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Eels And Mussels - An Unlikely Pair - Benefit Pennsylvania's River Ecosystems

By Kelly Arcieri, York County Master Watershed Steward

Eels and mussels seem like an unlikely pair. American eels are born in the Atlantic Ocean near Bermuda and migrate to freshwater to mature into adults. These catadromous, slithery fish are generalist predators that cover thousands of miles over their lifespans.

Mussels, on the other hand, are mostly sedentary filter feeders who live in the same section of river for decades. In fact, these two species enjoy a symbiotic relationship that benefits the ecosystem.

After hatching in the salty waters of the Sargasso Sea, the baby “glass eels” begin the arduous migration to freshwater habitats, swimming up the Delaware and Susquehanna Rivers.

They are tenacious about reaching their destination, sometimes scaling shear walls.

After spending approximately twenty years in freshwater rivers and streams, the adults migrate back to the Sargasso Sea to lay their eggs and die.

The damming of the Susquehanna has impeded the migration of the American eel.

While in our waters, the eels serve as willing partners for mussels. Once eggs are fertilized, female mussels lure fish with flaps of tissue that mimic macroinvertebrates.

When the fish gets close enough, the mussel shoots a larval stream of glochidia which attach to the fish. The baby mussels then get a ride to other parts of the watershed.

Once they mature, the mussels drop off the fish and rest on the river or stream floor where they filter water for the next eighty years.

Each mussels’ amazing ability to filter about a gallon of water a day makes them an integral partner in cleaning the water of the Chesapeake Bay watershed.

The most common mussel in the Susquehanna watershed, the Eastern eliptio, prefers to hitch a ride on the American eel.

In the early 2000s, scientists monitoring the Bay determined that the mussel population was aging. With the decrease in the American eel population, the baby mussels were not finding their way to the upper parts of the watershed.

A program was started to help the American eel bypass the Conowingo Dam. Scientists collect juvenile eels from below the dam and relocate them to streams upriver.

The good news is the American eel appears to be making a comeback in the northern tributaries of the Susquehanna River. They have been found as far north as New York and in many northern Pennsylvania streams where they were never stocked.

The theory is that these eels will transport more baby mussels further north, thus improving water quality in the Susquehanna watershed and ultimately, the Chesapeake Bay. The results so far are promising.

[Contact the York County Master Watershed Stewards or any Penn State Extension Master Watershed Stewards county group to see how you can become a Watershed Steward.]

(Photo: Aaron Henning, Senior Aquatic Biologist for the Susquehanna River Basin Commission, measures an American eel during a fish survey in the Conewago Creek, Lancaster County.)

(Reprinted from the Penn State Extension Watershed Winds newsletter. Click Here to sign up for updates from Penn State Extension.)

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