Helping a mysterious and slippery fish return home

"We've had about 800 [eels] come through this year," Eden Mill park manager Aimee Dunn told the Baltimore Banner. "They're slippery like worms and very strong."

Few native species carry as much mystery, or as much ecological weight, as the American eel. The fish and their relatives, European eels, begin life in the Sargasso Sea, a vast, current-bound expanse of Atlantic Ocean grasses, where scientists only confirmed spawning behavior in 2022. Larvae drift toward the coast, transforming first into slender and transparent juvenile "glass eels." As they move inland, the eels turn brown and become known as "elvers." When they reach adult size, another color change leads to yet another name: "yellow eels." After 10 to 20 years living as freshwater fish, these yellow eels receive some kind of signal - still unknown to science - that triggers an amazing transformation. Their eyes grow larger, they stop eating, and they become "silver eels." They swim back downstream, out into the Atlantic Ocean and across thousands of miles to gather in the Sargasso Sea, where they spawn and die.

Less mysterious is the American eel's important and well-established place in the freshwater food web. During different stages of their lives, they serve as both predator and prey, consuming insects, crayfish and small fish while also providing food for larger fish, birds and mammals. But one of their most valuable ecological contributions is their relationship with the eastern elliptio, a native freshwater mussel species that helps filter and improve water quality. Eels host the larvae of the eastern elliptio. As the eels move throughout a watershed, they help distribute mussels to new habitats, supporting healthier streams and rivers.