Tina McCrobie yelped. She'd no sooner stuck an electric probe in the water than four snakeheads floated belly-up to the surface of the Dogue Creek.

"There are a bunch of them," she said.

The biologist scrambled to net them before they recovered from the stun. Then she plucked the long, slimy fish out of the net and flipped them, one by one, into a nearby cooler.

Within an hour-and-a-half, she and her fellow scientists would have 45 of the fish writhing in the cooler. The bounty confirmed their fears: The snakehead, a pernicious predator native to China that officials have tried to eradicate from local waters, is thriving in the wild.

"This is pretty dramatic," said Matt Fisher, a biologist with the Virginia Department of Game and Inland Fisheries, while examining the haul. "This is a lot."

The group - mostly from the U.S. Fish and Wildlife Service and the Virginia Department of Game and Inland Fisheries - was there to determine the extent of the snakehead infestation on this tributary of the Potomac.

Only 70 snakeheads had been found in the Potomac before last weekend.

"We know it has a voracious appetite. We know it sits on the top of the food chain. But nobody knows how it is going to affect the Potomac. I don't know how we're going to stop it," said Ed Merrifield, the head of an environmental watchdog group for the Potomac River.

The snakehead gained notoriety in 2002 when one was hooked in a pond in Crofton, in Anne Arundel County.

It was nicknamed "Frankenfish" because of its rows of sharp teeth and its ability to slither short distances on land and to survive out of water for up to three days. The fish are imported mostly for aquariums but are also found on the menu of some specialty Chinese restaurants.

Maryland spent roughly $150,000 poisoning and draining the three Crofton ponds, killing six adults and more than 1,000 juveniles. Two years later roughly $10,000 was spent to empty a Wheaton lake where one snakehead was hooked last year. The state then passed a ban on the northern snakehead, the kind found in Crofton and the Potomac.

Scientists worry that the aggressive fish could gobble food that native species rely on. They are also concerned that the fish could reproduce rapidly and disrupt the ecosystem, possibly affecting the population of largemouth bass - a prized sport fish that flourishes in the Potomac.

Two years ago snakeheads were also found in the Potomac in small numbers. Those were genetically different from the Crofton snakeheads - indicating that the two infestations were not related.

John Odenkirk, a biologist with the Virginia Department of Game and Inland Fisheries, who led yesterday's snakehead census expedition, said the reason for the population surge remained a mystery.

"Did they come up here because it is a seasonal thing? Are they going to winter up here? We don't know," Odenkirk said.

On the river yesterday, he was in fast-paced data-gathering mode. He glided through the brush as if he were walking down a sidewalk - and didn't miss a beat when he needed to leap over a ditch or tree limb.

At one point he plucked a 12-inch snakehead out of his net and pried open the mouth to show off the fish's puppy-sharp teeth. The fish latched on to Odenkirk's finger for a moment before falling back into the net.

"I'd like to see you do that when we find a big one," shouted Amelia Sandiford, a technician who works with him. The fish can grow to 33 inches.

The biologists planned to kill most of the snakeheads they found, but the purpose of their trip was not to reduce the population. Odenkirk doesn't think it would be worth trying.