Determining the scope of impact: Evaluating the indirect effects of American eel stocking on eastern elliptio populations throughout the Susquehanna River Basin

Elaine Langone, Forum

Co-extinction among parasitic and symbiotic species is thought to be a leading cause of biodiversity loss. Freshwater mussels are one of the most globally imperiled faunas due to a variety of factors, including loss of their host fish which are needed for developing larval mussels to metamorphosis to free-living juveniles and adults. The U.S. Geological Survey Northern Appalachian Research Laboratory (NARL) and Maryland Fish and Wildlife Conservation Office (MDFWCO) have been examining the relationship between the common eastern elliptio mussel (Elliptio complanata) and its migratory host fish, the American eel (Anguilla rostrata), for over 10 years. American eel distribution has been severely reduced along the entire Atlantic coast in part by dams blocking their migration, which has resulted in cascading effects through the ecosystem. Research completed by NARL and MFRO has found that eastern elliptio populations have declined in abundance over the last 20 years and are experiencing limited recruitment (i.e. reproduction). In 2009 NARL and MDFWCO began an experimental stocking study to re-introduce the American eel to 2 sites in the Susquehanna River basin and assess the potential impacts on mussel recruitment. Five years after stocking we found that restoration of host fish improved recruitment, but results were not equivalent between stocking sites. While initial results show that mussel recruitment has improved at some American eel stocking sites, it was unknown if and how far these benefits extend outside of the direct stocking locations. Reports of stocked eels were observed to have traveled up to 80 km from initial stocking locations. Follow-up freshwater eel and mussel surveys throughout the basin were conducted in the summer of 2018 to determine how eastern elliptio populations have changed at previously surveyed sites over a period of 10 years and if populations outside of direct stocking locations experienced increased juvenile recruitment related to eel stocking in the watershed.