DOGUE CREEK, Va. -- With the boat throttle in one hand and a portable antenna in the other, John Odenkirk zeroes in on the pinging sound in his headphones.
Unseen but definitely heard is one of 20 northern snakeheads fitted with small radio transmitters that emit sounds like sonar in a World War II movie. It is fish No. 0024, which, at about 12 inches, is one of the smaller of the bunch.
Odenkirk, a Virginia Department of Game and Inland Fisheries biologist, adjusts his headphones and leans over the side.
"Right there," he says, grinning and pointing at a pile of branches hard by a dock with cabin cruisers tied to it. "Oh, man, he's blowing my ears off."
Instead of dropping a fishing line over the side, Odenkirk marks down the GPS coordinates, the water depth and temperature, then motors away, listening intently for another ping.
Faced with a congressional mandate to "contain and eradicate" the invasive species nicknamed "Frankenfish," Maryland and Virginia biologists are trying to pinpoint where it lives and how quickly it reproduces.
The number of reports from recreational fishermen this year indicates that the voracious predator is breeding and growing at prolific rates in Potomac River tributaries such as Dogue Creek.
Last weekend, anglers reported catching snakeheads in the Anacostia River near Bladensburg in Prince George's County and in Mattawoman Creek in Charles County.
"The bottom line is, we're not going to eradicate them, so we've got to figure out how to control them," Odenkirk says.
In mid-April before spawning season, biologists implanted in 20 fish radios slightly larger than a paper clip and capable of transmitting their positions for up to a year. Odenkirk and his partner, Steve Owens, say they hope the pings lead them to where snakeheads have their young. Then, they will overlay locations on a map to show habitat, water temperature and depth.
The torpedo-shaped fish with camouflage markings made national headlines in 2002 when a fisherman hooked an 18-inch snakehead in a tiny pond in Crofton. Jon Stewart did jokes on Comedy Central, hawkers sold T-shirts with the fish's toothy profile and Interior Secretary Gale A. Norton called it "like something from a bad horror movie."
Maryland officials blasted the pond with poisonous chemicals, killing hundreds of snakeheads, and the federal government quickly banned importation and transportation of snakeheads.
But the fish from Asia popped up in a Montgomery County pond and in the Potomac River.
Just south of George Washington's Mount Vernon home is "snakehead mecca," Odenkirk says. Last month, a Baltimore house painter and his brother landed a 7-pound, 14-ounce snakehead not far from where Odenkirk is prowling. That was the record until yesterday, when Odenkirk caught a 12-pound, 3-foot-long snakehead at the Mount Vernon Yacht Club basin, where Dogue Creek widens into a bay that empties into the Potomac.
Once a week, Odenkirk sets out from a Dogue Creek boat ramp. He keys in the frequencies of each radio into a receiver, then picks up a wire loop antenna shaped like a stop sign. When conditions are ideal, he can pick up a signal four football fields away.
"That seems like a lot, but look at all this water," he says. "Let's see if we can find the party."
Odenkirk's cell phone rings. A scientist from down south is trying to secure a fresh snakehead for study and, perhaps, grant money. "I'll see what I can do," says Odenkirk.
He throttles back and glides up to a bass boat, where two fisherman scouting for a weekend tournament flag him down. On the deck is a just-caught snakehead about a foot long that is trying to flop over the side.
"I don't want it. That's not what I'm fishing for," says Dave Whitehead of Great Meadows, N.J. "How do I get rid of it?"
That quickly, Odenkirk has his scientific sample, which he throws into his lunch cooler.
"They don't walk, but they can live out of water," he says, as the fish thrashes wildly. "You could wrap one in a piece of wet burlap and throw it in the back of your car for three days, and as long as it stays moist, it doesn't care."
Tales of the snakehead have been greatly exaggerated, but scientists know the ferocious fish has the potential to cause havoc among the local fish species.
The snakehead's range doesn't appear to be expanding in the Potomac as fast as its numbers are. But that could change as food and territory become scarce.
Steve Minkkinen of the U.S. Fish and Wildlife Service says his agency is preparing a national snakehead management plan that includes more study, early detection, education and stiffer penalties to deter people from transferring snakeheads into other ponds and rivers to create fishing opportunities.
"We know that 20 fish were caught in 2004 and 300 fish were captured or reported in 2005. Last year, the biggest fish was five pounds. This year, we're already at 12 pounds," he says. "What we do know is that they've adapted very well to this habitat."
In three weeks of looking for his prey, Odenkirk has learned several things: Snakeheads like shallow water; they seek cover, such as lily pads or docks, for protection from fishermen and birds; and some of them have decided to roam.
It takes him most of the day to find all but three of the tagged 20 fish, a situation that frustrates him.
"To do this right, we need graduate students to track them this summer. We need two to three times more fish [fitted with transmitters], with remote [control] stations at the head of the bay and at the headwaters to track when they leave and when they come back," says Odenkirk. "This is just a snapshot that we hope will lead to bigger things."