American eel (Anguilla rostrata)

GenusAnguilla (1)
SizeFemale length: up to 130 cm (2)
Male length: up to 60 cm (2)
Female weight: c. 7.5 kg (3)

The American eel has not yet been assessed by the IUCN.

Despite having been known for hundreds of years, the American eel (Anguilla rostrata) has a complex life history which, even today, is not fully understood (4). This enigmatic species belongs to the freshwater eel family, the Anguillidae, and is the only anguillid eel to occur in North America (5).

A highly distinctive species, the American eel has a long, cylindrical, snake-like body. Its characteristic dorsal fin begins well behind the pectoral fin and runs along the back, around the tail and along the underside of the body (3) (5) (6) (7) (8). The eyes of the American eel are small and round, and its snout is pointed. The eel has a large, gaping mouth which extends back past the eye (3) (6) (7), and its jaws are strong, with the lower jaw often protruding past the upper jaw (3) (6). The American eel is covered with a mucus layer which makes its skin slimy to touch, despite having many minute scales which develop at is grows (3) (5) (6) (8). 

The American eel varies widely in colour depending on its age and the surrounding habitat (5) (6), with individuals living in muddier habitats typically appearing darker than those from clearer, sandier habitats (5) (6). Generally, adults of this species are dark brown to yellowish, greenish or olive-brown on the upperparts, with yellow, green, orange or even pink tinges on the sides. The underparts are paler brown and yellowish, becoming creamy or yellowish-white on the belly (3) (5) (6) (8). Adult American eels with this colouration are commonly referred to as ‘yellow eels’. During the migration back towards the sea, adult American eels develop a bronze to black back with a metallic sheen and a light or silvery belly, after which they are known as ‘silver eels’ (3).

The larvae of the American eel are transparent. Juvenile eels of this species, commonly called ‘elvers’ or ‘glass eels’, are also transparent, gradually becoming grey-black as they age (3). The female American eel is larger than the male and may sometimes reach over a metre in length (3) (7).

The American eel has a fairly large range, being distributed along the Atlantic coast of the Americas, as far north as Greenland in North America, and south to Central and South America (2) (5) (9).

It also occurs in most major freshwater streams along the coast, and in some areas the female American eel may migrate considerable distances inland (8).

The American eel spawns in the Sargasso Sea, a large area of warm water in the middle of the North Atlantic Ocean (2) (7).

The American eel inhabits a diverse range of habitats (4), occurring in both freshwater and marine environments at different stages in its life cycle. The American eel spends most of its life in freshwater, where it inhabits mud-bottomed streams, rivers and lakes, as well as estuaries, brackish and freshwater tidal channels, coastal impoundments and small creeks (3) (5) (9).

A nocturnal species, the American eel typically shelters during the day close to banks, logs and boulders, in deep pools, among submerged aquatic vegetation, or buried in mud or silt (3) (5) (9).

The American eel is catadromous, meaning that it spends the majority of its life in freshwater but migrates to the sea to spawn and die (3) (5) (8) (10). Although very little is known about the spawning period of the American eel, it is thought to occur in the autumn, with larger females releasing somewhere between 15 and 30 million eggs to be fertilised by the male (3) (4) (8). The buoyant eggs of the American eel hatch into transparent, leaf- or ribbon-like larvae (4) (8), known as ‘leptocephali’, which drift with ocean currents for around a year before reaching the Atlantic coast (2) (3) (4) (5) (8).

Once they have reached a length of around 6 to 6.5 centimetres, the larvae transform into the first juvenile phase and will actively migrate into coastal estuaries in late winter and early spring (2) (5). Eels in the first juvenile phase are known as ‘glass eels’, as although they are shaped like adults, they lack pigmentation and are much smaller (4) (5). The glass eels feed very little, but within a few weeks of leaving the ocean they enter the second juvenile phase and begin to grow and feed, at which time they are called ‘elvers’ (4) (5).

Elvers resemble the adult eels and are grey to greenish-brown in colour (4) (5). The elvers generally migrate upstream to freshwater, although it is thought to be mainly the females that migrate far inland, while the males stay in brackish and estuarine areas (2) (3) (5) (8). The American eel is able to absorb oxygen through its skin as well as its gills, which enables it to travel over land to reach isolated water bodies (3) (4). The elvers in particular are well known for their ability to negotiate obstacles such as waterfalls and rapids (10), and they are even able to traverse vertical objects such aslow dams and canal locks (3) (5), as long as the surface is damp and textured (5).

The elvers develop into immature adults, known as ‘yellow eels’ (2) (4). Generally, the yellow eels live in estuaries, rivers and streams for 4 to 10 years before reaching sexual maturity, although some may not mature for 20 years or more (2) (8) (10). The American eel reaches maturity once it grows to a certain size, and faster-growing individuals mature earlier (2). It is thought that yellow eels remaining in estuarine areas around the coast typically mature earlier than those in freshwater (4).

The sexually mature adult American eel undergoes a number of significant changes, including enlargement of the eyes and pectoral fins (2). It also becomes characteristically silvery and is henceforth referred to as a ‘silver eel’ (2) (4). The mature adult eel ceases feeding and begins its migration back to the spawning grounds in the Sargasso Sea (2) (4) (10). The American eel is thought to die after it has spawned (2) (4).

The American eel may live for more than 50 years in the wild (3). 

The American eel is an opportunistic carnivore, feeding on a wide range of prey species (3) (5) (9). Adult American eels typically feed on small fish and invertebrates, including crustaceans, insects, worms and molluscs. Elvers and yellow eels feed mainly on aquatic insects and their larvae, as well as small crustaceans (5). The American eel may sometimes also take frogs and carrion (3).

The American eel population has been declining in recent years (8) (10). Dams and other obstacles in rivers have had one of the biggest impacts on this eel’s population, resulting in habitat loss and preventing migration up or downstream (4) (5) (8). Obstacles also cause stress to the eel and increase its susceptibility to predation (8). Turbines from hydropower plants are also a source of mortality, while declining water quality due to contaminants and pollution may also affect the health of American eel populations (4) (5) (7).

Habitat degradation, for example by dredging, is known to negatively affect the American eel. Dredging may result directly in injury or death, or can cause turbidity and reduced water quality which may impact this species’ ability to migrate (5). Furthermore, a rapidly spreading parasite, the Asian swimbladder nematode (Anguillicola crassus), is known to infect the American eel’s swim bladder and affect the eel’s ability to migrate (4) (5).

The American eel is harvested commercially for food (8), and overfishing and excessive harvest of juveniles in particular could have a harmful affects on its population in some areas (5). Increasing competition and predation from invasive or non-native species, such as the flathead catfish (Pylodictis olivaris) and the blue catfish (Ictalurus furcatus), are also threats to the American eel (5).

Seaweed harvest, particularly of Sargassum species, may adversely affect the American eel’s spawning habitat in the Sargasso Sea. The marine habitat of this species may also be further affected by pollution, for example from oils spills (5).

Climate change is likely to become a threat to the American eel in future, with warming temperatures and changing ocean currents likely to interfere with the movement and migration of larvae and juveniles (5) (7).

The American eel has not yet been assessed by the IUCN and therefore its conservation status is currently uncertain (11). However, the American eel is protected under Ontario’s Endangered Species Act (7), and in 2004 a petition was filed with the United States Fish and Wildlife Service (USFWS) and the National Marine Fisheries Service (NMFS) to have the American eel listed as an endangered species (5). As of 2011, the petition was still currently under review by the USFWS (12).

In 2001, an Interstate Fisheries Management Plan for the conservation of the American eel was developedin response to perceived population declines (2) (5). Under the plan, regulations for commercial and recreational fisherieshave been enforced, including gear restrictions and size and possession limits (2). Other management activities include counting eels as they migrate inland, and constructing ‘eel-passes’ and ramps around dams to allow this species to move freely up and downstream (5) (10).

In some areas, research is being conducted to determine American eel distribution, population size and the timing of downstream eel migration (5) (10). Further work should also be done to investigate aspects of the American eel’s life history that are currently poorly understood. Development of education and outreach programs that distribute information about the critical habitat needs, threats and potential conservation actions for the American eel would also be beneficial to this species (5).

Find out more about the American eel:

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  1. ITIS (September, 2011)
  2. Northeast Fisheries Science Centre, National Oceanic and Atmospheric Administration: Status of fishery resources off the north-eastern US - American eel (September, 2011)
  3. U.S. Fish and Wildlife Service: Factsheet - American eel (September, 2011)
  4. Fishes of Canada’s National Capital Region - American eel (September, 2011)
  5. South Carolina Department of Natural Resources: Comprehensive Wildlife Conservation Strategy - American eel (September, 2011)
  6. Bigelow, H. and Schroeder, W. (1953) Fishes of the Gulf of Maine. Fishery Bulletin 74, Volume 53. Fishery Bulletin of the Fish and Wildlife Service, United States Government Printing Office, Washington.
  7. Ministry of Natural Resource, Ontario - American eel (September, 2011)
  8. U.S. Fish and Wildlife Service: Connecticut River Coordinator’s Office - American eel (September, 2011)
  9. U.S. Fish and Wildlife Service: Maryland Fisheries Resources Office - American eel (September, 2011)
  10. IUCN Red List (September, 2011)
  11. U.S. Fish and Wildlife Service: Endangered Species Program (September, 2011)