The 300 snakeheads caught in a Potomac River tributary last month were more than an anomaly. They were proof, biologists say, that the predatory Asian fish has bred its way to the top of the river's food chain, and it is science's turn to adapt.

Leaping and fighting their captors' nets, the snakeheads caught in just a few days numbered more than four times the total ever found in the river -- evidence that not only are they thriving, but they also appear to be breeding faster than the river's most beloved game fish.

"I caught 62 in two hours, they were that thick," said John Odenkirk, a Virginia Department of Game and Inland Fisheries biologist. The snakeheads, he added, "are growing very fast. The spawning season is very protracted and likely due to repeat spawning. It's not what we would have wanted."

Worse, he said, "there's not a lot we can do."

Having lost the battle to eradicate the snakehead, Virginia and Maryland biologists and federal wildlife officials have recruited a team of researchers to study its adaptation. They hope that the fish will find a place in the Potomac's ecosystem without damaging the other species there.

But if, as they fear, it begins to crowd out other fish, officials are searching for methods -- such as disturbing its breeding cycle or controlled culling -- to keep its numbers in check and prevent it from spreading to other waterways.

"There's a lot we don't know about this fish," said Steve Minkkinen, a snakehead expert at the U.S. Fish and Wildlife Service. "If we understand something about their behavior, maybe we can exploit those behaviors. It's an opportunity for us to learn something about the fish that we can use."

What they know so far is this: The freshwater northern snakehead was probably introduced into the Potomac about three years ago, perhaps by someone who imported it from Asia, where it is a popular food.

With a mouth as big around as a fist and lined with rows of teeth and an ability to grow to three feet over an estimated life span of a decade, the adult fish has no known predators in the Potomac. Of special worry is its potential to diminish the population of largemouth bass, a species that fuels a multimillion-dollar tourism and fishing industry.

Although it will likely be at least a decade before the snakehead's population reaches full strength, the freakish catch on Virginia's Dogue Creek in October confirmed to biologists that predatory species has reproduced more quickly and plentifully than expected, Minkkinen said.

Their apparent quick adaptation, he said, supports forecasts that the northern snakehead "would have the ability to colonize a large portion of the U.S. and even southern Canada," he said. Consequently, he said, "We may not be able to control snakeheads in the Potomac, but we'd certainly want to control their spread to other areas."

In the Potomac, the bass and snakehead could compete for food and nesting areas, and they would eat each other's young. The snakehead's breeding habits seem to give it an edge, researchers say. Snakeheads appear to have a breeding season twice as long as the bass's, and its eggs have a shorter hatching period. The predators can breathe air and move through water so muddied it would immobilize other fish, allowing them to nest in far poorer-quality habitat than can bass.

Yet although it seems to be thriving near Dogue Creek, not much is known about the fish's migration patterns, nor its long-term tolerance for waters far from its home.

Invasive species "tend to proliferate and overwhelm a system and then they drop back, but by that time, they may have already had a drastic impact on the system," said Steve Early, a biologist with the Maryland Department of Natural Resources' Fisheries Service. "We just don't know if this is going to be one that takes off and causes a problem or is this going to be one that doesn't survive."

In 2002, this region's first snakeheads were found in a fetid pond in Crofton. Wildlife experts poisoned the pond and counted the fish. Six adults and more than 1,000 juveniles were proof that the fish bred successfully even in the pond's poor habitat. Already, they outnumbered the bass there.

In May 2004, an angler caught the Potomac system's first snakehead, in Little Hunting Creek near Alexandria. Biologists and anglers caught 20 snakeheads in 2004. This year, they've caught more than 300, 270 during the week-long run at Dogue Creek. DNA tests showed that the juveniles among the 20 fish caught in the Potomac in 2004 were all descended from one or a few related females, making it possible that the Potomac's snakehead problem originated with the introduction of only a few fish.

The research has revealed a few possible strategies for controlling the snakehead's spread. For instance, the fish make their nests by tearing a column into a thicket of aquatic grass then lay a clutch of about 50,000 eggs, which both parents guard, said Walter R. Courtenay Jr., fishery biologist with the U.S. Geological Survey.

To control them, "We maybe could target [and] disrupt spawning behavior," Minkkinen said.

Elsewhere, invasive fish have been controlled with repellent chemicals or hormones. In the Great Lakes, an electrified "fence" bars Asian carp from entering through the Mississippi River.

But the Potomac system's size could be a problem for such methods, Minkkinen said. Now, the fish appear to be most heavily concentrated around Dogue Creek. Tests have shown they can't tolerate the Chesapeake Bay's salt water. The river's natural barriers mean "it's very unlikely they're going to get above Great Falls, unless some stupid idiot moves them," Courtenay said.

So far, fishing is the only method of controlling them.

"It grows fast, bites anything that comes by, fights hard and taste great," with firm, white flesh similar to perch, Odenkirk said.

Minkkinen, too, eats some of his test subjects, but "in the management plan I'm going to be very careful of how we talk about that," he said. When anglers grow fond of a fish, they are more likely to introduce it elsewhere: That is how the walleye, a Midwestern game fish, invaded the Columbia River, Courtenay said.

"I absolutely don't want to encourage that," Minkkinen said.

Officials fear that the snakehead, above, could diminish the number of bass, hurting a multimillion-dollar fishing industry.