How scientists are helping mysterious, beneficial eels reclaim Maryland rivers

The young eels thrashing in a bucket at Eden Mill Nature Center faced a pivotal moment in their epic journey.

They had hatched among the thick grasses of the Sargasso Sea, transforming from larvae to tiny, transparent glass eels, then lengthening and darkening as they swam from the Atlantic Ocean up the Chesapeake Bay to the shallows of Harford County’s Deer Creek.

For two centuries, the eels’ ancestors had been stymied by a dam at Eden Mill, dying by the hundreds each year.

But these eels would survive — at least for now.

A newly installed device called an eel ladder, one of the first in the state, had enabled the glistening, finger-length fish to slither out of the water and crawl over the dam to the safety of a waiting bucket.

“We’ve had about 800 come through this year,” said Eden Mill’s park manager, Aimee Dunn. “They’re slippery like worms and very strong.”

The ladder, which was installed in October, resembles a covered sliding board rising up at an angle from the water. Inside, a stream of hose water flows through a tangle of fibers that the young eels grip as they swim upstream.

At the top of the ramp, the eels fall down a pipe into a mesh bag suspended in a bucket of water.

Once a day, park workers and volunteers empty the bag, weigh and count the eels and then plop them back in the water past the dam, where — environmentalists hope — the endangered species could begin to repopulate the tributaries of the bay, fostering a cascade of ecological benefits.

On a recent morning, park employee Chastity Curry peered into the bag to inspect the creatures who had made the climb that day. The eels migrate upstream from late spring to late summer, and are most active during new moons.

Inside were two elvers — eels in the third of five life stages — wriggling like thick, bulbous-eyed earthworms. Despite the resemblance to worms, American eels are a type of fish with elongated, snakelike bodies and truncated fins.

The population of American eels, which were once ubiquitous in waterways throughout the eastern United States, has plummeted in the past five decades.

Although humans have caught and eaten eels since ancient times, the creatures remain shrouded in mystery, said Michael Wilberg, a University of Maryland Center for Environmental Science professor.

“The most mysterious thing about eels is that there is a lot of their life cycle we’ve never observed,” said Wilberg.

After the elver stage, the creatures grow up to two-feet long and become yellow eels, apex predators of streams, feeding on insects, crawfish and small fish. Then, after a period lasting from eight to 40 years and dependent on unknown factors, the eels change again, turning silver and heading back to sea to breed and die.

Since antiquity, humans have marveled over the mystery of eel reproduction, since yellow eels don’t have external genitalia. Aristotle assumed that eels were generated by the “entrails of the earth.”

Modern scientists long suspected that both American eels and their cousins, the European eels, spawned in the Sargasso Sea, a two-million square mile patch of aquatic grasses bound by currents in the Atlantic Ocean, but were unable to prove the theory until 2022.

Humans have yet to witness eels breeding, perhaps because the process transpires on mountains jutting up from the ocean floor, Wilberg said. Somehow the American and European eels, despite sharing the same watery love shack, manage to not mix genes.

“We don’t know how they don’t interbreed, but they don’t,” Wilberg said.

It’s also unclear how American eels pick which waterways to migrate to as juveniles, but it does not appear that they head to their ancestors’ territory, he said.

What is clear is that the eels bring a bevy of environmental benefits to freshwater habitats. As young elvers, they are an important food source for many fish, aquatic mammals such as otters, and even other eels.

“They’re both predator and prey at different life stages,” said Alex Vidal, a U.S. Fish and Wildlife Service aquatic conservation biologist, who helped secure about $75,000 for Eden Mill’s eel ladder.

As adults, eels play an even more important, if indirect, role in preserving the ecosystem.

“This is really going to blow your mind,” said Jim Thompson, a fish passage coordinator for the Maryland Department of Natural Resources, who helped bring the Eden Mill eel ladder to fruition.

A native mussel, the Eastern elliptio, is one of nature’s most powerful water filters, and a healthy population can fully filter a body of water six times a day, Thompson said. But the young mussels must mature on the gills of a fish, and their preferred host is the American eel.

Because the eels traverse a lot of territory and are bottom-feeders, they are particularly good at spewing baby mussels along riverbeds, Thompson said.

“If you could filter out the Susquehanna six times a day, imagine how much cleaner the Bay would be,” Thompson said.

The population of American eels has dropped by 90 percent since the 1970s in some rivers, said Wilberg, the UMD professor. Pollution, habitat loss, and especially hydroelectric dams, are the presumed causes, he said.

Thompson, who calls himself a “dam buster,” works on identifying and removing defunct structures. But some dams, like that at Eden Mill, can’t be removed without major changes to the surrounding areas.

Larger eel ladders, which were constructed at Conowingo Dam in 2008 and Octoraro Dam in 2015, have helped more than 3.4 million eels navigate the Susquehanna, according to USF&WS data. In October, Constellation Energy pledged $28 million to improve fish and eel ladders along the Susquehanna.

A smaller, more DIY eel ladder traverses the Patapsco River at Daniels Dam near Ellicott City, Thompson said.

Unlike the passageways along the Susquehanna, Eden Mill’s eel ladder is open to the public. The project has drawn a lot of interest from visitors who are eager to learn about eels and aid their comeback, Dunn said.

“Everyone I see around town asks me about the eel ladder,” she said.