European EeL
Monitoring

The European Eel

The European Eel (Anguilla anguilla) is a long-lived fish species that are known for their extraordinary life cycle and lengthy migration. They were once prevalent in UK estuaries and rivers but are now listed as a Critically Endangered species on the International Union for the Conservation of Nature (IUCN) Red List of Threatened Species.

Lifecyle of The European Eel

European eel are catadromous, which means they spawn in the sea and migrate to freshwater to mature. They breed in a region of the Atlantic Ocean near the Caribbean known as The Sargasso Sea, but spend the majority of their lifetime in brackish and freshwater on continental Europe.

STAGES OF THE LIFE CYCLE

Small spherical eggs are deposited at different depths throughout the water column and rise towards the light at the surface.

They hatch into transparent leaf-shaped larvae, known as a leptocephalus, about 5mm in length. These larvae are carried on ocean currents as they begin a one to two year migration to Europe.

As they approach Europe they metamorphose (change in form from one life stage to the next) into thin, see-through glass eels, approximately 8cm long.

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Glass eels enter estuaries and rivers and become pigmented to provide camouflage against predators. At this point they are known as elvers and grow up to 12cm in length. Elvers find their way up rivers. creeks and into wetlands where there is suitable habitat and food to grow.

As the elvers mature, they grow and undergo another metamorphosis, into a yellow eel. The eel develops brown, yellow and grey pigmentation, becomes more muscular and serpentine, develops a wide powerful jaw and thin, soft fins along the back and belly. Yellow eels are solitary and an individual can navigate many miles of streams, ditches, lakes and swamps before finding a suitable home. They then remain within the same area for the majority of their lifetime, which can be as long as fifty years.

At some point, an eel will begin to prepare for reproduction, though the exact triggers for this are unknown. They develop a silver colouration, the fins grow longer, the eyes larger, the stomach dissolves and the reproductive organs begin to develop - they are now a silver eel. They navigate back to the sea for the return migration to the Sargasso where they will breed.

The European eel is a semelparous species, individuals breed once in their lifetime, and are thought to die after spawning.

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why should we care about eels?

Eels were once prevalent in river systems across Europe, including the Tees, with great ecological, cultural and historical importance.
They are an indicator species for the state of our estuaries, rivers and wetlands.
They are a critical food source for many birds, and predators such as otters.
They form an important link between marine and freshwater ecosystems.
They are a unique species with an incredible life history!

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They are Critically Endangered as a result of multiple impacts including barriers to migration, climate change, parasites, pollution and exploitation!