Small American eels trying to make their way up the Potomac River drainage last year found something unique in the Bay watershed—a fish passage of their very own.

While dozens of fish passages have been constructed on Bay tributaries, the movable aluminum structure over the Millville Dam on the Shenandoah watershed is the first “eelway” constructed in the Chesapeake watershed.

Biologists say it may be the first eelway built in North American outside Canada.

The $75,000 structure was built and installed after officials with Allegheny Energy Supply, which owns the dam, learned of the success of similar devices in Canada. “We decided, let’s give it a try,” said Charles Simons, a biologist with the utility.

After the device was installed last spring, more than 300 eels were observed passing the dam, Simons said, and more likely went unseen.

The Millville Dam is the first blockage on the Shenandoah upstream of the Potomac. The utility is planning a passage at the next upstream dam, at Warren, in about two years. “We will just keep working our way up the Shenandoah,” Simons said.

The nontidal portion of the Potomac River has not been a major target for fish passages because Great Falls, just north of Washington, created a natural barrier to fish migration. Eels, though, have always been able to slither around the falls as they move upstream. Colonial reports show the presence of eel fisheries in the Potomac beyond the falls.

Some eels have been able to get around the dams on the Potomac, although biologists believe the obstructions also prevent many from moving upstream. “This just offers them an easier passage to find their way upstream,” Simons said.

Allegheny Energy this year is planning to pay for eelways at two downstream dams on the Potomac as part of a relicensing agreement with the Federal Energy Regulatory Commission.

Compared to traditional fish passages, which can cost hundreds of thousands of dollars— and in the case of large dams, millions—to build, eelways are relatively inexpensive. “You don’t have to get into concrete,” Simons noted.

The Millville eelway is a simple, covered aluminum trough about 2 feet wide. Inside are a series of vertical PVC pipes which allow the small eels to slither up the slope. The eelway is placed over the top of the dam to the river below, and is removable.

While the Potomac watershed is the first in the Bay region to get eelways, they may be appropriate in other areas as well, said David Sutherland, a biologist with the U.S. Fish and Wildlife Service’s Annapolis Field Office. He said it’s unknown how effective traditional fish passages are in passing eels upstream, but eels would probably need to be 2–3 years old to get up a traditional fish ladder.

Also, the ideal placement for an eelway may be different from that for a fish passage. Migrating fish are attracted to strong flows as they swim upstream, but eels seek the easiest way over an obstacle. The entrance to the Millville eelway, for instance, is placed in a pool of relatively calm water.

“There is a need for this specialized passage,” Sutherland said.

And while eels can slither around some smaller dams, larger structures may close rivers altogether.

The Maryland Department of Natural Resources’ Biological Stream Survey found that the average density of American eels in the Susquehanna River basin below the 100-foot-high Conowingo Dam was about 500 per square mile. Above the dam, the survey found only one eel at 11 sites surveyed in Maryland.

Extrapolating those figures for the entire upstream habitat, the survey made a “conservative estimate” that the upstream watershed would have supported at least 13 million eels without the dam.