MOUNT VERNON, Va. // 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.
"We'd kill millions of fish, and we probably wouldn't get all the snakeheads. What would be the point of that?" Odenkirk said.
The biologists could pick out the snakehead at a glance: It has a torpedo-shaped body and snakelike diamonds on its side.
The biologists used an electrofishing barge to help catch the fish yesterday. The contraption is about the size of a go-cart and is an aluminum raft with a generator on board. When the device is on it makes a high-pitched beeping noise similar to the sounds made by trucks backing up.
A long black wire connects the power source to three hand-held probes - which the scientists walking in front wave back and forth in the water. As if by magic, hundreds of fish appear in front of them, belly up.
The people in the water wore rubber waders, which insulated them from the shock. But, as the water got deeper, the biologists kept yelling: "Keep your elbows out of the water!"
As the line of scientists passed, they left a wake of hundreds of small, stunned fish in their path. It would take the fish about 20 to 30 seconds to "wake up" and disappear again into the dark water.
Once out of the water, the fish look harmless enough. They are speckled and deep green and - despite their reputation - even pretty.
The snakeheads in the cooler were to be weighed and measured so biologists could get a rough idea of the age of the overall population.
Their inner ears will be removed because they contain growth rings. The snakehead creates a new ring every day, so by counting them scientists can determine the day the fish hatched, said Scott Herrmann, a Virginia biologist.
Also, their stomachs will be sliced open so biologists can see what and how much they've been eating. Thus far, Odenkirk said, they favor inch-long killifish. But snakeheads seem to eat a lot - Odenkirk found four killifish in the belly of an 8-inch snakehead.
Some fish will be studied for their tolerance to saltwater. Preliminary results from that study showed a snakehead died in water with salt levels of 10 parts per thousand, said Mike Mangold, a biologist with U.S. Fish and Wildlife.
That's good news for the Chesapeake Bay, he said. The salinity level at a data buoy at the mouth of the Patuxent River was 13.8 parts per thousand yesterday, according to the Chesapeake Bay Observing System Web site.
After an hour-and-a-half of wading through the creek yesterday, the scientists stopped for lunch.
The fish were counted, and one was put on the grass. Despite its reputation as a "walking fish," the snakehead just flipped and flopped on the ground - the way one would expect a fish out of water to behave.
Most biologists spread out on the grass and ate sandwiches, but Odenkirk had a different meal.
He showed off a plastic baggie containing bits of fried, breaded snakehead - his own special recipe.
"Try some," he offered. "It tastes almost like a pork chop."
firstname.lastname@example.org For previous stories about snakeheads in Maryland, go to baltimoresun.com/snakeheads.