Anguilla anguilla

Common name. European eel

Diagnosis. Distinguished from other freshwater fishes in Mediterranean, Caspian, and Black Sea basins by: ● body very elongate, slender, and cylindrical / ● dorsal, caudal, and anal long and confluent / ● flank plain olive yellowish to dark-brown or black, silvery grey from jaw to anus / ○ dorsal origin clearly in front of anus / ○ pelvic absent. Size usually about 400–600 mm TL, rarely larger, although individuals up to 2000 TL are reported.

Distribution. All rivers flowing to Mediterranean, Black, North, and Baltic Seas and Atlantic as far south as Canary Islands. Stocked in many inland waters and occasionally caught on Caspian coast and in rivers of Iran. Occasional individuals reach Volga drainage through canals. Very rarely in White and Barents Seas, recorded as far east as Pechora. Small numbers enter Black Sea, migrating east to Kuban drainage. Parts of population remain at sea (north-west Atlantic and Mediterranean).

Habitat. All types of benthic habitats, from small streams to banks of large rivers and lakes. Part of population remain at sea. Occurs naturally only in waters connected to sea, stocked elsewhere. Hypothesised to spawn only in Sargasso Sea, in western subtropical Atlantic (about 26°N 60°W). Spawning area is about 2000 km wide.

Biology. Catadromous. Spawning peaks in early March and continues until July. No concrete data on spawning sites, but presumably at depths of 100–200 m and at about 20°C. Adults die after spawning. Mechanisms by which Leptocephali reach European coasts are not well understood. Evidence shows that larvae swim actively and that drifting plays a minor role. A statistical re-analysis of data does not support the well-known classical scheme of larval migration in Gulf Stream. Leptocephali reach continental slope at about 70 mm and metamorphose into glass eels (almost adult appearance but transparent body), which enter estuaries. Glass eels are observed on Portuguese coast in autumn, in North Sea in winter, and in Baltic Sea in spring. Pigmentation increases during upstream migration, and juveniles are called elvers. Feeding phase lasts 5–8 years for males and up to 12 years or more for females. Males are rarely recorded more than 200 km upstream. Downstream migration begins in late summer or autumn, and adults arrive at spawning sites after spring. In male, eye diameter increases significantly before migration. Migrates in open water layers of ocean, at about 300–700 m depth. Feeds on a wide variety of benthic organisms. Heavily infected by nematode Anguillicoloides crassus, which feeds on blood in swim bladder, this parasite was introduced from Japan with experimental stocks of Japanese eel A. japonica.

Conservation status. CR; has strongly declined. Upstream and downstream migration barriers, including mortality from hydroelectric turbines, are a major threat. Furthermore, accumulation of lipophilic chemical contaminants stored by fish and released as fat stores are broken down during migration. These have toxic effects on migrating adults. Anguillicoloides crassus is thought to affect ability of European eel to reach their spawning grounds. European eel is still heavily fished. Exports outside Europe are banned, all trade within Europe (for consumption, culture, and stocking) is regulated, and quotas are in place. However, poaching and illegal trade are believed to occur throughout European eel fishery and are a major wildlife crime.

Further reading. Schmidt 1925 (breeding site); Boëtius & Harding 1985 (migration); Tesch 1991, 1999 (biology); Wirth & Bernatchez 2001 (genetics); Dekker 2003 (decline); Pike et al. 2020a (biology, distribution, conservation status).