200 Years. 200 Stories. Story 109: “Migrations ”

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photo of electrofishing in a stream
Fisheries biologists Shane Moser (left), Cassie Ornell (center), and David Keller use backpack electrofishing to sample for young eels in Ridley Creek, Pennsylvania.


The American eel (Anguilla rostrata) is a migratory species that spawns in the Atlantic Ocean but spends most of its life in the streams, rivers, estuaries, and lakes of eastern North America. As these eels move from marine to estuarine and freshwater habitats, they transform from transparent ribbon-like glass eels to yellow-colored juveniles. They spend about 20 years in this yellow phase, feeding and growing until they are ready to return to the ocean to spawn. Monitoring eel populations is important because eels are a commercial food source. Eels can be the most abundant fish species in streams.

To understand the status and population trends of this species, the Academy’s Ruth Patrick Chair in Environmental Science, Dr. Richard Horwitz, led an Academy team that assessed eel abundance upstream and downstream of potential migration blocks such as dams in southeastern Pennsylvania. From 2007–2008, Rich and colleagues re-sampled historically sampled sites and compared new data to the historic data. They concluded that there was no consistent trend in the abundance of American eels in southeastern Pennsylvania between 1990 and 2010, except where dam removal increased upstream passage, although there may be a recent local increase in some areas. Monitoring locations of newly settled glass eels has provided information on dispersal of eels, which can be used to improve eel monitoring programs. 

Find out more about the work of the Patrick Center.

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