The fishing boat crept through the warm green water, past sprigs of hydrilla and milfoil, into a lush shoreline nook of the Pohick Bay.
The depth finder ticked down -- four, three, two feet. A metal claw hung out from the bow, trailing its stainless steel tendrils in the slack water. Two men looked into the murk, holding their nets, poised to pounce.
"If I were a snakehead, this would be a perfect spot," said John Odenkirk, a Virginia fisheries biologist. "A little cove, a lot of grasses. It's protected."
He stepped on a pedal and sent six amps of electricity pulsing through the steel wires into the water. Stunned fish suddenly rose to the water's surface. White perch, long-nosed gar and common carp, among others, flopped onto their sides or floated belly-up, easy prey for the nets. In all, more than 200 fish of 21 species were scooped up in nets, measured and tossed back into the tidal waters of the Potomac River one morning last week.
None was a snakehead.
The electro-fishing technique, used for years to survey fish populations in lakes and rivers, is now being used to search the Potomac and its tributaries for the northern snakehead, the notorious Asian import that can breathe air, wriggle over land and devour scores of native fish.
The four northern snakeheads found in the Potomac and its tributaries in the past few weeks have raised concerns that the predatory fish could disrupt the ecosystem and harm fish populations in the river.
Scientists are intent on finding out whether there is a reproducing population or simply a limited number of individuals.
"Right now, it's just pretty much poking around blind," said Steve Early, an assistant director at the Maryland Department of Natural Resources. "The biologist part of me says . . . 'There's got to be more.' I want to find out as quickly as I can."
Maryland and Virginia officials plan to deploy gill nets and haul seines on the Potomac starting this week to assess the snakehead population, as well as continue electro-fishing surveys. The nonlethal shocking method, which runs off a 5,000-watt generator in Odenkirk's boat, is intended to temporarily immobilize the fish's muscles, to "maximize capture efficiency and minimize injury," he said.
It is a powerful enough jolt that one of his colleagues was nearly knocked out cold a few years ago by an accidental blast of electricity as he stood near the boat.
"He got laid out flat. He was stiff as a fish," he said.
The hunt for snakeheads last week focused on the shallow shoreline waters of the bay and nearby Pohick Creek, within a mile of Little Hunting Creek, where the first Potomac snakehead was caught May 7. Scientists believe the fish prefers shallow water amid shoreline vegetation. In its native China, the snakehead is believed to prepare its nest by clearing a circular area in the grasses about one meter in diameter, then building a cone-shaped structure and laying its eggs on top, said Steve Owens, a fisheries biologist who accompanied Odenkirk on the electro-fishing outing.
"But there's not a lot of information out there that pertains to the Western Hemisphere," he said. "We're assuming that's how they would interact with the environment here, but we really don't know."
The lack of knowledge about the snakehead, or what it could do to the Potomac habitat, prompted a sense of alarm that some officials say may be unwarranted. In South Florida, where the bull's-eye snakehead was found four years ago, surveys of fish populations show that the intruder did not wipe out native species, said Paul Shafland, director of the nonnative fish research laboratory of the Florida Fish and Wildlife Conservation Commission.
"There's no evidence that [snakeheads] decimate fish communities and then walk to another fish population," he said. "Are they problematic? Most certainly . . . but they're more of an unknown entity. And the fact that the impact is unknown does not mean it's inherently satanic."
Odenkirk, who works for the Virginia Department of Game and Inland Fisheries, described the local reaction to the arrival of snakeheads as "hysteria." Since environmental authorities canvassed marinas and public boat launches along the river this month telling fishermen to report any snakeheads they might find, they have been inundated with calls from anglers.
Usually the suspected snakehead turns out to be hake or lamprey. Odenkirk recently drove to meet one man who was convinced he had snagged the real thing.
"I took one look inside the brown paper bag and walked away. I told him, 'It's an eel,' and he just started crying," he said. "It's crazy."
In the Potomac, many nonnative species of fish and plants have established populations. A frequent sighting on Wednesday's trip was the common carp, a large exotic fish cultivated in Europe and Asia that was intentionally introduced in North American waters in the 19th century to be a sport fish.
The channel catfish, blue catfish and, most recently, the flathead catfish have also infiltrated the Potomac. Odenkirk said they are changing the population makeup of local waters, but some fishermen enjoy the opportunity to hook a 50-pound fish. Even the hydrilla, the rapidly growing invasive aquatic weed that sparked widespread concern in the early 1980s, turned out to have some ecological benefits, he said, including preventing erosion, cleaning the water and providing habitat for fish and waterfowl.
"The snakeheads are disconcerting, but I'm not ready to push the panic button," he said. "And I hear they're good to eat."
Nevertheless, the campaign to determine the extent of the snakehead's presence in the Potomac is moving forward quickly. A snakehead task force has been established to coordinate efforts between state and federal authorities.
And fishermen are doing their part, finding more snakeheads than the scientists have turned up.
On Thursday, in the same spot the electro-fishing boat patrolled a day earlier, a commercial fisherman snagged a northern snakehead, the fourth specimen caught in an eight-mile stretch of the Potomac within the past month.