They can't drain the Potomac.

When a 19-inch female northern snakehead was caught in Wheaton's Pine Lake last month, authorities emptied the entire lake. No more were found.

Maryland officials will have to come up with another way to determine how many are making a home in the river. Another northern snakehead, a voracious Asian fish that can breathe air and wriggle across land, was caught on the river's Maryland shore Wednesday, state officials announced yesterday.

The 12-inch fish was hooked by an angler in Charles County, just across from the spot in Virginia where one was caught last Friday.

The two fish were about the same size and the same age. Maryland officials said they worried it was not a coincidence.

"One snakehead in the river doesn't bother me a whole lot," said Steve Early, associate director of the fisheries service for the Maryland Department of Natural Resources. "But when you catch two of them in such close proximity, it really worries me that about how many more there might be."

The newest snakehead was caught by a fisherman at the boat ramp for Marshall Hall, a historic mansion near the Prince George's County line. Early said the fisherman was carrying the snakehead when he ran into state natural resources personnel, who were in the area on a fish-stocking project.

They confirmed it was a northern snakehead, a native of China and Korea with rows of large teeth. The fish was female, likely a year old and not ready to spawn this year, Early said. It was not clear if the fish had spawned last year.

The Virginia snakehead was caught Friday by angler Robert J. Hinds in the Little Hunting Creek, a Potomac tributary near Mount Vernon. Early said it was possible the two fish were placed in the river at the same time.

But he said it was also possible that they were part of a larger, reproducing population.

"It just kind of begins to defy your imagination that these are the only two fish out there," he said.

If there is a larger population of snakeheads in the river, Early said, it's unclear what it would mean for the Potomac's other fish. Snakeheads might prey on the river's stocks of rockfish or largemouth bass, he said.

The northern snakehead is kept as an aquarium fish and also sometimes bought as food. The fish made its first appearance in this area in 2002, when two adult fish and hundreds of babies were discovered in a pond in Crofton.

Officials say they have no systematic way of determining how many snakeheads, if any, remain in the Potomac. Their only option, they said, is to rely on reports from anglers. Early said the state is asking them to kill any snakeheads they catch. Anyone who catches one is asked to call the Department of Natural Resources at 410-260-8320.

There is no cash reward for the outlaw fish, Early said. But officials are designing a snakehead hat.

"If we get a bona fide snakehead, the reward is the hat," he said.

Yesterday at Marshall Hall, angler Dennis Purcell said that he has always been amazed by what turns up in the Potomac.

In his weekly fishing trips to Marshall Hall, the 51-year-old retired sheriff's deputy from Waldorf said he has caught overgrown ornamental koi, seen a giant sturgeon, and has heard tales of alligators pulled from the river.

"People dump everything in the water," he said.