The American eel (Anguilla rostrata) is a facultative catadromous eel found on the eastern coast of North America. Freshwater eels are fish belonging to the elopomorph superorder, a group of phylogenetically ancient teleosts. The American eel has a slender, supple, snake-like body that is covered with a mucus layer, which makes the eel appear to be naked and slimy despite the presence of minute scales. A long dorsal fin runs from the middle of the back and is continuous with a similar ventral fin. Pelvic fins are absent, and relatively small pectoral fins can be found near the midline, followed by the head and gill covers. Variations exist in coloration, from olive green, brown shading to greenish-yellow and light gray or white on the belly. Eels from clear water are often lighter than those from dark, tannic acid streams.

The eel lives in fresh water and estuaries and only leaves these habitats to enter the Atlantic Ocean to make its spawning migration to the Sargasso Sea. Spawning takes place far offshore, where the eggs hatch. The female can lay up to 4 million buoyant eggs and dies after egg-laying. After the eggs hatch and the early-stage larvae develop into leptocephali, the young eels move toward North America, where they metamorphose into glass eels and enter freshwater systems where they grow as yellow eels until they begin to mature.

The American eel is found along the Atlantic coast including the tributaries of the Chesapeake Bay, the Delaware River, and the Hudson River, and as far north as the Saint Lawrence River. It is also present in the river systems of the eastern Gulf of Mexico and in some areas further south. Like all anguillid eels, American eels hunt predominantly at night, and during the day they hide in mud, sand, or gravel very close to shore, at depths of roughly . They feed on crustaceans, aquatic insects, small insects, and probably any aquatic organisms that they can find and eat.

Eels were once an abundant species in rivers and were an important fishery for aboriginal people. The construction of hydroelectric dams has blocked their migrations and locally extirpated eels in many watersheds. For example, in Canada, the vast numbers of eels in the St. Lawrence and Ottawa Rivers have dwindled.

Description

thumb|Juvenile eels

thumb|30"-long American eel captured from the Missouri River near [[Yankton, South Dakota|Yankton, SD]]

American eels can grow to in length and to in weight. Females are generally larger than males, lighter in color, with smaller eyes and higher fins. The body is elongate and snake-like. Its dorsal and anal fins are confluent with the rudimentary caudal fin. It lacks ventral fins but pectoral fins are present. The lateral line is well-developed and complete. The head is long and conical, with rather small, well-developed eyes. The mouth is terminal with jaws that are not particularly elongated. The teeth are small, pectinate or setiform in several series on the jaws and the vomer. Minute teeth also present on the pharyngeal bones, forming a patch on the upper pharyngeals. Tongue present with thick lips that are attached by a frenum in front. Nostrils are superior and well separated. Gill openings are partly below pectoral fins, relatively well-developed and well separated from one another. Inner gill slits are wide.

The scales are small, rudimentary, cycloid, relatively well embedded below the epidermis and therefore often difficult to see without magnification. The scales are not arranged in overlapping rows as they often are in other fish species but are rather irregular, in some places distributed like "parquet flooring". In general, one row of scales lies at right angle to the next, although the rows immediately above and below the lateral line lie at an angle of approximately 45°. Unlike other bony fishes, the first scales do not develop immediately after the larval stage but appear much later on.

Several morphological features distinguish the American eel from other eel species. Three morphological characteristics persist through all stages from larvae to maturing eels: the total number of vertebrae (mean 107.2), the number of myomeres (mean 108.2), and the distance between the origin of the dorsal fin to the anus (mean 9.1% of total length).

Distribution and natural habitat

Geographic range

The distribution of the American eel encompasses all accessible freshwater (streams and lakes), estuaries and coastal marine waters across a latitudinal range of 5 to 62 N. Their natural range includes the western North Atlantic Ocean coastline from Venezuela to Greenland and including Iceland. Inland, this species extends into the Great Lakes and the Mississippi River and its tributaries as far upstream as Minnesota and Wisconsin.

Nonindigenous occurrences of this species in the United States were recorded from Lake Mead on the Colorado River and on the Arizona border. It was stocked on a few occasions in Sacramento and San Francisco Bay, California, in the late 1800s. No apparent evidence of survival on these occasions was noted. It was also stocked and unintentionally introduced in various states, including Illinois, Indiana, Nebraska, Nevada, Ohio and Pennsylvania. Stockings of this species also occurred in Utah in the late 1800s, but soon disappeared.

Natural habitat

thumb|right|200px|American eel in Long Pond, [[Littleton, Massachusetts, in 2021]]

Eels are bottom dwellers. They hide in burrows, tubes, snags, masses of plants, other types of shelters. Individuals during the continental stage occasionally migrate between fresh, salt and brackish water habitats and have varying degrees of residence time in each. During winter, eels burrow under the mud and enter a state of torpor at temperatures below , although they may occasionally be active during this period. Habitat availability may be reduced by factors such as habitat deterioration, barriers to upstream migration (larger eels), and barriers (i.e. turbines) to downstream migration that can result in mortality. In 1926 Marie Poland Fish described the collection of eggs that she observed hatch into eels, which she expanded in her taxonomic description of the larval egg development. From there, young eels drift with ocean currents and then migrate inland into streams, rivers and lakes. This journey may take many years to complete with some eels traveling as far as 6,000 kilometers. After reaching these freshwater bodies, they feed and mature for approximately 10 to 25 years before migrating back to the Sargasso Sea in order to complete their life cycle.

The yellow eel is essentially a nocturnal benthic omnivore. Prey includes fishes, molluscs, bivalves, crustaceans, insect larvae, surface-dwelling insects, worms, frogs and plants. The eel prefers small prey animals which can easily be attacked.

Predation

Little information about predation on eels has been published. It was reported that elvers and small yellow eels are prey of largemouth bass and striped bass, although they were not a major parts of these predators' diet. Leptocephali, glass eels, elvers, and small yellow eels are likely to be eaten by various predatory fishes. Older eels are also known to eat incoming glass eels. They also fall prey to other species of eels, bald eagles, gulls, as well as other fish-eating birds. American eels also make up the entirety of the diet of adult rainbow snakes, lending the species one of their common names: eel moccasin.

Commercial fisheries

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The major outlet for US landings of yellow and silver eels is the EU market.

Conservation

According to the International Union for Conservation of Nature, the American eel is at very high risk of extinction in the wild. which is typically found below dams. Contaminations of heavy metals, dioxins, chlordane, and polychlorinated biphenyls as well as pollutants from nonpoint source can bioaccumulate within the fat tissues of the eels, causing dangerous toxicity and reduced productivity.

The U.S. Fish and Wildlife Service reviewed the status of the American eel both in 2007 and in 2015, finding both times that Endangered Species Act protection for the American eel is not warranted. The Canadian province of Ontario has cancelled the commercial fishing quota since 2004. Eel sport fishery has been closed. Efforts have been made to improve the passage in which eels migrate across the hydroelectric dams on St. Lawrence River.

As of August 2023, A. rostrata is under consideration for protection under the Canadian Species at Risk Act. The Committee on the Status of Endangered Wildlife in Canada reported that the species was threatened in 2012. On December 2, 2025, the government of Canada chose not to list the American eel on the Species at Risk registry, finding that "the population's abundance across Canada has remained stable over the past two decades."

To help A. rostrata migrate safely, scientists are working on LED lights specifically designed to drive the eels away from dangerous obstacles or paths in their migration routes, such as hydro-electric facilities. In 2010, Greenpeace International has added the American eel to its seafood red list.

References

  • ESA protection