About 185,000 migrating eels on the Susquehanna River hit the road this summer with the aid of biologists who trucked them around the Conowingo Dam so they could complete their journey up the East Coast’s largest river.

That was fewer than the 275,479 collected and moved last year, but still the second-best haul since biologists from the U.S. Fish and Wildlife Service began trucking eels upstream in 2008.

Steve Minkkinen, who heads the USFWS Maryland Fisheries Resources Office, said the reduced number likely was the result of the late spring. Eels tend to move upstream at night during a new moon phase — when there is no moonlight — and the migration normally occurs in pulses that span several of those phases.

“We just basically had one big pulse of eels,” Minkkinen said. That pulse took place in mid-July, when most of the migration is usually over, “but everything has been two or three weeks late this year,” he said.

The operation could be hugely expanded in coming years, as the so-called “truck and transport” program for eels is part of the Federal Energy Regulatory Commission’s recommendations for improving fish passage at the 100-foot-high dam.

FERC is in the process of issuing a new operating license for the dam to its owner, Exelon. In preparation for that change, Exelon this year gave the USFWS $25,000 to continue its trucking operation.

Right now, eels are only collected at a small ramp built by USFWS biologists on the west side of the river below the dam. But FERC is calling for the construction of more sophisticated collection sites on both sides of the river, each of which would be capable of collecting up to 50,000 migrating eels a day. The eels would then be trucked around Conowingo and three upstream hydroelectric dams before being put back in the river to complete their migration.

It’s hard to know how many eels may be helped upstream with the new system but, Minkkinen said, “it could be in the millions.”

Truck and transport was used to move American shad upstream prior to the construction of fish passages at the dams on the lower Susquehanna in the 1990s. Trucking actually proved be a more effective means of getting shad upstream than the multimillion dollar fish passages built at all four dams in the lower 55 miles of the river. This year, just eight shad got past all of them.

FERC is also calling for an eel passage to be completed at Conowingo by 2030, but — in part because of the experience with shad passage — commission staff also recommended that the truck and transport option be continued past that date in the event that eel passages are not as effective as hoped and because it allows biologists to stock eels in specific watersheds.

Much of the new interest in bringing back eels is driven by recent research that has shown that eels are critical for the reproduction of the Eastern elliptio, a common freshwater mussel. The larval stage of freshwater mussels needs to live for a time on a fish “host” before it drops off and begins to grow on its own.

Laboratory research has shown that eels are the main host for the Eastern elliptio. The Conowingo Dam totally closed the river to eel migration when it was built in 1928–29, and without eels, the Susquehanna elliptios are disappearing, although they continue to be the most numerous mussels in the nearby Delaware River. Young mussels are largely absent from the Susquehanna and there is little sign of reproduction.

That connection led biologists in Minkkinen’s office to begin experimenting with collecting eels below the dam in 2004, which then led to a broader effort to transport them to upstream Susquehanna tributaries to see if reintroduced eels could help spur mussel reproduction.

Since 2008, about 700,000 eels have been trucked upstream and placed in more than a dozen locations. Monitoring shows they are beginning to spread to other areas.

“If we hadn’t started collecting eels at the base of the dam, I’m totally convinced that eels wouldn’t even be part of the license negotiation — that everyone would assume there just weren’t many eels below the dam because the fish lifts weren’t catching them,” Minkkinen said. “You needed a concerted effort to catch eels there, so this has forced the whole issue.”

This year, surveys offered a hint that those efforts may be paying off in Pine Creek, in Pennsylvania’s Tioga County, not far from the New York, where stocking began five years ago.

“We saw a whole bunch of small mussels there,” Minkkinen said. “That’s promising because that’s what you would expect to see through time.”

The transport program could also help the broader eel population, whose numbers have declined so dramatically in recent years they that are being considered for Endangered Species Act protection.

Eels are born in the Sargasso Sea in the middle of the Atlantic Ocean, but live most of their lives in the coastal areas and rivers before returning to the Sargasso to spawn, usually after 10 or more years.

The sex of an eel is not determined until they are several years old, and the farther upstream eels migrate, the more likely they are to become egg-bearing females.

“Basically, all of those eels should become females by allowing them to move upstream beyond all four dams,” Minkkinen said. “It could be the most important management option that you do for eels.”