A northern snakehead fish swims in a tank at the Academy of Natural Sciences in Philadelphia. (William Thomas Cain/Getty Images)

A decade or so ago, no snakehead fish were in the Washington region.

Today, more than 20,000 are in the Potomac River, experts say, a population approaching that of the largemouth bass, which was introduced to the region in the 19th century.

The invasive species just achieved another beachhead: The National Park Service announced Monday that since last fall, snakeheads have been spotted numerous times above Great Falls in the nontidal portion of the Potomac, where they have not ventured before.

“This is just absolutely something we didn’t want to happen. But it did,” said Joseph Love, the tidal bass program manager at the Maryland Department of Natural Resources.

To try to stem the spread of the snakehead, Love’s department is sponsoring a contest for fishermen who kill snakeheads in any of the ways the state suggests: rod and reel, bow and arrow, spear and more.

State officials emphasized in a news release that snakehead fish can be legally caught in any season and at any size, unlike some other fish. To qualify for the contest, anglers must agree to not catch and release the fish; they must kill the snakehead.

“We’d like it to be harvested if anyone catches it. We’d like it if they took it home and possibly ate it,” Love said. “Anglers and archers enjoy fishing for them, which is great. And they enjoy eating them, which is great.”

Scientists believe snakeheads are a threat to local waterways, even if they are uncertain how, Love said. They are studying the consequences of the population boom and are fearful of what they might learn.

“Part of the reason we should be worried about it is we don’t really know what the impacts are going to be,” Love said. “We do know that in some cases, invasive species cost millions of dollars in damage to the ecosystem.”

In particular, he said, the snakehead and the largemouth bass eat many of the same foods, including crayfish and mice and other small mammals that end up in the water. So a spreading snakehead presence might starve the bass.

Scientists believe the fish were most likely introduced in Washington about 2002 because of live fish markets, which imported them from Asia. Federal laws now prohibit possession of live northern snakehead fish in the United States, but then some people bought them — and let them go.

“You never know what you’re going to get when you release one of these live animals into the waterway,” Love said. “I’m hoping that people are also becoming more aware of the dangers of just releasing a pet or a live animal they bought at a fish market.”

Love said that, in recent years, fishermen seem to have effectively culled the fish. But their expansion is causing concern.

Each spring, the snakeheads that remain swim farther when they are looking for a place to spawn, Love said. They quickly acquire more nesting spots, even if their overall numbers stop increasing. Snakeheads have been reported around Baltimore and Maryland’s Eastern Shore, in addition to the Potomac.

“I don’t think we’re ever going to get rid of the snakehead,” Love said.