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The USGS Northern Appalachian Research Laboratory is a science research facility located in Asaph, PA. One of the primary researcher questions that we have been addressing for the last 10 years is the decline of freshwater mussels, with particularly focus on the eastern elliptio mussel in Pine Creek and throughout the Susquehanna River Basin. The United States is home to nearly 300 species of freshwater mussels that can be found in streams, river, and lakes. Mussels are important to our stream ecosystems for a variety of reasons. They are filter feeders, meaning they reside at the bottom of the river and filter particles (like sediment and algae) out of the water. Because mussels are often found in dense beds with 10’s of thousands of individuals living together, their filtration can be important for maintaining stream water quality. They also provide habitat to other aquatic species (algae and bugs) that serve as the base of the food web for many stream fish.

Freshwater mussels have a complicated and unique life cycle.  When mussel larvae (babies known as glochidia) are newly released from female mussels, the glochidia have to attach to a host fish’s gills or fins. The fish’s body builds a cyst around the baby mussel who is protected while it metamorphoses into a juvenile mussel, complete with organs and a foot. Once the mussel metamorphoses into a juvenile it can fall off of its host fish and live on its own in the bottom of the river, growing to be as old as >160 years (in some cases); we know this because mussel shells have growth rings just like trees that allow us to figure out how old they are. The kind of fish required for each mussel species varies: some species can use many different host fish species while others can only use one or a few species.  The common eastern elliptio mussel in the Susquehanna Basin needs one particular fish host for its babies: the American eel.

USGS researchers have been conducting widespread surveys for eastern elliptio throughout the Susquehanna River drainage. One thing they noticed was that the large majority of eastern elliptio mussels were old: very few (and in some cases no) juveniles could be found. The USGS researchers hypothesized that this might be due to the fact that American eels have been eliminated from the Susquehanna River. This is partly because eels also have a unique life history. Baby eels are born in the Sargasso Sea (near the Bermuda triangle). They are carried by ocean currents to the streams and rivers along the east coast and migrate into freshwater where some individuals can live for 20 years or more. While they live in freshwater, the eels help eastern elliptio mussels reproduce, serve as important top predators in our aquatic ecosystems, and are even a valued food fish in many places. Then, adults migrate back to the Sargasso Sea to spawn and die.
  
Unfortunately, many dams (and other migration barriers) are preventing eels from reaching freshwater habitat along the entire east coast. Because eels are unable to navigate to our streams and rivers, eastern elliptio babies cannot develop into juveniles, which is why we have not been able to find young mussels in our surveys. USGS researchers have been working closely with their US Fish and Wildlife Service partners as well as with local groups to experimentally reintroduce the eel to Pine Creek to evaluate whether or not this could improve eastern elliptio populations. Our initial findings suggest that this strategy can be successful! After several years of eel stocking, we have been able to locate juvenile mussels in Pine Creek. Unfortunately, this does not seem to be the end of the story. While eel stocking was successful in Pine Creek, in other nearby creeks (namely those with poorer water quality) we did not see a dramatic increase in juvenile eastern elliptios. We do not know why yet, but that is next on our list of things to investigate– right along with ways to sustainably restore American eel populations to the streams and rivers along the east coast.