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

Small spherical eggs are deposited at different depths throughout the water column and rise towards the light at the surface.
The eggs hatch into transparent leaf-shaped larvae, known as a leptocephalus, that are about 5mm in length. These larvae are then 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 glass eels, approximately 8cm long. These glass eels appear like small see through versions of their adult counterparts.

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.

Glass Eel

Elver

Leptochephalus

Ocean

river

Eggs

estuary

Spawning
In the Sargasso Sea

Silver Eel

Yellow Eel

As the glass eels enter estuaries and rivers, they 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 upriver systems and into tributaries where there is suitable habitat and food to grow.
As the elvers mature, they grow and develop yellow colouration on their bellies and become yellow eels. They can remain at this life stage for over 20 years.
As they start to reach sexual maturity, eels develop a silver colouration and prepare for migration back across the Atlantic to the Sargasso Sea. They are a semelparous species, meaning they breed once in their lifetime, and are thought to die after spawning.

Why Monitor eels on the tees?

Eels were once prevalent in river systems across Europe, including the Tees. They are of great ecological, cultural and historical importance. For example, elvers are an essential food source for many birds, and predators such as otters, bitterns and herons eat eel for their high fat content. However, since the 1980s, populations have declined by as much as 98% across all life stages (Aarestrup et al., 2009). This means that they are now classed as Critically Endangered (Pike et al., 2020).

Did you know?

In medieval England, eels were a form of currency and were often used to pay tax and rent!

Threats that have contributed to this decline include:

Barriers to migration (dams, weird, culvets)
Habitat loss
Overfishing
Climate change: warming and changing ocean currents
Pollution
Parasites

So, what are we doing?

Our Projects

Tidal Barrage
Elver Monitoring

Running since 2012, this is our monitoring program for elvers and glass eels using the tidal barrage elver pass. Run by a dedicated team of Trust members and volunteers, and helping us understand elver recruitment up the Tees.