American Eels in the Potomac Watershed

By Nick Walker, NPS Aquatic Ecology Intern

An American eel captured during NPS Inventory & Monitoring work at Catoctin Mountain Park.

NPS/Nortrup

American eels are found everywhere along the Atlantic Coast from Iceland to Suriname and can swim up the Potomac River’s Great Falls! But many aspects of these fish remain poorly understood. They are perhaps one of the most mysterious fish in the Potomac watershed.
In pristine times, American eels were one of the most abundant fish in Atlantic coastal streams. If you could catch and weigh all the fish present then, eel would have accounted for 1 of every 4 pounds! They were a valuable source of food for ospreys, raccoons, herons, and striped bass as well as a staple food for American Indians and early colonists. Today however, American eels have been reduced to less than 1% of their historic levels.

Threats

American eels are declining over much of their native range and the range itself is shrinking. Threats to this species and its habitats include hydroelectric dams, stream fragmentation, pollution, parasites, overfishing, changes in ocean temperature, and changes to the eel’s spawning grounds in the Sargasso Sea.

Eel Biology & Behavior

American eel (Anguilla rostrata) are catadromous fish that can live for 20 years or more in rivers and lakes before migrating to the ocean to spawn. They follow the opposite path of anadromous fish, like salmon, which swim upstream to spawn. Eel spawning grounds are in the Sargasso Sea, a strangely calm and warm area near Bermuda that Jules Verne described as a lake in the Atlantic Ocean because of the lack of wind. After hatching, larvae begin a long, slow journey toward the Atlantic coast. After reaching the mouth of an estuary or stream, larvae metamorphose to finger-size glass eels, then change to elvers, and then into yellow eels. A large portion of the eel life cycle takes place in the yellow phase, when the eels usually migrate upstream toward headwaters. Yellow eels eventually change color to dark green and then go through a final metamorphosis to become silver eels, which is when they become sexually differentiated. As inland distance increases, it is more common to find larger eels and more females. These silver eels migrate back to the Sargasso Sea in late summer and fall, although migration has also been observed at other times.

Potomac Population & Fishing

Despite their resilience, the American eel population has declined steeply since 1980. By 2008, the number of eels in the Potomac River was estimated at less than 2.4 million, with recruitment at less than a fifth of 1980 levels. One of the main reasons for this decline was overexploitation. From 1950 to 2008, the Chesapeake Bay accounted for more than half of the U.S. yellow eel harvest, with 16% of that coming from the Potomac River since 1964. Female silver eels migrating downstream from the Potomac River to the Sargasso Sea decreased by an estimated 94% between 1980 and 2008. At the start of 1992, the Potomac River Fisheries Commission imposed a 15 cm minimum size for American eel in commercial and recreational fisheries. Between 2001 and 2013 however, the Potomac River yellow eel fishery declined from over 200,000 pounds harvested to fewer than 50,000.

Protection Improvements

Fishing for American eel has been restricted in much of North America since 2000. Since then, eel landings have increased across the mid-Atlantic region, with the exception of Delaware and the Potomac River. For the last decade, Maryland fisheries managers have been trying to restore the once-abundant Potomac River eel population. The Maryland Department of Natural Resources and the U.S. Army Corps of Engineers have studied the feasibility of removing dams along the Potomac River to improve fish passage and stream connectivity. Evidence from the Rappahannock River suggests this will have a positive effect on eels—when the 22-foot Embrey dam was removed, eels were able to return to habitats 150 km upstream in less than a year. According to the Atlantic States Marine Fisheries Commission, as of 2000, there were 443 dams on the Potomac River. This reduced the American eel habitat from 28,140 km, the historical length of the Potomac River, to 3,281 km.

Fish Ladders

Eels are able to get through many barriers that other fish cannot. They are even capable of swimming upstream through Great Falls, which has several waterfalls of over 6 meters. Eels are also able to get past barriers over land for short distances. A slimy mucous coating allows them to slither along the ground, especially when it is saturated. That said, eel abundance is still greater in the lower sections of the Potomac than above natural and manmade barriers. Fish ladders may provide an option for passage over such barriers. While eels seek slower current spots to get past obstacles (unlike anadromous fish that seek faster currents), they are also able to make use of fish ladders. For instance, eel distribution was significantly higher in Rock Creek following the 2006 construction of the Peirce Mill Dam fish ladder. If eels are able to get upstream of a hydroelectric dam however, the return migration will present a real challenge since turbine blades can chop up an eel like a blender. One option to avoid this is for dam operators to turn off the turbines at night during migration time, although this can be an expensive solution. The first fish ladder built specifically for eels opened in 2003 at the Millville dam on the Shenandoah River just upstream from Harpers Ferry. It has been described as looking like a long pegboard and allows the eels to slither up and over a dam. Across the Atlantic coast, more than 2,500 miles of streams have re-opened through fish passage projects, with about 1,500 of those miles being re-opened since 2000.

Eels Near You

While eating an eel from the Potomac River may not be the best option—both the Virginia Department of Health and the D.C. Department of Energy and Environment advise against this because of polychlorinated biphenyl (PCB) contamination—there are other ways to get in touch with eels. To look for them in your local stream, take your flashlight and head out on a dark night. Look near rocky crevices where eels like to hide and you too may be able to catch a glimpse of the mysterious fish that swims into our world before venturing back out in the Sargasso Sea.

For More Information

To learn more, visit the NCRN's stream biota monitoring webpage.
This article originally appeared in the Summer 2016 issue of the NCRN Natural Resource Quarterly. Issues are available at: https://www.nps.gov/im/ncrn/quick-reads.htm