It was a cloudless morning along the Potomac River.

At the Sweden Point Marina in Smallwood State Park on Friday, ducks were floating on the tranquil waters of Mattawoman Creek, a Potomac tributary. In the distance, bass fishermen cast into the quiet shallows.

But as Herman Williams set down his rod and icebox on a shoreline picnic table, he said was afraid.

"It frightens me to death; they're notorious," he said. "I don't want them to wipe out everything."

Williams, 57, a retired government employee from District Heights, was referring to the northern snakehead, a ferocious fish that can breathe air and wiggle across land, and one that has recently infiltrated the Potomac River.

On Wednesday, a fisherman off Marshall Hall in Charles County snagged a snakehead, the second such catch in the vicinity in less than a week. In response, environmental officials in Maryland and Virginia raced along the Potomac last week, tacking up signs at marinas and boat launches that warned fishermen not to let any snakeheads escape.

Usually, the Maryland Department of Natural Resources encourages anglers to catch and release mainly to avoid depleting stocks and to allow more fish to reach mature growth.

Not so for the snakehead.

"Have you seen this fish?" reads the flier. "Please do not release. Please kill this fish by cutting/bleeding or freezing."

Environmental officials said any fishermen who believe they have caught a snakehead, a species native to China that is sold in some pet shops, should hold onto it and contact the Department of Natural Resources. They warned that the snakehead threatened to damage the ecosystem of the Potomac River by eating small fish and the bigger largemouth and striped bass.

"We're very concerned that there could be a spawning population [of snakeheads]," said Steve Early, associate director of the fisheries service for the Maryland Department of Natural Resources. He said if snakeheads were reproducing in the open water, there could be a "sudden explosion in their population."

That possibility poses a major threat to the Potomac fishery, which supports both recreational and commercial anglers and is a major element of tourism in Charles County.

When other snakeheads surfaced in ponds in Maryland over the past two years, authorities chose to poison the fish or drain the water. In the Potomac, authorities said, they are helpless to stop the spread of snakeheads because of the size of the river. The fish would probably flourish in the slow, brackish waters of the Potomac because the shoreline vegetation is suitable and it can survive in varying temperatures and at low-oxygen levels, officials said.

"There's no effective fish management method we could use," said Steve Minkkinen, the project leader at the Maryland Fisheries Resources office of the U.S. Fish and Wildlife Service. Snakeheads could eat shad and herring, which the bass rely on for food, he said.

"It certainly could have detrimental impacts on the populations of several other species," he said.

Along the river in Charles County, fishermen worried about the effect the predator might have on the local fish populations. Jay Harford was pushing off from the dock at the Sweden Point Marina for some bass fishing when he was told that there were snakeheads in the Potomac.

"Oh jeez. Ouch, that's bad," said the retiree from Annandale. "You can't control them. That's really a shame."

Down the dock, Roby Johnson sat on a piling in his regular fishing spot. The 70-year-old from Oxon Hill, who fishes nearly every day, took out his pocket pliers to demonstrate what he would do to a snakehead if he caught one.

"I'll punch his head in if I see him," he said. "I'll kill him deader than hell."

Johnson was worried that the snakeheads would kill both bass and rockfish along the river. "If they eat like they say they do, it will have a great impact on the bass in the river," he said.

Marina Manager Louie Galeano, 32, said he bought four snakeheads from a pet store in Waldorf about three years ago but returned them after three months because they grew too fast and were too vicious.

"They're so aggressive, they kill anything that moves," he said. He looked out over the water and considered how much damage they could do in the wild.

"It scares me. I know they're breeding."