European eels

European eels (Anguilla anguilla) have a long and complex life cycle. They are a migratory, catadromous fish, feeding and growing in fresh water but returning to the sea to spawn. Their spawning grounds are believed to be in the Sargasso Sea resulting in a 4,000 mile migration from the UK.

Once spawned, eel larvae (leptocephali) are carried by oceanic currents back to continental shores where they are distributed into waterways throughout Europe.

By this stage they have transformed from the flat, leaf-like shape of the larvae to a small transparent eel known as glass eels.

As unpigmented glass eels or newly pigmented elvers, they enter UK estuaries with the spring tides in Spring, migrating upstream into freshwater where they stay and mature for up to 20 years.

Elvers and adult eels are an important prey source for birds, fish and mammals.

Elver recruitment in Northern Europe has declined by approximately 90% in the last three decades possibly due to a combination of increases in man-made structures acting as barriers to migration upstream, a reduction in freshwater habitat, pollution, fisheries, changing oceanic currents and exposure to the parasite Anguillicola crassus.

Fast facts

• Once eels have left the rivers to return to the sea to spawn, they stop feeding, and so have to rely on stored energy alone. Their body undergoes dramatic changes: the eyes start to enlarge in size, the eye pigments change for optimal vision in dim, blue, clear, ocean light, and the sides of their bodies turn silvery, to create a counter-shading pattern to make them difficult to see by predators during their long open ocean migration.
  
• Eel migrations are affected by a number of factors. Their movement upstream is mainly triggered by water temperature.

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  Eels are capable of surviving for periods of time out of water and can cross land and damp meadows in their search for water systems.

• Mature eels prefer to move seawards when it is dark and large migrations are known to occur on wet, stormy autumn nights especially when the half-moon is on the wane.

Projects

ZSL has been monitoring eel migrations in the River Thames and its tributaries since spring 2005 as part of the Tidal Thames Conservation Project. Traps have been placed in several tributaries of the Thames in order to estimate recruitment and to identify potential restrictions. Find out more