On a sun-drenched July morning, a pair of biologists scanned a 60-foot expanse of broken rock along the Susquehanna River, just below the Conowingo Dam, looking for eels about the size of a pencil.

Normally, the biologists would spot the eels crawling up the rocks along a small stream of water from a hose. But over the weekend, a pump had shut down, and the flow was barely a trickle.

"We have a couple of them climbing on the rocks," said Ian Park, a biologist with the U.S. Fish and Wildlife Service, pointing to the slithering eels about 20 feet away. "They are just trying to find wherever the water is coming from."

The trickle of water was, it would turn out, their ticket around the dam. They were about to become part of a 10-year experiment to see if returning eels to the Susquehanna could also help return mussel populations - and the river - to health.

That would be in their future. It had already been a long journey for these eels, starting almost two years earlier when they were hatched someplace in the Sargasso Sea, a 5-million-square-mile expanse east of the Bahamas and south of Bermuda.

The thin, flat larvae - shaped somewhat like willow leaves - then did what their ancestors had done for the past 50 million years: They headed west, sometimes swimming, sometimes drifting with the currents.

Along the way, they transformed into translucent, wormlike glass eels. Then, as they began working their way up estuaries along the East Coast of North America, they gained pigmentation and became "elvers" measuring up to a half-foot long. It was hard work to get this far: swimming along the slow-moving water near the shore, floating upstream on flood tides, holding their place along the bottom during ebb tides.

Nearly two years of hard work had rewarded them with, literally, a concrete wall: the Conowingo Dam. Eels can slither over or around some obstacles, such as Great Falls on the Potomac River, but not a 100-foot-high dam.

Defeated, eels on the Susquehanna typically dispersed into the streams that feed into the river in the 10 miles between the dam and the Chesapeake. Figures from the Maryland Biological Stream Survey show that survey sites in those streams have the largest numbers of eels of any waterways in the state.

But for eels making the trip this year, the trickle of water over the sun-baked riprap - a hazardous trip where they were prime pickings for herons and other birds - seemed to offer an alternative way to fulfill their continued drive to move upstream.

After crawling up a metal ramp for the final 20 feet of the trip, they plunged into a bucket where they were sedated, measured and counted by Park and his colleague Dylan Carey. Then they were poured into an 80-gallon cattle tank - something that was certainly never part of their ancestor's itinerary.

They were among the nearly 86,000 eels that biologists helped around the Conowingo Dam this summer. The eels were trucked north for release into two Pennsylvania waterways: Buffalo Creek, about halfway up the river near Lewisburg, and Pine Creek near Wellsboro, not far from the New York state line.

In addition, 64,000 glass eels, captured near Ocean City, MD, were also released in those creeks.

It's the second year of an experiment to see if biologists can prove in the wild what they've seen in the laboratory: that American eels represent a critical cog in the river ecosystem.

Laboratory research by the U.S. Geological Survey Northern Appalachian Laboratory near Wellsboro and by the U.S. Fish and Wildlife Service Maryland Fishery Resources Office in Annapolis shows that eels are the primary host for the eastern elliptio, the most abundant freshwater mussel in many other northeastern rivers.

In the Susquehanna, eastern elliptios are considerably less abundant, and remaining populations are dominated by large, old mussels, with little sign of reproduction.

Biologists speculate the reason is the lack of eels. Mussels release their larvae, or glochidia, into the water where they attach to fish "hosts." The larvae live on the hosts as parasites until it transforms into a tiny mussel and falls to its home in the stream bottom where it can live up to 100 years.

Many mussels use a variety of hosts, but lab work indicates that the eastern elliptio in the Susquehanna River rely primarily on eels. For millions of years, that was a safe relationship as eels were one of the most common freshwater species - often accounting for a quarter of all fish biomass in some streams.

But the construction of the Conowingo Dam in 1928 violated one of Aldo Leopold's rules for conservation: "To keep every cog and wheel is the first precaution of intelligent tinkering." Before the dam was built, historic reports show that a million or more pounds of eels were harvested each year from the river. Within a couple of decades, the last of the long-lived eels had died out.

Unlike many other rivers where eels could work their way over, or around, obstructions, the Conowingo dam rendered 43 percent of the Bay watershed devoid of eels.

Mussel populations have hung on in part because they can live for decades. And, they may have been unwittingly helped by sporadic efforts by the Pennsylvania Fish Commission to stock eels from 1938 to 1980. But no eels have turned up in fish surveys on the river since 1985.

"There are lots of large, old mussels, but very few small ones," said Steve Minkkinen, who heads the USF&WS Maryland Fishery Resources Office.

Tests show that some other fish may serve as hosts for elliptio larvae, but they are nowhere near as effective as eels.

Lab tests alone can't prove eels are essential to elliptio mussels. Other factors not accounted for in the lab, perhaps changes in water quality, might also be stemming elliptio mussel reproduction.

"To really nail that theory down we've got to do it in the field," said William Lellis, who found the apparent link between elliptios and eels while studying endangered mussel populations at the USGS Northern Appalachian Research Laboratory.

"We had to go places where there is no recruitment, where there are only old mussels, and the only thing we change is stock eels on top of them. If they start recruiting, we've got it."

Thanks to funds from a riverbank mitigation project along the Susquehanna near Sunbury, PA, biologists began stocking eels last year in Pine and Buffalo creeks, where surveys had found the most abundant mussel population. Significantly, Minkkinen noted, those were also creeks where the state fish commission had stocked eels in past decades.

Initial evidence suggests biologists are on the right track. Julie Devers, a fisheries biologist in Minkkinen's office, conducted a laboratory study to introduce eels to glochidia from Buffalo Creek and found that an average of 87 eastern elliptio glochidia transformed on each eel. In addition, an eel collected from Buffalo Creek this spring had 20 glochidia attached to its gills.

"That's exactly what this stocking is intended to do," Minkkinen said. "We should see an explosion in recruitment."

But final proof - that the eels help the glochidia become mussels - is still years away. The mussels that start growing after they fall off the eels are about the size of a grain of sand. Biologists think it will take at least five years to detect them in the wild.

"We have to be really patient," Minkkinen said. "We can't go out and look for a grain of sand."

Biologists plan to stock more eels next year, then survey for young mussels in the fifth and 10th year of the project.

It's not just an academic exercise. Both eels and mussels are critical ecosystem components.

Lellis has estimated that in the neighboring Delaware River, an estimated 250 million eastern elliptio mussels have the potential to filter 2 billon to 6 billion gallons of water and remove 78 tons of sediment from the water each day.

If so, eels might prove to be an even more important cog in the river system than even Leopold might have imagined. "I think it's going to have a huge impact on the river," Minkkinen said.

Besides serving as temporary hosts for hitchhiking mussel larvae, eels also become top predators of freshwater streams as they age and transform into "yellow eels" with lengths of 2–3 feet. Crayfish are an important prey source, and Minkkinen speculates that restored eel populations could help control the invasion of rusty crayfish from the Midwest, which are outcompeting native species.

Still, the task of returning eels throughout the river would be a huge job. They would need passageways over dams on the river: Those built for shad are "completely ineffective" for eels, Minkkinen said. Small eels migrate where the flows are weakest, while adult shad migrate where currents are strongest.

And once eels get upstream, there is the problem of getting back downstream. Unlike anadromous shad, which live most of their lives in the ocean but return to freshwater to spawn, eels are catadromous and will live most of their lives -10 to 20 years - in freshwater and make a one-way trip back to the Sargasso to spawn.

While juvenile shad can migrate through turbines of hydroelectric plants with minimal problems, it's a different issue for a 3-foot-long adult "silver eel."

Discussions about whether, and how, to provide passages for eels are part of ongoing relicensing agreements with the hydroelectric dam owners on the river. "They are natural parts of the ecosystem," Minkkinen said. "And if they weren't blocked by dams, they would be swimming here on their own."

One July morning, Minkkinen and Park drove a batch of more than 4,390 eels north to be released into Buffalo Creek - ironically named for another species that's been extirpated from the Susquehanna basin.

A couple dozen people, including members of a local watershed group and local media turned out to watch as a large hose was hooked to the blue tank filled with eels.

A valve was opened, and the water - and eels - drained into the creek. The last step of their journey had taken only a minute.

Except, that is, for a few stragglers that were still lingering in the tank and had to be flushed out with buckets of water. As the water drained out the bottom, they were trying to climb the slick walls of the tank - still trying to fight their way upstream.

USF&WS to review status of American eel

The U.S. Fish and Wildlife Service is planning to re-examine whether American eels should be protected under the federal Endangered Species Act.

The service rejected such protection in January 2007 after reviewing the issue, contending that while the East Coast eel population was declining in some places, its overall population appeared to be within the range of natural fluctuation.

But after reviewing a new petition calling for listing, this one from the Council for Endangered Species Act Reliability, a nonprofit environmental advocacy group, the service said on Sept. 28 that it was launching a new review of the eel's status. A recommendation is due in one year.

The service states that new information presented in the petition raised questions about the health of the eel population. Of particular concern were recent studies which suggest climate change might be affecting eels, both by shifting their spawning location within the Sargasso Sea, and by altering the currents that eel larvae use to help carry them to the coast.

The research cited in the petition indicated that such factors may be affecting the European eel, which also spawns in the Sargasso. The researchers suggested similar impacts might be felt by the closely related American eel.

Although the service's 2007 review had concluded that the overall eel population was stable, many researchers believe the population has been declining in recent decades. Surveys conducted in the Chesapeake have shown a sharp decline for more than a decade. And a stock review by the Atlantic States Marine Fisheries Commission in 2006 concluded that eel populations were at or near record lows in many areas.

Overfishing, in part to supply foreign demand; poor water quality; infections by nonnative parasites; and dams that inhibit migration to historic habitats are among factors that could contribute to population declines.

Eels are the only catadromous species along the East Coast. They spawn in the mid-Atlantic's Sargasso Sea, but spend their adolescent and adult lives in freshwater. They live for one to two decades, before returning to the Sargasso to spawn.

The eel joins a growing list of Chesapeake Bay fish that are either protected under the Endangered Species Act or being considered for protection.

The shortnose sturgeon is listed as endangered, and the National Marine Fisheries Service has recommended that the Atlantic sturgeon be listed as endangered, with a final decision expected this fall.

The Natural Resources Defense Council this summer petitioned the NMFS to consider listing river herring - blueback herring and alewife - as threatened species.