On the banks of the Potomac River yesterday, three scientists went fishing for eels, baiting tube-shaped traps with clams to catch the slithery creatures.

"Bingo," Maryland state biologist Jim Thompson said as he pulled a trap out of the shallows. Four eels were wriggling around inside.

In the scope of the Chesapeake Bay watershed -- where an army of scientists is always monitoring the water, counting the fish, even measuring the algae -- this was a minuscule effort.

But in the world of eels -- a native species that has long been the Chesapeake Bay's forgotten fish -- it was something big.

The eel is routinely harvested by watermen who sell some of their catch overseas for Asian and European diners and keep the rest for bait. But it was never as important to watermen as rockfish, blue crabs or oysters, and was shunned by recreational anglers. So scientists never spent much time learning about eels.

In recent years, though, as the eel population has declined, scientists have begun to study the fish -- and to realize how confounding the creatures can be.

"It's one of those deals where the more we know, the more we don't know," said Marcel M. Montane of the Virginia Institute of Marine Science.

The American eel is not as fearsome as it's more famous cousins: It is not electric, and doesn't have rows of sharp teeth. It eats small animals and fish, can grow to be several feet long and can live more than 20 years.

Even so, the eel has one of the most mysterious life cycles of any creature in the bay: Adult eels end their lives by swimming out to the Sargasso Sea -- a part of the Atlantic Ocean near Bermuda -- to spawn and die, researchers said.

"It's amazing that we've known [the eel's] life cycle since the early 1900s," when watermen were catching eels during their migrations to and from the bay, said David Secor, a professor at the University of Maryland Center for Environmental Science. "But the key details continue to elude us."

For instance: Scientists don't know exactly where the eels spawn in the Atlantic, or how their young find their way back to the Chesapeake and other coastal waters. They know that eels can wriggle on land near dams, and even find their way through the wooden lock gates on the C&O Canal -- but they don't know whether that is how all eels make their way upstream.

Researchers also are trying to understand the decline in the eel population in the past decade. The commercial take of eels in the bay fell from 992,739 pounds in 1993 to 597,509 pounds in 1999.

Scientists suspect an illegal trade in "glass eels," as young, transparent eels are called. Officers with Maryland's Department of Natural Resources have tried to stop watermen who were catching and selling the eels to be raised in captivity in Japan.

Another possible explanation is the general rise in popularity of sushi, which spread the popularity of eel, called unagi in Japanese. And scientists are studying the bay pollution's effect on the species.

As in the case of rockfish and blue crabs, the drop in population might have revealed the dangers of over-fishing.

"That's the first signal, when people start to see a decline," said Derek Orner, a fisheries biologist with the National Oceanic and Atmospheric Administration. "They said okay, we need more research."

Virginia scientists have tried to measure the population of young eels, called "elvers," coming into the Chesapeake Bay. Another unusual step was taken last year: the opening of the area's first "eel ladder," at a hydroelectric dam on the Shenandoah River in Millville, W.Va.

The eel ladder, which looks like a long pegboard, gives the eels something to grip as they slither 20 feet up and over the dam. In the past three months, more than 1,000 eels have climbed over it, far more than scientists expected.

Biologists would like similar eel ladders built on the upper Potomac, on National Park Service dams between Maryland and West Virginia.

Dave Sutherland, of the U.S. Fish and Wildlife Service, said he's pushing for a screen that would keep eels from wandering into the Dalecarlia Reservoir, which provides some of the District's drinking water.

Now, he said, some eels grow to adulthood there -- only to realize they can't get to the ocean to spawn. He said he's heard accounts of giant eels seen wriggling around in frustration in the reservoir, eventually dying.

"That's pretty much a really wrong turn for them," Sutherland said.

Yesterday, Sutherland and Thompson were at the C&O Canal National Historical Park, just downstream from where water from the Potomac is diverted to the reservoir. Last year, scientists began to capture eels on the river and inject them with a small electronic tag. The aim is to track the eels' movements in the river.

The results have been modest: 31 eels have been captured and tagged this year, 12 of them more than once. But Sutherland said it's a significant step, especially "compared to nothing."

Pat Eby, an Anne Arundel Community College student assisting the efforts, used a towel to hold up an eel -- the fish are too slippery to hold with bare hands -- and admired it.

"They're very, very cute," she said. "But they're very, very . . . "

She paused, looking for the right word.

" . . . slimy."

An American eel is caught as part of scientists' efforts to study its mysterious life cycle in the bay.A passive integrated transponder, or PIT, tag is implanted in the eels to track them. Researchers Dave Sutherland and Jim Thompson release six eels they caught, measured and tagged along the Potomac River at Great Falls. "The more we know, the more we don't know," a Virginia marine scientist says of studying the American eel, which can grow to several feet.