As states have been grappling with the impact of climate change, Maryland had a sign of hope — its American eel population in the Susquehanna River and Chesapeake Bay reached a record-high in recent years, a sign of improved climate resiliency, according to state officials.

The eels help distribute baby mussels that naturally filter the water and improve the health of the ecosystem. Those mussels act as “amazing water machines of nature,” said Maryland Secretary of Environment Ben Grumbles.

For more than a decade, the state has actively trapped and trucked baby eels upstream and around the Conowingo Dam along the Susquehanna River in Harford and Cecil counties in Maryland. The dam blocks the eels’ typical migration patterns and their access to parts of the watershed. After the efforts produced record eel populations, the state and the dam’s owner, Exelon, are now planning to expand this effort by building a way for the eels to get across the dam themselves.

Eels used to move freely throughout the Chesapeake Bay and the Susquehanna River. They often served as food for Indigenous people and early settlers who lived around the area, said Aaron Henning, a fisheries biologist for the Susquehanna River Basin Commission.

But once the Conowingo Dam was put in place in 1928, it eliminated the migration path for the eels and their ability to enter the system.

“You lose eels, and you’re starting to lose mussels,” Henning said. “So, now you’re losing the ecosystem’s natural ability to cleanse itself to some degree.”

Baby eels act as a carrier for mussels. Once adult mussels are ready to produce, they put out a lure that attracts bigger, more predatory fish, like the American eel. The mussels then shoot up thousands of larvae into the gills of the eels. The mussels are dispersed as the eels swim from the bay upstream to freshwater where the eels must go to grow into adults.

The state and its partners began tracking eel passage near the Conowingo Dam in 2007. At the time, fewer than 50,000 eels had been moving through the dam, according to the state. A few years later,Steve Minkkinen of the U.S. Fish and Wildlife Service and his colleagues built a ramp on the west side of the dam. In partnership with Exelon, U.S. Fish and Wildlife Services began trucking eels upstream between 2009 and 2016. After that, Exelon took over the operation under an agreement with the state.

Eels swim up the ramp, get trapped and are picked up by workers in a truck. Those workers then drive around the dam and move the eels upstream to the Susquehanna River.

“It’s hard to get a fish in or out of a dam,” Henning said. “You’re either going to have to, you know, build something that hopefully they will find and use or in this case, with the eels, you basically move them around with human intervention.”

In late August of this year, the state estimated about 537,000 eels were transported up the dam into the Susquehanna River — a record high in the past decade. The previous record was roughly 293,000 eels in 2013.

“We see this as a very positive trend,” Grumbles said. “And it is a trend that we hope will happen in rivers throughout the Chesapeake Bay watershed, particularly the Potomac River.”

Maryland is planning to add a second ramp on the east side of the dam through an agreement between the state and Exelon. That ramp would allow the eels to get to the Susquehanna River on their own, rather than relying solely on human intervention to relocate them. It’s also working on expanding a mussel hatchery in Brandywine, Md.

Grumbles said he believes the data shows the value of pursuing a holistic approach that relies on more environmentally friendly infrastructure, or “gray infrastructure” — such as gutters, pipes and dams — and nature-based solutions to improve climate resiliency.

“There’s such promise in our ecosystem’s health by relying on nature-based solutions,” Grumbles said.

This story has been updated.