American Eel
Anguilla rostrata
Illustration by Ethan Nedeau. Gulf of Maine Council on the Marine Environment. 2007. American Eels: Restoring a Vanishing Resource in the Gulf of Maine. www.gulfofmaine.org. Used by permission from artist.
Common name: American eel, common eel, freshwater eel, elver, glass eel, unagi
Scientific name: Anguilla rostrata
Range: The American eel is native to Maine. A wide-ranging species, populations can be found in the Western Atlantic from northern South America all the way to the North Atlantic. Their distribution is entirely based on the oceanic currents that transport the larvae out of the Sargasso Sea (east of the Bahamas and south of Bermuda) where their lives begin.
Identification: American eels have an elongated snake-like body with true jaws, pectoral but no pelvic fins, and skin coated with a layer of mucus. Adult “yellow eels” are olive-green to brown on the top, with yellow-green sides and gray-white bellies. However, upon reaching sexual maturity, they transform into "silver eels", which are black on top with silver-white bellies. Adult females normally grow to between 25 and 40 inches in length, but may grow to up to 50 inches. Males do not get as large as females, usually growing between 18 and 24 inches in length.
Life history: The American eel is the only catadromous species in the Gulf of Maine, which means they spend most of their life in freshwater but migrate to the ocean to reproduce. American eels start life as eggs laid in the Sargasso Sea, which hatch after several days and develop into a larval stage called a leptocephalus. The larvae then drift in the ocean for several months at the mercy of the ocean’s currents. The eels that end up in Maine have been picked up by the Gulf Stream and carried north toward the North American continent.
As they approach the continental shelf, the larvae transform into miniature transparent "glass eels". When the glass eels enter estuaries, develop pigmentation, and prepare to ascend rivers, they become known as “elvers”. This migration can occur anytime throughout late winter, spring, and summer, and can take months (or even years) to complete. The timing of the glass eel migration depends on water temperature. The earliest glass eels enter Maine’s coastal waters shortly after the ice thaws each spring.
Sexually immature adult “yellow” eels live in lakes and rivers for a long time; anywhere from 3 to 40 years. During this time they can grow very large, particularly the females. Then, in the fall, individuals migrate downstream as sexually mature “silver” eels, all the way back to the Sargasso Sea. Their digestive tract degenerates during this outmigration, and spawning occurs in late winter, after which the adults die. However, the specifics of eel reproduction remain a mystery; the depth and exact location is not known, and scientists have never observed spawning behavior in the wild.
From a biological perspective, much is still unknown about American eels. Information is limited about their abundance, status at all life stages, and habitat requirements. Scientists still don’t know exactly where in the Sargasso Sea the eels reproduce, despite decades of research. One thing we do know, however, is that all migratory eels have a single breeding population. Individuals from the Gulf of Mexico, Greenland, and anywhere in between may breed with each other. Thus, there are no distinct watershed or regional “stocks” as there are for anadromous species such as Atlantic salmon.
Ecological importance: Eels are an important prey species for many fish, aquatic mammals and fish-eating birds.
Cultural importance: Fisheries for yellow and silver eels have a long history in Maine, since long before the earliest colonial settlements. Once abundant in virtually all Maine rivers with connection to the ocean, eels were an important food source for residents of Downeast Maine, especially during the fall and winter. Eels were a major component of the traditional Wabanaki diet during the fall, when they were caught in the highest abundances, and in the winter when preserved eels could be relied upon until other species became available in the spring. Eels were most commonly smoked for storage, while their skin was used in medicine or to make tools.
Certain place names in Maine are derived from words or phrases that reference eels. For example, Kenduskeag Stream, which passes through the city of Bangor, was once known as Kadesquit, a word derived from the Maliseet word kat, meaning eel. Kadesquit can be translated to mean “eel weir place”.
Threats: Eel stocks are at or near historically low levels due to a combination of historic overfishing, habitat loss and mortality due to dammed rivers, food web alterations, predation, environmental changes, toxins and contaminants, invasive species, and disease. The effects of dams and pollution, however, seem to be the hardest hitting.
Eels have difficulty getting over dams due to inadequate passage structures, and their downstream journey through hydroelectric turbines often results in severe injury or death. American eels are long-lived, have a high fat content, and spend much of their time near the bottom of rivers and ponds. These characteristics make them particularly vulnerable to persistent heavy metals and other toxic substances that accumulate in sediments.
The American eel population is classified as depleted in U.S. waters according to the 2023 stock assessment, but does not go so far as to warrant protection under the US Endangered Species Act. Given the limited information on their abundance, status at all life stages, and habitat requirements, assessments like these have to rely on factors such as declining harvests to make a recommendation.
Restoration efforts: Today, the Atlantic States Marine Fisheries Commission (ASMFC) closely monitors American eel fisheries, making adjustments to the management program as necessary to ensure stock rebuilding. Restoration of eel populations will require managers to focus on getting eels up and downstream safely.
Similar to other diadromous fish, the best way to help the eel population recover is to remove barriers to their migration, like dams, so they can successfully complete their life cycle. However, in cases where dam removal isn’t feasible or desirable, dams can be retrofitted with fish passage structures, or adjustments can be made to reduce mortality during periods of peak migration, such as by temporarily disabling turbines at night to allow eels to pass safely downstream. Unfortunately, traditional fish ladders, which work for species like alewives, aren’t very effective for eels due to their shape and swimming behavior. Structures designed to promote eel passage usually incorporate a rough surface to help elvers climb over or around dams, but the percentage of eels that are able to make it past such barriers is unknown.
Restoration efforts need to take an informed approach, like focusing on migration barriers that meet certain criteria: dams that are up for relicensing, as well as barriers that restrict access to the largest amounts of high quality upstream habitat or are located in watersheds with few or no barriers further upstream.
Fishery: Eel populations support valuable commercial fisheries as well as small scale recreational and subsistence fisheries. There are two distinct fisheries for eels in Maine which relate to different life stages: A spring (March-May) glass eel/elver fishery, and a year-round yellow eel fishery. Historically, there was also a fall silver eel fishery in September-November, but it was closed in 2014 under the ASMFC’s Fishery Management Plan.
Elvers are caught in tidal streams with hand-held dip nets or fyke nets, funnel-shaped nets made of fine mesh. Dealers buy elvers from individual fishermen, which are then shipped overseas, primarily to Asia. In recent years, market demand for elvers has increased dramatically. Elvers are highly valued in Japan, China, Taiwan, and Korea, where they are cultured and reared to adult size to be sold live, fresh, and frozen. Elvers have become Maine’s most valuable marine resource by weight, fetching over $2000 per pound in recent years and earning Maine nearly $20 million in 2023. Presently, the elver fishery in Maine is regulated using an annual quota of 9,688 pounds, with just 16 available licenses granted through a lottery system. Competition is steep, with more than 4,500 entries submitted in this year's lottery.
Sources:
https://seagrant.umaine.edu/maine-seafood-guide/eel/
https://www.asmfc.org/species/american-eel
https://www.maine.gov/ifw/fish-wildlife/fisheries/species-information/american-eel.html
http://www.downeastfisheriestrail.org/fisheries-then/american-eel-2/
https://www.fws.gov/species/american-eel-anguilla-rostrata
Gulf of Maine Council on the Marine Environment. 007. American Eels: Restoring a Vanishing Resource in the Gulf of Maine. www.gulfofmaine.org.