Amazing lifecycle of the European Eel

By Kevin Collins

By Kevin Collins


The life cycle of the European Eel is amazing. In January, baby eels arrive in Irish estuaries, having drifted across the Atlantic for about 12 months. At this stage their bodies are 5 to 7 cm long and transparent and they are known as ‘glass eels’. They must be invisible to predators at this stage. They spend about a month in the estuaries while their bodies adjust from a salt-water to a fresh-water environment.

Then, when the water temperature is high enough, they start to wriggle their way upstream and a yellow pigment appears in their skin, so these are known as ‘yellow eels’. The peak month for this movement is May, and millions of elvers set off on dark, moonless nights to avoid predators. They remain in the rivers and lakes as they grow and mature from 10 to 20 years. They feed on the beds of lakes and rivers where they can catch fresh-water snails, midge larvae, caddis fly larvae, mayfly larvae, fresh-water shrimps and fish. Adult eels even eat elvers. Most of their hunting is done at night. Adult eels are active during spring, summer and autumn and they hibernate in the mud at the bottom of lakes for the cold winter months.

A mature female eel can grow up to a metre in length but most are about 60cm long with the males being smaller. When they are fully mature they turn silver in colour and are known as ‘silver eels’. As they have no reproductive parts in this part of the world, and nobody ever found eggs, there were many myths about how eel reproduced. It wasn’t until the 1920s that the truth was discovered. Our mature adult eels leave the rivers and lakes in the autumn and they swim across the Atlantic to the warm waters of the Sargasso Sea, which is near Bermuda. There they spawn and die.

Genetic studies have shown that the river in Northern Europe that a glass eel returns to is completely random.

The population of wild European Eels has been declining and in 2009, the then Minister Eamon Ryan, banned all eel fishing until June 2012, while a scientific review was carried out. The European eel is now considered critically endangered and the reasons for its decline usually involve water pollution, building of dams and weirs and hydro-electric power stations and possibly over-harvesting. The number of young eels reaching European rivers is down to about 1% of its peak in the 1970s.

Because of its unique life cycle, the problem of declining eel numbers has to be dealt with on a European wide basis. Studies are on-going where adult eels are fitted with electronic tags. Because part of their life cycle is in the open ocean, climate change and changes in ocean currents may have a large part to play.