One fish was a worry. Two fish were a troubling trend. Now that the total is up to nine, some scientists say they're close to conceding: The northern snakehead is in the Potomac River, and likely to stay.

Nine times over the past seven weeks, the Asian transplant that can breathe air and scoot slowly over land has been caught in a 14-mile stretch of the Potomac or its tributaries.

Even though eggs and babies -- definitive proof of a breeding population -- have not been found, scientists say the number of catches and the swath of the river involved suggest that the fish is established in open water that can neither be poisoned nor drained.

"It's time to say, 'Yeah, we have a naturally reproducing population in the system, and they've probably been there for two or three years,' " said John Odenkirk, a fisheries biologist at the Virginia Department of Game and Inland Fisheries.

The arrival of the northern snakehead, a native of China and Korea that feeds chiefly on other fish, has alarmed some scientists, who say it could throw the Potomac ecosystem out of balance.

"It's the first act of the nightmare, if you will," said Mike Slattery, assistant secretary of the Maryland Department of Natural Resources.

It also has intrigued some fishermen, eager to hook the elusive creature that already has provided fodder for some whopping fish tales. One angler reported his catch walking on its fins a foot across the floor. Another told how his was still flopping after four hours out of the water.

Not everyone is certain that the fish is breeding in the Potomac. The Smithsonian is conducting DNA testing to see if any of the fish caught are related.

Until those tests come back, or until baby snakeheads are found, some scientists say it's still possible that all of the snakeheads were dumped into the river and none of them was born there.

"As scientists, we don't have enough information to go on yet," said Becky Wajda of the Virginia Department of Game and Inland Fisheries.

The snakehead was first discovered in the Washington region in 2002, after a man bought a pair of them to make soup, then changed his mind and released the snakeheads into a Crofton pond. They bred, and Maryland officials eventually used poison to kill the hundreds of baby snakeheads in the pond.

Another snakehead appeared this year in Pine Lake, a fisherman's pond in Wheaton. Again, officials took drastic measures, draining the lake to make sure no others remained.

But then, in early May, a bass fisherman caught one in Fairfax County's Little Hunting Creek -- an inlet of the Potomac near Mount Vernon.

As the weeks went on, there were more catches around the Potomac: in Fairfax's Pohick Bay, near Marshall Hall in Charles County, and twice more in Little Hunting Creek. The most recent catch came Sunday in Kane's Creek near Occoquan.

The biggest of them all was caught June 17 in Little Hunting Creek. Cliff Magnus, 44, a woodworking artist from Waldorf, was practicing for a bass tournament when he saw a large bulge of water pushing through the lily pads.

Magnus cast, and the fish hit. Then things turned ugly.

"He tried to wrap me up in the motor a few times. He tried to swim under the boat a couple times," Magnus said. "He put up a big fight, a very good fight."

Magnus hauled out a two-foot-long snakehead, weighing nearly six pounds.

"As soon as I saw it, I knew it was a monster," he said.

Magnus tried to kill the fish by hitting it over the head with a pair of pliers, but gave up after four or five blows. In all, the fish spent more than four hours out of water by the time Maryland Department of Natural Resources officials picked it up at Magnus's home on June 17.

It was still flopping, Magnus said.

Scientists examined bones in the fish's head called otoliths, which grow rings for each year of life. They found at least four in Magnus's fish, making it older than most of the other snakeheads caught in the river.

Magnus's fish had another interesting feature: The last two rings were much bigger than the earlier ones. Scientists said this likely indicates that the fish had lived in captivity and then been released into the food-rich environment of the Potomac.

"The last few years when it was wild it was living high on the hog," Odenkirk said.

In all, scientists say that the nine Potomac snakeheads represent three different generations: Some are 2 years old, some are 3, and at least two of the fish are 4 or older.

Since the northern snakehead has not taken hold anywhere else in the United States, scientists have only a vague idea of how it might behave if a breeding population were established in the Potomac.

This much is certain, according to Walter R. Courtenay Jr., a snakehead expert with the U.S. Geological Survey: The breed would not be able to spread downstream into the Chesapeake Bay because snakeheads cannot deal with salty water. The fish also could not make it above the rapids at Great Falls, a few miles upstream from Washington, he said.

In between, there are plentiful fish that a snakehead might feed on -- including gizzard shad, white perch and pumpkinseed sunfish -- plus the young of species like rockfish and largemouth bass.

In 10 or 20 years, Courtenay said, it's possible that there will be enough snakeheads to affect the populations of some of these or to crowd out native predators. That has some fishermen very worried.

"It's scary, it really is. I've heard they can really devastate a river," said Bill Bennington, 42, who was drying off his bass boat at the Sweden Point Marina in Charles County. "If they get established, they'll eat everything in their path."

Other fishermen, though, see the alien species as a challenge.

Already, there is a snakehead fishing tournament scheduled for July, and Bass Pro Shops Outdoor World at Arundel Mills in Hanover plans to offer a bounty on northern snakeheads -- $10 to $50 gift certificates, depending on the length of the fish.

Doug Stanton, who hooked the eighth Potomac snakehead in Mattawoman Creek on Thursday, compared the snakehead to another of the Potomac's nonnative species, the blue catfish. This transplant from the Midwest now is accepted as one of the Potomac's big-time game fishes.

Today, Stanton said, "you're just going to have days when you catch a lot of catfish."

In the future, he said, "you're going to have days when you catch a lot of snakeheads."

"As soon as I saw it, I knew it was a monster," fisherman Cliff Magnus said of the snakehead he caught June 17 in Little Hunting Creek.The largest northern snakehead caught in the region's waters measured 24 inches. "He put up a big fight, a very good fight," Cliff Magnus said. Cliff Magnus caught one of nine northern snakeheads taken from the Potomac and its tributaries in the past seven weeks, leading many to believe the fish has become established here.