Stream erosion may contribute to decline of American eels

Research by USDA Forest Service Southern Research Station (SRS) scientists and collaborators suggests that erosion in small freshwater streams could be a contributing factor in the decline in American eel populations recorded over the last two decades.
The observations are a result of a project by the SRS Coldwater Streams and Trout Habitat unit in Blacksburg, VA, the Virginia Department of Game and Inland Fisheries, and the George Washington and Jefferson National Forests to learn more about the American eel in its freshwater habitat.

The American eel (Anguilla rostrata) is found over a very large geographic range that extends from Greenland to South America. Though scattered so widely, American eels spawn in only one location–the Sargasso Sea in the middle of the North Atlantic Ocean. After hatching, eel larvae drift for months on ocean currents, finally making their way to coastal areas. Now known as glass eels, they move into freshwater estuaries where they develop into the elvers that migrate up streams and rivers to transform first into yellow eels, then into the mature silver eels that migrate back to the Sargasso Sea to spawn and die. Because it actually spends most of its lifespan in streams, the American eel is considered a freshwater fish.

Since the mid-1970s, numbers of American eels have been declining in both Canada and the U.S., prompting concern over the status of this species. Although eels have historically occupied all of the Atlantic watersheds, little is known about their seasonal behavior or distribution patterns in headwater mountain streams. Barriers to headwater habitats have been identified as a possible factor in eel decline.

This past summer, SRS researchers and their collaborators began assessing the abundance, habitat use, growth, and activity of American eels in the headwater tributaries of the James and Potomac rivers in Virginia, using radiotelemetry to track the daily and seasonal movements of individual eels along a stream network. Preliminary results show that the daily activity of eels is strongly influenced by seasonal changes, and somewhat unexpectedly, that the eels actually winter in the smaller headwater streams.

The scientists found that the eels were most active during the 3 to 5 hours just after sunset in the summer months; during the fall, activity was much more sporadic and varied more between individuals. Although researchers expected the eels to move out of the smaller streams into larger, deeper streams for the winter, the radiotelemetry studies showed that the eels actually spent most of their time underneath the boulders and undercut banks of the headwater streams, moving little.

These findings have important implications for how the streams that provide habitat for American eels are managed. High sediment loads from flooding or erosion could fill in the cracks and undercut banks occupied by the eels during the winter, smothering them or forcing them to migrate to less desirable habitats.