The sun had barely risen when Steve Early tacked up the wanted poster on the wood piling along a Potomac tributary:

"Have you seen this fish?" read the flier, complete with a picture of the northern snakehead. "Please do not release. Please kill this fish by cutting/bleeding or freezing."

Up and down the shoreline yesterday, Early, an assistant director at the Maryland Department of Natural Resources, and other Maryland and Virginia fisheries officials canvassed marinas and boat launches, warning fishermen that they are the first line of defense against the voracious alien species plucked out of the Potomac River and its tributaries twice in the past eight days.

With no way to rid the river of the snakehead, scientists and fishermen fear the fish could undermine other species there, gobbling up the food supply that such large fish as striped and largemouth bass rely on.

"We're very concerned that there could be a spawning population" of snakeheads, Early said. Mating snakeheads, he added, could generate a "sudden explosion in their population" and feast on fish of several sizes.

"It certainly could have detrimental impacts on the populations of several other species," said Steve Minkkinen, the project leader at the Maryland Fishery Resources Office of the U.S. Fish and Wildlife Service. "There's no effective fish management method we could use there."

That leaves the fishermen. "No resource agency has the power of the many, many thousands of recreational anglers that are out there on the river," Early said.

Bass fishermen, many of them gathered for a tournament in Southern Maryland yesterday, said they are taking the threat seriously.

"I'll kill him deader than hell," said Roby Johnson, 70, of the snakehead while fishing on the dock at Smallwood State Park's Sweden Point Marina in Charles County. "There's just no way you can control them. You're going to see a lot of dead bass floating around."

The first snakeheads found in Maryland were in ponds, which could be drained or poisoned to stop the spread. The Potomac, which flows more than 380 miles, offers no such options.

In the Crofton pond where the alien fish appeared in 2002, authorities found fewer than 10 adults but more than 1,000 juveniles. Each female can hold 15,000 eggs. There were more snakeheads in the pond than any other type of fish by the time the pond was poisoned.

The discovery of a single snakehead last month at Wheaton Regional Park led authorities to drain the pond, but no other snakeheads were found.

The fish appeared again, in a Potomac tributary near Mount Vernon, and then five days later off the Maryland shore in Charles County. Fisheries officials say the snakehead could flourish in the brackish waters of the Potomac.

The river houses one of the nation's top largemouth bass recreational fisheries and is an important spawning ground for striped bass, officials said. The snakehead, which can grow to 33 inches in length, could also feed on smaller shad and herring, or even soft-shell crabs, authorities said.

"This fish has the potential to change the way the habitat operates," Early said. The fish, however, poses no threat to swimmers or pets, authorities say.

Even if the snakehead population is growing in the Potomac, Minkkinen said, it is unlikely to spread into the Chesapeake Bay because of the higher salinity. "But once you have an introduction, over time they can spread out very far," he said.

The recent catches in the Potomac are not the first time the snakehead has appeared in open waters in the United States. The bull's-eye snakehead, a tropical cousin to the northern snakehead, has lived in lakes and canals in South Florida for six years or more.

Florida officials faced a similar management quandary trying to stop a population that is now estimated in the hundreds or thousands. But despite the notorious reputation, Florida officials said their snakeheads have had a negligible effect on local fish.

"There's still native fish present in that area," said Water R. Courtenay Jr., a biologist with the U.S. Geological Survey. "They haven't eaten everything."

The arrival of the snakehead was the talk among fishermen along the Potomac yesterday. At Marshall Hall, where the fish was caught Wednesday, some came out just to look at the river and contemplate the creature. A commercial fisherman brought in a load of catfish and called out: "No snakeheads. Not today. Not yet."

Farther south, at the Sweden Point Marina, manager Louie Galeano, 32, said he knows the power of snakeheads firsthand.

Three years ago, Galeano bought four snakeheads from a pet shop in Waldorf, kept them in a 35-gallon tank, and fed them 20 plump goldfish twice a week. They grew like Godzilla, he said, and mealtime became a slaughter. He wouldn't put his hand near the water and, after three months, he took them back to the store.

"They scare me," he said. "They just kill and eat and eat and kill."

Fisheries officials say they believe snakeheads living in local waters were dumped by aquarium owners who have tired of the voracious eaters. Pet stores can still sell the fish, but it is now illegal to import them.

Yesterday, Bureau of Immigration and Customs Enforcement agents arrested a Glendale, Calif., man for allegedly smuggling live snakeheads into the country. The agency said Sung Chul "Daniel" Rhee, 46, had hidden the fish in containers labeled "sea bass" in 2003 and sold them in Los Angeles for $14.99 a pound.

The parent company of Rhee's supermarket is Rhee Brothers Inc., headquartered in Columbia. But a spokesman for the federal agency said it has uncovered no evidence that any of Rhee's snakeheads were brought into Maryland.

A northern snakehead recently caught in the area. The fish, native to Asia, can breathe air and walk on land and has a voracious appetite. Michael Mill helped the Maryland Department of Natural Resources post notices about the snakehead at marinas in the Fort Washington area.