Across Pennsylvania, fishermen thought 1904 was a tough year for eels. Of the 316 licensed eel fishermen, the third highest catch came from a Lancaster County man who trapped 8,000 eels, weighing just 1,928 pounds.

"Complaints are made this year that owing to the lowness of the streams the run of eels was very small and the catch not nearly up to the average," said a report from the state Department of Fisheries.

The fishermen had no idea how bad things could get.

Twenty-four years later, the Conowingo Dam sealed the fate of eels on the Susquehanna, blocking all but the first 10 miles of the river to migratory fish.

But last year, for the first time in decades, American eels were back in Lancaster County. They came in cattle tanks, loaded onto the back of pickup trucks.

Biologists monitoring eels at Conowingo caught so many they decided it would be a waste to toss them back into a river closed to migration. They secured a small grant from the U.S. Fish and Wildlife Service and hauled them to Conestoga Creek instead, ultimately releasing about 20,000 small elvers that measured 4-6 inches long.

If funding is available, they hope to move 100,000 this summer. Eventually, they hope to see "eelways" built at the four hydroelectric dams located in the first 55 miles of the river, potentially allowing millions of eels to slither upstream.

"I'm convinced it's the most important management action that we could do for American eels," said Steve Minkkinen, head of the USF&WS Maryland Fisheries Resource Office. "I just think the impacts could be so far-reaching."

He and others think bringing back large numbers of eels will trigger the recovery of depressed native mussel populations, and even help control the invasion of the nonnative rusty crayfish.

The eels themselves also need help.

The Atlantic States Marine Fisheries Commission-a state-federal agency that regulates migratory fish along the East Coast-recently reported that eel population is "at or near" record low levels.

Federal agencies even reviewed whether it should be considered for threatened or endangered species status. That review, completed in February 2007, concluded that the overall eel population was not in danger of extinction anytime soon, but said eel numbers had declined, and loss of upstream habitat could be a contributing factor.

Eels are the only catadromous species in North America. That means, Minkkinen said, "they do everything backward." Unlike anadromous fish such as shad, which spawn in rivers but spend their lives in the ocean, eels spawn in the mid-Atlantic Sargasso Sea, but live most of their lives in streams, often moving into the farthest headwaters.

Eel numbers were once so great in many streams that they often accounted for one-quarter of all fish biomass.

It's unclear why eels have declined. Theories include overfishing, loss of habitat, infections by a nonnative parasite and changes in ocean currents that transport larval eels to coastal rivers. Scientists believe a combination of factors could be involved.

Habitat lost to dam closures was likely a significant "body blow" to eel populations, said Leonard Machut, who now works at the Virginia Institute of Marine Science but previously studied eels in New York. "You are losing just huge tracts that are no longer available to eels," he said.

In some places, eels can still get past dams. Tiny, transparent "glass eels" can climb over small structures, or even slither around obstacles through wet grass.

In a study of the Hudson River watershed, Machut found the presence of barriers was the dominant factor in predicting upstream eel abundance. "There was roughly a 90 percent decrease in the population abundances from just below a migratory barrier to above that same barrier," he said. When a river has multiple dams, that cumulative toll can greatly diminish upstream populations.

There is no shortage of dams in East Coast rivers. A study cited in the endangered species review counted 15,115 in rivers between Maine in Florida. As a result, access to 84 percent of the potential historic stream habitat was either impeded to some degree, or prevented altogether.

But lost habitat is only part of the problem caused by dams. The sex of an eel is not determined until later in life, and research suggests that eels in areas with dense populations tend to be males, while those that reach sparsely populated headwaters are almost exclusively females.

By congregating eels below dams, the structures may skew more eels toward becoming males, and limit the number of females. In addition, eels trapped downstream are often in poor condition, possibly because of increased competition with other eels for food and habitat.

Eels may well be crowded below Conowingo, where Minkkinen believes "millions and millions" of eels bump up against the dam in a fruitless migration, before dispersing into downstream creeks.

Minkkinen said surveys by the Maryland Biological Stream Survey in small tributaries below Conowingo showed eel numbers in "an order of magnitude higher than there are in any other tributary in Maryland."

"It's pretty phenomenal when you go electro-shocking in those tributaries," he said. "You hit the switch and there are eels everyplace."

In October, when the ASMFC considered efforts to protect eels, the only recommendation it made was to improve passage at dams. Other actions were put off until a new population assessment is completed.

The ASMFC called on resource agencies to seek "special consideration" for upstream and downstream eel passages at hydroelectric dams during the Federal Energy Regulatory Commission's relicensing process.

At smaller dams not licensed by FERC-95 percent of all dams -ASMFC said states should seek to remove the structures where feasible. If not, they should try to improve passage.

That's already happening in the Bay watershed, as state and federal agencies and utilities have been removing, or building eelways over, dams on the Potomac and Shenandoah rivers.

But when it comes to reopening habitat, the big prize is the Susquehanna, the East Coast's largest river, which drains 40 percent of the Bay watershed. Three of the four large hydroelectric dams on the river come up for relicensing in 2014, and biologists hope to make eelway construction a condition of the license.

FERC relicensing was used to secure the construction of $100 million in fish passages for American shad in the 1990s, but that effort has had difficulty moving significant numbers of shad upstream. In fact, the 20,000 eels stocked in the Conestoga last summer far exceeded the 2,795 shad that got that far.

But moving shad is more problematic. They have to be lured into elevators which physically lift them over the dams.

Biologists think eels passage would be more successful. Eelways are relatively inexpensive to build, with costs running in the tens of thousands of dollars rather than millions. Instead of being lifted over dams, eels can slither past them if given the means to do so.

"Fish passage for eels is not nearly as expensive or as complicated as it is for American shad," said Mike Hendricks, a fisheries biologist with the Pennsylvania Fish and Boat Commission. "The structure you need is much simpler, much lighter. It is a whole different order of magnitude in terms of what you have to do."

Eels are willing to follow a small trickle if it gets them upstream. Biologists monitoring at Conowingo captured eels with a small stream of water flowing down rip-rap near the base of the dam. This summer, nearly 50,000 small eels followed that stream up nearly 60 feet of rip-rap, into the scientists' collection trays-jerry-built devices made from aluminum cable trays that builders use to enclose electrical wires that run through office buildings.

Of those, 20,000 were hauled north to the Conestoga, where their growth and survival will be monitored for several years.

"We're trying to get some impression about what kind of impact they're going to have, reintroducing them to tributaries up in the watershed," Minkkinen said.

Scientists believe reintroducing eels may be especially important in the Susquehanna. Laboratory work by the U.S. Geological Survey's Northern Appalachian Research Laboratory indicates that the survival of a 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 it can transform into the juvenile life stage. Some mussels can use multiple fish species as hosts, while others are specialists. Studies at the lab 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, but less common in the Susquehanna. That was reaffirmed last year in surveys conducted on the Susquehanna several miles upstream of the Conestoga.

"What's really striking is the size distribution of elliptios up there," Minkkinen said. "As you get above the dams, the elliptios are huge, and there are no little ones."

In the adjacent Delaware watershed, elliptios are range from 10 millimeters to 100 mm. "In the Susquehanna, we never saw one less than 70 mm, and most of them were 90 or 100," Minkkinen said. "It suggests that for some reason, there is a complete lack of reproduction. And potentially, that could be host-driven."

Another benefit, Minkkinen said, might be the control of rusty crayfish. The large, reddish crayfish from the Midwest have recently invaded Pennsylvania waterways-probably transported as bait. They are large and aggressive, quickly pushing out native crayfish, and potentially altering the food web in freshwater streams.

Eels have voracious appetites for crayfish, and are large enough to take on the invader. "Eels are going to chow down on those things," Minkkinen said.

A major question, though, is whether eels that move up the Susquehanna will contribute to the lagging spawning stock. When they mature and move downstream, which could be 10 years or more after reaching the river, outgoing eels would likely be around 3 feet long.

Getting big fish past four hydroelectric dams with lethal, power-generating turbines, could be much harder than getting 6-inch eels upstream. "If you start with 100 eels at Harrisburg, and you have 15 percent mortality at each dam, you end up with roughly half of the eels that started out being dead," Machut said. "This cumulative mortality, I think, is going to be very important in the Susquehanna."

Sheila Eyler, of the USF&WS Maryland Fisheries Resource Office, has been studying outmigration of eels at dams on the Shenandoah, where power companies have agreed to shut off generation during the night, when eels move. With the turbines turned off, the eels can spill over the tops of the relatively small dams, which seems to protect them on the Shenandoah.

But Eyler has found that dams may create another problem-they seem to delay outmigration for some eels. Usually eels leave rivers in the fall on their journey to the Sargasso Sea. Eyler found some moving past the last dams as late as May.

That could remove them from the spawning population as effectively as a turbine. "We don't know it for sure, but we suspect it's an issue," she said.

The challenges on the Susquehanna dams could be more daunting. Water doesn't flow over the top of most of them, and even if it did, most are high enough that the fall could be lethal. The dams are also more significant power producers than those on the Shenandoah, so simply shutting them off could be an issue.

Minkkinen said that if biologists can better understand the environmental cues that trigger outmigration, they can identify a narrow window of time, and an alternate mechanism, to get eels past the Susquehanna dams.

While some mortality may occur, he still sees benefits. "They are all going to be egg-bearing females, and those are the fish that are very, very valuable to the population," he said.

If so, then last year's trucking program may eventually be viewed as the first small step toward a healthier Susquehanna-and a healthier eel population. "There is no better way to increase spawning stock biomass than to allow these animals access to the headwaters," Minkkinen said, "because all of those fish that go upstream are going to become females."

If successful, the eel runs on the Susquehanna may someday be described the way a typical run was described in the 1904 report: "Simply enormous."