European Eel Monitoring
European Eels
European eels (Anguilla anguilla) are a long-lived, semelparous (breed once in their lifetime) and catadromous (migrate from river to sea to spawn) fish species. They can be found in fresh and salt water and are found across Europe.
They breed in a region of the Atlantic Ocean near the Caribbean known as the Sargasso sea. Adults die once spawning is completed, whilst their leaf-shaped (leptocephali) larvae begin their oceanic migration, which takes around two years to complete. As the larvae near land they develop and become glass eels, which lack pigmentation. Glass eels swim into estuaries and river systems and begin to pigment becoming elvers.
As the elvers grow and move further upstream they develop a green/yellow colouration. This period of their life can last between 8-15 years during which they are known as yellow eels. Once eels reach sexual maturity they develop a silver colouration and prepare for migration back across the Atlantic to the Sargasso Sea.
Why
Despite once being prevalent in river systems across Europe, including the Tees, a variety of pressures including modifications to migration passages, fishing and pollution has led to a 95% reduction in recruitment since the 1980s. This means that they are now classed as critically endangered by the IUCN.
What
Each week between April and November a dedicated team of Trust members and volunteers count the number of elvers and glass eels taken from a small removable section of an elver pass. This pass consists of a metal tube with titanium mesh inside which restricts the water flow and enables the eels to migrate up-stream. The first 50 individuals are measured and classified as either glass eel, elver or yellow eel. Then, the total number of eels is counted and they are rereleased into the river. We hope that by collecting this information we can understand how the elver population is changing overtime in our river so that we can better protect them.
