The European eel, Anguilla anguilla, is a snake-like fish that lives in fresh water and breeds in the sea. It can reach in exceptional cases a length of 4.92 ft (1.5 m), but is normally much smaller, about 2.3 ft (70 cm), and rarely more than 3.28 ft (1 m). They are generally believed to spawn in the Sargasso Sea and the larvae (Leptocephalus) migrate towards Europe in a three-year-long migration. As glass eels they reach the coasts of Europe and enter estuaries. Before entering fresh water, the glass eels metamorphose. They spend most of their lives in freshwater, although recent studies on Japanese eels (Anguilla japonica) show that some populations never migrate into freshwater but spend their lives in marine or estuarine habitats. Those eels living in freshwater undergo changes in pigmentation; their bellies turn yellow. It is assumed that the yellow-coloring acts as a protection from predators as it makes it harder to visually detect the animals. The slimy coating of the eel is thought to protect the fish against changes in salinity.
Since the 1970s, the numbers of eels reaching Europe is thought to have declined by around 90% (possibly even 98%). It is unclear whether this is part of a normal long term cycle, or whether this reflects a decline in eel numbers generally. Potential causes include overfishing, parasites such as Anguillicola crassus, river barriers such as hydroelectric plants, and natural changes in the North Atlantic oscillation, Gulf Stream and the North Atlantic drift. Recent work suggests that PCB pollution may be a major factor in their decline.
Eels have been important sources of food both as adults (including the infamous jellied eels of East London) and as elvers. Elver fishing using basket traps has been of significant economic value in many river estuaries on the western sea-board of Europe.