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Reuniting Eels and Mussels May Unlock Water Quality Improvements in the Susquehanna River

September 06, 2017

Reuniting Eels and Mussels May Unlock Water Quality Improvements in the Susquehanna River

Eastern elliptioA revolutionary partnership between federal agencies is capitalizing on an evolutionary partnership between freshwater mussels and American eels to improve water quality. Through the agencies’ work, both species are back in the Susquehanna River watershed.

Most people think of eels as snakelike, slimy, and maybe even scary. American eels, native to the Susquehanna River, are indeed long, like snakes, and slimy to the touch. But luckily for swimmers, they are shy and have teeth barely larger than sandpaper.  And luckily for everyone who depends on clean water, they are making a comeback.

Eels are a migratory fish species that start and end life in the Atlantic Ocean. Baby eels are brought to our shores by the Gulf Stream current. They then swim up our freshwater streams, where they stay up to 20 years before heading back downstream to the Atlantic Ocean to spawn. 

While in residence, eels serve as predators and prey, as well as partners for freshwater mussels. Freshwater mussels live in streams and filter our water, but rely on the presence of fish in the ecosystem to reproduce. Many mussel species evolved to rely on specific fish species in a symbiotic relationship. 

The relationship is complex. Mussels lure in fish by displaying realistic copycats of mayflies, crayfish, or minnows that are really flaps of tissue made by the mussel.  The fish thinks this lure looks like food, but when it takes a bite, the mussel shoots out a stream of tiny larvae that attach to, or “infect,” the fish’s gills. Tiny mussel larvae hitch a ride on the gills of fish until they mature into baby mussels. 

Eastern elliptio larvaeIf the right fish isn’t present in the stream to serve as a temporary host for the larvae, the mussels cannot reproduce, and populations die off. The most common freshwater mussel in the Susquehanna River, eastern elliptio, needs the American eel.

Eels are amazing at getting where they want to go, sometimes climbing up sheer walls. However, the 90-foot-high Conowingo Dam, near the mouth of the Susquehanna River, is insurmountable. Dams in the Susquehanna River started blocking eels from swimming upstream around 90 years ago. This curtailed a lucrative commercial eel weir fishery. Most of us have never eaten an eel, but for our ancestors who lived along the Susquehanna and Delaware Rivers, eels were regular supper fare.

When eels stopped swimming up the Susquehanna River, the eastern elliptio mussels in the river and its tributaries stopp​ed making babies. Scientists estimate that most of the remaining eastern elliptio mussels in the watershed are over 75 years old. 

“The eel/mussel relationship and the sediment and nutrient cycling ability of the mussel is a great example of the ecological services a functioning stream can provide,” said Jason Fellon, Watershed Manager in DEP’s North-central Region. “Mussels can very effectively reduce the pass-through of nutrients to downstream reaches.”

To determine if reintroducing eels would help these mussels reproduce and re-establish in the Susquehanna River watershed, beginning in 2010, the U.S. Fish and Wildlife Service and the U.S. Geological Survey collected eels from below the Conowingo Dam, transported them upstream, and stocked approximately 120,000 eels in Buffalo Creek, Union County, and 120,000 eels in Pine Creek, Tioga County.

P8300438.jpg“DEP staff performing field work in the West Branch Susquehanna River and its tributaries have seen a dramatic increase in eel populations in recent years, including two- to three-foot long adults,” said Fellon. “These eels appear to be dispersing and finding new habitat.”

As researchers hoped, the return of the eels has sparked a return of the mussels. After five years, the number of juvenile mussels found in Buffalo Creek doubled, and the number of juveniles found in Pine Creek went from zero to 25% of the total eastern elliptio mussel population. 

Mussels born today will spend the next 80 or more years filtering nutrients and sediment from our water. As mussel populations increase, they will essentially increase the capacity of the watershed to heal itself, helping Pennsylvania meet its federal obligations to restoring the Chesapeake Bay. 

“The increased presence of these mussels, through their filtering ability, will allow more nutrients to be stored or converted to forms that benefit other aquatic life, ultimately improving water quality,” said Fellon.   

Exelon Energy, owner of the Conowingo Dam, is now required to truck eels around four large hydropower dams and release them in the Susquehanna River. Each American eel’s arrival upstream is a step toward restoring the ecosystem and improving water quality for future generations.

Thanks to all of the contributors to this project!

  • Maryland Department of Natural Resources
  • Pennsylvania Fish and Boat Commission
  • City of Sunbury
  • U.S. Army Corps of Engineers
  • Exelon Corporation
  • Bucknell University
  • Pennsylvania Game Commission
  • Normandeau Associates
  • Tiadaghton Audubon Society
  • Merrill W. Linn Land and Waterways Conservancy
  • Susquehanna River Basin Commission
  • Pennsylvania House Redevelopment, LP

Julie Devers is a fish biologist with the U.S. Fish and Wildlife Service, in Annapolis, MD.​​​


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