The 2004 Snakehead Roundup was about to get under way. The roundup was sponsored by the Marina Operators Association of America to remind boat owners to take care not to transport unwanted species from one place to another—as hitchhikers on their boats or trailers, for example—and to let them know what northern snakeheads look like. Although 16 adult snakeheads had been caught in the Potomac by that time, no one knew whether they’d been born there or whether someone had just tossed them in—or even how common they were.

I tagged along in a 19-foot white-and-blue ski boat with three managers from a family-owned company whose boss didn’t seem to mind that the information technology division was running itself that day. “We’re conducting an offsite meeting,” software designer Brian Turnbull explained. Turnbull’s father-in-law, who is Vietnamese, asked him to bring a snakehead home. “He says if you catch one, you don’t have to hand it over to the state. It’s a delicacy.” Fortunately, Turnbull wasn’t required to choose between duty to family or to society because he didn’t catch a snakehead. Neither did anyone else on the boat, and neither, we found out when we later pulled up at the marina, did anyone else in the roundup.

A few weeks later, John Odenkirk, a biologist from the Virginia Department of Game and Inland Fisheries, seemed to be imitating the sheriff in Snakehead Terror, who kills his murderous lakeful of snakeheads by electrocuting them with a downed power line. Odenkirk, driving an aluminum boat through Dogue Creek, a Potomac tributary, was “electrofishing,” which involved running about 1,000 volts through a boom that protruded from the bow and trailed wires in the water like tentacles. “High voltage . . . The next best thing to explosives,” read the small print on the back of Odenkirk’s green “Snakehead Task Force” T-shirt, which he designed to sell to colleagues for $12 apiece.

Electrofishing, a common sampling method in fisheries research, isn’t meant to kill fish. But it may knock them out for a while. (It’s not considered sporting and requires a special permit.) Odenkirk nosed the boat in and out of the empty slips at the Mount Vernon Yacht Club a couple of miles downriver from Little Hunting Creek. Tiny fish leapt out of the water as others lolled gracelessly on their backs, stunned, just below the surface. Biologist Steve Owens and technician Scott Herrmann leaned over the bow clutching long-handled nets. Afish’s response to the electrical current depends on its skeletal structure, scales, size and how close it is to the wires. “Snakeheads are—they’re kind of bad-asses,” Odenkirk said. “They don’t like the juice and they try to avoid it.” Still, a snakehead that got close to the trailing wires would be stunned and surface, for Herrmann or Owens to snag. At least, that was the theory. We sped back up the Potomac past Mount Vernon to Little Hunting Creek, where the first Potomac snakehead was caught by a fisherman back in May. At the end of an hour and a half of electrofishing, the catch included many carp, several species of catfish, a bunch of goldfish, a long-nosed gar, a turtle— and zero snakeheads. Odenkirk said he’s always conflicted after an unsuccessful day of snakehead fishing. On the one hand, he said, he was disappointed he’d failed to catch one. On the other, “you’d be happy if you never saw one again.”

Though we didn’t see any snakeheads that day, Odenkirk says he’s sure the fish is established in the Potomac or soon will be. “It’s just not even an option that we’ve caught them all.” He says the fish probably nest in wide, shallow expanses of lily pads and wetlands. “We just can’t get back in those areas.”

But other officials say they’re not convinced the fish are here to stay. Steve Early, assistant director in the fisheries service at the DNR, worked on the Crofton pond in 2002 and has handled some of the Potomac snakeheads. He thinks the fish were only very recently dumped in the river, perhaps after Virginia’s 2002 ban on snakehead ownership. He points out that most of the snakeheads caught this year have been 2 to 6 years old, and that if they’d been living in the Potomac for years, surely someone would have caught one before. Early remained unpersuaded even after a baby snakehead was found in a Potomac tributary this past September. It was the 20th northern snakehead caught in the Potomac watershed, and the first juvenile. “Well, it’s not good news,” he says of the discovery, but points out that if some snakeheads do manage to reproduce, they may never thrive in the big river. Their future also depends on whether other fish in the Potomac develop a taste for snakehead fry.

For now, scientists are working on figuring out how the adults got there. It’s a critical question—if the fish were just recently dumped in the river, there’s a chance they’ll die without having generated a self-sustaining population—but it will require more than a rod and reel or a stun gun to answer.

Behind a door at the National Museum of Natural History in Washington, D.C. rest specimens from the world’s largest fish collection. Smithsonian ichthyologist Thomas Orrell walked down an aisle between rows of gray metal shelves containing jars with labels such as “China 1924.” Orrell held up a jar marked Channa argus, the northern snakehead. “They’re really beautiful fish,” he said.

Orrell is trying to learn if the northern snakeheads caught this past summer in the Potomac were born there. He’s analyzing DNA from 16 fish; if some of the Potomac specimens are closely related, it’s likely that the fish bred in the river. If they’re not kin, they were likely dumped in the river. Orrell is also comparing the DNA of Potomac fish with that of those caught in the Crofton pond, testing the idea that someone might have captured juveniles before the pond was poisoned and released them in the Potomac.

Orrell led me down a bare stairwell into the museum’s basement, past sandbags piled near an entrance in case of heavy rain and a walk-in freezer that smelled of long-dead fish, containing, among other things, an enormous tuna frozen since the 1960s. He lifted the top of a nearby freezer chest, rooted around and pulled out a long, black lump. “Watch out for flying debris,” he said, unwrapping a black garbage bag and scattering pieces of frozen blood. Inside was one of the most recent Potomac catches: a dark, diamond-patterned snakehead more than a foot long, now solid as a rock. After showing it off, Orrell shrugged, wrapped it up, laid it back in the freezer and washed his hands. He already knows whether the snakeheads are reproducing in the Potomac, but he isn’t telling; adhering to scientific protocol, Orrell declines to share his data until they’ve been reviewed by other experts and published in a scientific journal.

If northern snakeheads do have some ecological impact in the Potomac, largemouth bass are likely to suffer, says U.S. Geological Survey fishery biologist Walter Courtenay, who in 2002 wrote a snakehead risk assessment for the agency. The two species have similar habitats and would probably eat each other’s young. Capt. Steve Chaconas, one of only a few full-time fishing guides on the Potomac, does not like snakeheads one bit. “Of course, I’m worried about what potential it could have to impact the fishery,” he says. “Also because I’m a businessperson and my business relies entirely on people coming here to fish.” Even now, he says, customers ask how much the snakeheads have hurt fishing. It’s hard to estimate the extent of the snakehead’s impact on largemouth bass and other Potomac species. The northern snakehead was introduced to rivers in Japan in the early 20th century, but there has been little study of its ecological effects there. (The largemouth bass, native to North America, was introduced to Japanese waters in 1925 and is reportedly terrorizing native fish and snakeheads alike.)

In southern Florida, a close relative of the northern snakehead, the bullseye or cobra snakehead, has been living for a few years in the canals of BrowardCounty. The fish, which is native to rivers in South Asia and Southeast Asia, can grow to four feet or longer, but there are not yet enough data to know what effect the bullseye snakehead has had or will have on Florida ecology. Courtenay says the fish probably first got into Florida waters through ritual animal release, a common practice in East Asia that some immigrants have continued in their new land. (A study conducted in Taiwan in the 1990s, for instance, found that 30 percent of Taipei citizens— most of them Buddhists—had released animals as part of a prayer.)

Florida is home to dozens of introduced fish. Paul Shafland, a fisheries scientist with the Florida Fish and Wildlife Conservation Commission, has worked with invasive fish for 30 years, but he isn’t as troubled by them as most biologists. “We have philosophically, largely determined that exotics are inherently bad, and that’s fine,” he says. But, he adds, some introduced fish might fill up some part of the food web that was previously unoccupied.

In fact, introduced fish are just about everywhere. Rainbow trout, native to the western United States, have been transplanted into cold waters all over the Midwest and East. In the Great Smoky MountainsNational Park, on the border between Tennessee and North Carolina, rainbows have taken over at least 70 percent of the native brook trout’s territory since the 1930s. In the late 1960s, the walking catfish, an Asian species that really can move over land, escaped into the Florida wild. They’ve walked their way into warm waters throughout the southern half of the state, without causing major damage so far, Shafland says.

Lake Michigan, says Philip Willink, an ichthyologist at Chicago’s FieldMuseum, is also infested with nonnative fish. “Out of eight species of salmon here, six are introduced,” Willink says. But, as in the Potomac, some native fish still hang on in the lake, and he says it’s worth fighting new invasions. “We’re just trying to preserve what is left, because once it’s gone, it’s gone.” Since the Lake Michigan snakehead was found in a fairly deep harbor with little vegetation—an unlikely snakehead habitat—Willink surmises that the fish was probably just tossed into the water. Scientists did some electrofishing in the harbor to look for more snakeheads but didn’t turn up any.

The Potomac is not the river it was when George Washington looked upon it from Mount Vernon and made good money selling native shad. Goldfish, carp, channel catfish—none is native to the river. The now-widespread common carp, brought over from Europe, was put in the river in the late 1800s. Carp stir up a riverbed and make the water too cloudy for some other fish. The largemouth bass, native to other U.S. rivers, was introduced into the Potomac in the 1800s. And the blue catfish, a sharp-spined transplant from the Mississippi River basin that arrived in the Potomac late in the 20th century, is a headache for fishery managers now, who fear it could interfere with the commercial fishing of channel catfish—which were introduced from the Mississippi basin decades earlier. Descendants of released pet goldfish flourish in the Potomac, as they do virtually all over the world. But the other introduced species aren’t the point; the native fish are, says Dan Simberloff, an ecologist at the University of Tennessee, Knoxville. If the snakehead is different enough from the predators that natives have evolved with, it might drive some natives to extinction. It’s hard to predict what will happen, though. “Most invasive species don’t cause a huge amount of trouble, but some fraction of them do, and we haven’t been too good at predicting that,” he says. About the snakehead, he says, “I’m not optimistic.”

Cliff Magnus is a semiprofessional fisherman (he says he’s been sponsored for the past ten years by “Team Spouse,” a.k.a. his wife, a lawyer), but the attention he got from catching a snakehead last June in a Potomac tributary has brought him sponsors willing to pay his entry fees for bass tournaments. Magnus may have witnessed a seminal moment. He says he saw two northern snakeheads swimming around each other in Little Hunting Creek in late July. The fish were chasing and nipping at each other. The scene, which he described to some fisheries biologists, wasn’t exactly the makings of a Snakehead Terror sequel, but it was definitely ominous. The way the biologists see it, the fish were getting ready to spawn.

There Goes the Neighborhood

If the northern snakehead does establish itself in the United States, it’ll join a rogue’s gallery of introduced species that threaten native plants and animals in and around water. Clockwise from top: A denizen of the East, the bullfrog now inhabits the West, where it eats more delicate local frogs. Nutria, imported from South America as a source of fur, devour plant roots in marshes along the Gulf of Mexico and the Chesapeake. Asian zebra mussels, accidentally carried into U.S. waters aboard ships, have caused more than $1 billion in damage to pipes in the Great Lakes and Mississippi River basin since 1988. Amazonian water hyacinths clog Southern waterways. Aggressive mute swans from Eurasia take over native waterfowl’s feeding and nesting territories on the East Coast and Great Lakes.