An angler competing in a bass fishing competition on Saturday pulled a 13-inch northern snakehead out of the Potomac River, marking the third time in nine days that the exotic species has been captured in or near the waterway and heightening fears that the Asian predators could become permanently established.

If the snakeheads are reproducing in the Potomac, game biologists said, it might be impossible to eradicate them. State game workers ousted the species from a pond in Crofton two years ago with poison and from another in Wheaton this year by draining the pond. But the river is considered far too large for any such solutions.

"Here we go again," said Donald Boesch, president of the University of Maryland Center for Environmental Science. But he added that "the range of options for controlling a nonnative fish becomes much more constrained in a place as large as the tidal Potomac."

Saturday's catch occurred several miles downriver from the spot near Marshall Hall, Md., where a 12-inch snakehead was taken Wednesday and about 10 miles downstream from the site off Little Hunting Creek where a 12-inch snakehead was caught May 7.

"We now know they are spread out over about 10 miles," said Steve Early of the Maryland Department of Natural Resources. "This raises my level of concern."

Adult snakeheads eat mostly small fish and sometimes frogs and insect larvae. Scientists believe the snakehead has the potential to devastate local waterways if it becomes populous enough to displace large numbers of game fish, a multimillion-dollar industry in the region. But they're not sure whether that would happen.

"I think we're in trouble on the Potomac River because evidently they're pretty widespread," said Bill Haire, director of the Virginia Bass Federation's Northern Virginia region. "Evidently there's quite a few of them out there. It's my understanding they'll impact all species; they eat just about anything and everything."

The National Audubon Society issued a statement yesterday calling for passage of a federal law to ban nonnative species from American waterways. "We need to get a handle on this growing threat to our irreplaceable native ecosystems like the Potomac River, the Great Lakes, and the Chesapeake Bay," said Mike Daulton, assistant director of government relations for the group.

The first critical question is whether the fish have reproduced in the river or whether the three that have been found were dumped there. Some snakeheads discovered previously in the region were believed to have come from markets where live fish are sold for food.

If game officials discover large numbers of juvenile fish -- unlikely to have come from a market -- it would indicate that snakeheads have been reproducing, Early said, and could be on their way to making a permanent home in the river.

Scientists said there is not enough evidence to decide either way.

"The fact that we've found three fish of about the same size would tend to increase the likelihood that there was some reproduction in that system," Boesch said. But "it doesn't prove it by any stretch of the imagination."

"Three fish doesn't make a population," said Paul Shafland, director for the nonnative fish research laboratory of the Florida Fish and Wildlife Conservation Commission and a member of Maryland's Snakehead Scientific Advisory Panel. "These could be incidental releases. However, there's certainly a suggestion there."

The distances between the snakehead catches in the Potomac make it difficult for game officials to track down any more specimens or determine their origin. "It's like looking for a needle in a haystack," Early said. "We're looking for a hot spot."

For now, they are relying on anglers to report such finds and guide the research. Maryland and Virginia fisheries officials have been canvassing marinas and boat launches in recent weeks, warning anglers that they are the first line of defense against the snakehead invasion.

"Have you seen this fish?" ask fliers featuring a picture of the northern snakehead. "Please do not release. Please kill this fish by cutting/bleeding or freezing."

The northern snakehead -- which is edible and considered a delicacy by some -- is native to eastern Asia and is known in Japan and western Asia as well. In the past few years, some have been found in Florida and Massachusetts, according to a report from the snakehead advisory panel.

It is difficult to predict what kind of environmental harm might be caused if the northern snakehead were to become established in the Washington region.

"Adult northern snakeheads are large predators that would likely affect the populations of other fish, amphibians and invertebrates," according to a report of the snakehead advisory panel.

But scientists simultaneously caution against thinking it would be an environmental catastrophe. "I certainly think it's far-fetched to conclude that we could see the collapse of the ecosystem," Boesch said.

A different species of snakehead has established itself in Florida's waterways but has not caused significant problems, Shafland said. The first snakehead documented there was in October 2000.

"We feel it's going to be here forever," Shafland said. But "we have not seen the catastrophic effects that are so regularly speculated upon. Our rivers are more accommodating of these exotic species than some people think."

Steve Early posts a notice at Fort Washington Marina in Maryland, asking anglers to kill snakeheads.