American Eel

The American Eel (Anguilla rostrata) is a catadromous fish in the family Anguillidae, found along the Atlantic coast of North America, from Greenland and Canada south to South America, and inland through rivers, lakes, and streams. Unlike most fish, it spends the majority of its life in fresh and brackish water but returns to the ocean to spawn, undertaking a single reproductive migration to the Sargasso Sea before dying. Its life cycle includes several distinct stages, leptocephalus larva, glass eel, elver, yellow eel, and silver eel, each with a different appearance and habitat. Once extremely abundant, American Eel populations have declined significantly due to dams blocking migration routes and habitat loss, leading to increased monitoring and conservation attention across its range.

American Eels have a long, cylindrical, snake-like body that tapers toward a pointed tail, with smooth skin that appears scaleless, though tiny embedded scales are present but not visible.

Key field marks:

• Continuous fin running from mid-back around the tail to the vent, with no separation between dorsal, caudal, and anal fins
• Small, rounded pectoral fins just behind the head; no pelvic fins
• Coloration shifts with life stage, from yellow-brown to olive in the freshwater yellow eel stage, to a darker back with a bronze-silver belly in the migratory silver eel stage
• Small eyes and a pointed snout with a slightly protruding lower jaw

Adults typically measure 60-120 cm. It is nearly identical in appearance to the European eel and can only be reliably distinguished by vertebrae counts or genetics.

American Eels use an unusually wide range of habitats across their life cycle. Larvae hatch in the Sargasso Sea in the western Atlantic and drift with ocean currents toward the coast, where they transform into glass eels and enter estuaries. Juveniles and adults then spend years, sometimes decades, in freshwater rivers, streams, lakes, and ponds from Labrador to the Gulf of Mexico and Caribbean, as well as in brackish coastal marshes; some individuals remain in estuarine or coastal waters their whole lives. They favor muddy or vegetated bottoms with cover such as rocks, logs, or burrows. When mature, silver-phase adults leave fresh water entirely and migrate back to the Sargasso Sea to spawn.

American Eels are mostly nocturnal and spend daylight hours hidden in burrows, under rocks, or amid vegetation, emerging at night to forage for insects, crustaceans, small fish, and carrion. They can tolerate a very wide range of salinities and even travel short distances over wet land to bypass obstacles. After years of growth in fresh or brackish water, eels undergo a physical transformation into the silvery, large-eyed silver eel stage and make a single long migration back to the Sargasso Sea, where they spawn once and then die. This catadromous life cycle is the reverse of anadromous fish like salmon, and the eel's decline has drawn conservation concern due to its ecological role as both predator and prey in freshwater systems.