Biologists are hoping that a group of pathfinding eels will show the way for improved upstream migration on parts of the Potomac River that have been largely off-limits to the slithery fish for nearly two centuries.
In early May, they released 50 eels about a third of a mile below each of two dams upstream of Harpers Ferry. Each had a tiny transmitter inserted so that biologists can track their movements. They especially want to learn the exact location where the eels approach each dam — effectively pinpointing the spots where eel passages are needed and should be built.
The two dams — Dams 4 and 5, both near Williamsport, MD — were built in the 1830s to divert water for the adjacent Chesapeake and Ohio Canal. Although a small number of eels continue to slither out of the water and over or around the dams, the structures have largely blocked more than 17,000 miles of upstream habitat since their construction.
That’s about to change. David Sutherland, a biologist with the U.S. Fish and Wildlife Service Chesapeake Bay Field Office, said the service will seek proposals for the construction of an eelway at Dam 4 in early June. “We have every intention of construction at the end of summer or early fall,” he said.
That passage, expected to cost between $150,000 and $165,000, would be the first step forward to improve eel passages in the Potomac basin in nearly 15 years.
In the early 2000s, Allegheny Energy, as part of a relicensing agreement, completed eel passages at all of its hydroelectric dams on the Shenandoah River, which converges with the Potomac at Harpers Ferry.
But the river still remained largely closed to eels trying migrate farther up the Potomac. At that time, Allegheny Energy contributed money for passages at Dams 4 and 5, where electricity is also generated. But those historic dams are owned by the federal government as part of the C&O Canal National Historical Park, and the government never came up with enough matching money to build the eel passages.
Sutherland said the National Park Service and Fish and Wildlife Service have now come up with the funding to complete the long-delayed eelway at 18-foot-high Dam 4. “The sun, the moon and the stars I think are starting to come into alignment,” said Sutherland, who has been working on the project since it was envisioned nearly two decades ago.
That passage will give eels access to some sizable tributaries, including Conococheague Creek, a 100-mile waterway that stretches north into Pennsylvania.
But Dam No. 5, about 22 miles upstream of Dam 4, still blocks most of the 17,000 miles of the river’s upstream tributaries, Sutherland said.
Right now, he said there is no money to build a passage at Dam 5, and biologists are concerned it could be more than twice as expensive as the passage at Dam 4, largely because they believe the migratory path eels take upstream is near the electricity-generating powerhouse, which would require more work to avoid.
Sutherland, though, held out hope that the eel-tracking effort could lead biologists to a less costly route over the dam.
Meanwhile, Sutherland said that after years of inaction, efforts are also under way to improve eel passage around the intakes to the Washington Aqueduct, just outside the District of Columbia.
Biologists from the Fish and Wildlife Service and Army Corps of Engineers, which owns the aqueduct that supplies water to the DC region, are working on designs that would improve upstream and downstream migration.
The dam that crosses the river at the aqueduct impedes but does not entirely prevent eel migration. Biologists have been concerned that the eels migrating upstream are bypassing the aqueduct through the canal, where they are easy pickings for predators, and that those migrating downstream are being drawn through water intake pipes.
Once abundant, eel populations are near historic lows along the Atlantic Coast. Concern about eels — and getting them upstream in major rivers — has been growing in recent years.
Eels spawn in the Sargasso Sea in the Atlantic, but their larvae drift on ocean currents and are dispersed along the East Coast, with many migrating far up rivers where they spend most of their lives — often a decade or more — before returning to the Sargasso to spawn.
Getting eels beyond dams is important because those that swim farther upstream tend to become the largest females and can contribute the most eggs to the spawning stock. Eels also serve as essential hosts to early life stages of certain mussels, so their presence can bolster the populations of water-filtering bivalves.
As a result, eels are considered a priority species for restoration, especially for fish passage efforts, under the 2014 Chesapeake Bay Watershed Agreement.