After swimming more than 1,000 miles from the Sargasso Sea and up the Chesapeake Bay, eels reaching the Susquehanna River seem undaunted by the Conowingo Dam.

Rather than stop at the foot of the 100-foot dam, they are willing to follow a dribble of water up a long pile of concrete rip-rap.

For the first time in decades, their efforts are being rewarded. Biologists this summer have been collecting the eels, then giving them a ride around the dam.

It's the second year that humans have intervened to help the eels fulfill their migratory impulse. Last year, 20,000 eels were moved upstream; by early August this year, more than 10,000 had made the trip.

"They have great desire to move upstream," said Steve Minkkinen, who heads the U.S. Fish and Wildlife Service's Maryland Fisheries Resource Office. "That dam, unfortunately, is too much for them to get around."

Eel populations along the East Coast are at an all-time low for reasons not entirely clear. But there are many dams, like the Conowingo, that block historic habitat and are considered to be one of the major problems. Other factors such as overfishing, nonnative parasites or changes in ocean currents could also play a role.

Although they are born in the Sargasso Sea in the middle of the Atlantic Ocean, eels will migrate to the farthest headwater streams to spend most of their lives. Historically, biologists say eels accounted for as much as a quarter of the total biomass in many streams.

Even when abundant it's a fact that would be missed by most people: Eels are nocturnal and live in sediment or under rocks.

To stem their decline, efforts to restore eels are gaining momentum around the Bay-where a number of eel passages have been built in recent years-and along the East Coast. As a rule of thumb, the farther eels swim upstream, the larger they become and the more eggs the produce. Eels that are 3-4 inches when they enter the Susquehanna could be 5 feet when they migrate out a decade or two later, on their way to reproduce in the Sargasso.

And perhaps nowhere would the return of the eel be more important than on the Susquehanna where the Conowingo Dam, located just 10 miles upstream from the Bay, closed the largest East Coast river to migration in 1928. The dams block eels from 43 percent of the Chesapeake watershed.

Laboratory work by the U.S. Geological Survey's Northern Appalachian Research Laboratory indicates that the survival of a normally common freshwater mussel in the river, the eastern elliptio, may depend on eels getting upstream.

Mussel larvae, or glochidia, need to parasitize a host fish before they can transform into the juvenile life stage. Some mussels can use multiple fish species as hosts, while others are specialists. Studies by USGS scientist Bill Lellis have shown that eastern elliptio mussels appear to depend on American eels.

Those are the most common mussels in nearby rivers such as the Delaware. The 280 million elliptios in the Delaware have the potential to filter 2 billion to 6 billion gallons of water and remove 78 tons of sediment per day, according to estimates by Lellis.

The Susquehanna has lost that filtering ability. Recent surveys have found only remnant populations of elliptios, and all of them appeared to be decades old. That reinforced the lab finding that the mussels depend on the eels to successfully reproduce.

Now scientists are testing that hypothesis in the Susquehanna.

Last year, biologists developed their techniques to capture eels at the Conowingo Dam and move them upstream, ultimately trucking about 20,000 to the Conestoga Creek in Lancaster County, PA, and sending another 1,000 to the USGS lab.

But Minkkinen said Conestoga Creek turned out to be too large for the project. Eels quickly dispersed and became difficult for biologists to find.

This year, they switched efforts to the smaller Conowingo Creek, just a few miles above the dam. Based on the number of eels found in other Maryland streams, Minkkinen said biologists believe the creek would reach its carrying capacity for eels once 60,000 are stocked. He plans to spread the stocking over three years.

Surveys have found only a few elliptio mussels in the stream, so beginning next year, Minkkinen plans to begin placing eels in tanks filled with elliptio glochidia so they become "infected" with the larvae before being released in the creek. When the glochidia detach, they should spawn a new mussel population.

Biologists can also see if the mussels affect the stream, thanks to the presence of several monitoring sites of the Maryland Department of Natural Resources' Biological Stream Survey located on Conowingo Creek.

"If they do jump-start the mussel population, and mussels start filtering the river more, there is actually a process set up there to monitor it," Minkkinen said. "It is a perfect match for monitoring this type of thing."

Other eels have gotten a longer road trip: they were trucked to the USGS lab in Wellsboro, PA, more than 200 miles upstream.

Unlike Minkkinen, who is working in a creek with no mussels, Lellis is placing eels where remnant mussel populations still release glochidia into the water, but no young mussels can be found.

The objective is to see if glochidia latch onto the eels, and ultimately lead to the production of young mussels. "It is pretty clear that there is no reproduction going on in this system," Lellis said, "so any small mussels that show up will be pretty indicative that lack of a host was the problem."

One concern is whether the eels will stay near the mussel populations when they are released in larger river systems. Lellis kept eels he received last year at the lab until this summer, hoping to quell their desire to move farther upstream.

"When we get the eels they are in a migratory phase, and they are moving all the time in the lab," Lellis said. After keeping the eels in tanks for a while, Lellis said the eels appeared more willing to "settle in" and were then released in parts of Pine Creek west of Wellsboro.

Tags were placed in the eels so they could be detected in the future to determine whether they remain near the mussels. Lellis expects to release another batch next year.

The work is being done on a shoestring. Lellis' eel release work was supported by a $1,500 grant from Pennsylvania Audubon, which also provided volunteers. Minkkinen's work at Conowingo got no financial support at all.

Lellis said it will take time to learn whether the project was worth the effort. Young mussels will be hard to detect because they are only about the size of a grain of sand.

"I think it will be three to five years before we will be able to pick up recruitment," Lellis said. "They will have to get large enough that we can detect them."