European Eel: a Slippery Mystery

The European eel lives in wetlands across Europe and North Africa. It lives the majority of its life in brackish wetlands - freshwater bodies that are connected to the sea and thus have a higher salinity - such as rivers, estuaries or lagoons, and can be found all the way from Scandinavia to Morocco.

European eels are catadromous fish: despite spending most of its life in freshwater, it will brave the open sea when the time comes to reproduce. Far more than any poet has promised to venture for love, the European eel travels up to 6,000 km across the Atlantic Ocean to spawn in the Sargasso Sea, just to the east of the Bahamas.  

But what actually happens once under the Bahamas sun? No one knows. This is one of the European eel’s biggest mysteries. No one has ever seen it spawn. All that is known is that the adult eels gather between late winter and early spring. It is assumed that they die once they have reproduced. Then again, European eels have been observed to live up to 85 years, with one ('Åle') even living to the supposed age of 155 in Sweden. So, who truly knows?  

Once born, baby eels (called leptocephalus) drift around the sea between seven months and three years before making their way back to Europe, hitching a ride on the Gulf Stream. At this age they are small leaf-shaped larvae; flat and transparent.

By the time they reach continental waters, they have grown to become cylindrical though they remain transparent. They are known at this stage as glass eels. As they move into freshwater, they begin to gain colour becoming elvers - not to be confused with fantasy elves!  

As they continue to grow, they enter their longest growth phase. Lasting between 6 to 20 years, the European eel begins to develop its characteristic yellowish underbelly, earning it the name of yellow eel. They develop quite the appetite during this period, feeding on fish, crustaceans, cephalopods, and molluscs. They have even been known to leave the water entirely to cross wet grass in order to catch worms and slugs, as well as to climb over obstacles and navigate around dams during their upstream migrations. Omnivorous, opportunistic and overcoming!

Once in freshwater, European eels will quite literally live under a rock. They spend their days buried in mud or tucked under stones, only emerging at night to hunt. No wonder they haven’t heard of Taylor Swift!  

The yellowing phase is very important, and the ecosystem in which European eels go through it will have a defining impact. Glass eels enter freshwater as sexually undifferentiated individuals, meaning they are neither female nor male. During their yellowing phase, European eels will develop sexual organs. Whether they become female or male is largely determined by environmental factors; high population density, for instance, tends to produce more males.  

Once they have become sexually mature, they will leave their freshwater habitats. At this point, the European eel enters the final stage, becoming a silver eel. A metallic sheen, wider eyes, the European eel is ready to reproduce. It stops feeding and heads back out to the ocean, all the way back to the Sargasso Sea. Overall, their average lifespan is typically 15 to 20 years.

The European eel’s journey through life spans long distances and brings the eel face-to-face with a multitude of obstacles. Beginning in open waters, the eel faces the many threats of the ocean from a young age - and unfortunately “fish are friends, not food” is not every sea creature’s motto. Once it has arrived in continental waters, it must make its way upstream inland. A tough swim to begin with without considering the fragmented state of many wetlands - dams and barriers block many species’ migration.

Pollution, habitat loss, climate change and the spread of invasive parasites have compounded these pressures. And, if that wasn’t enough, the European eel faces enormous pressure from overfishing, as it is a traditional dish in many European countries and has become a delicacy in East Asia. Restaurants continue to serve it despite its threatened status. One way you can consume responsibly is by making sure not to order European eel in establishments that serve it.

The Anguilla anguilla is one of the most threatened eels in the world. Categorised as Critically Endangered (the closest to extinction) by the IUCN Red List, its population has been in decline since the 1980s. In 2007, the European eel was listed in Appendix II of CITES (Convention on International Trade in Endangered Species), regulating its international trade, and since 2010, import and export from the EU has been prohibited.

Efforts are underway across Europe to reverse the decline, including the removal of obsolete dams, installation of fish passes, stricter fishing regulations, and national eel management plans. Progress is being made but the eel's recovery will require sustained, coordinated action across borders.

Until next month!  

In awe of nature’s resilience,

Talia, from the IUCN Med communications team    

Sources:

• IUCN Redlist

• IUCN ECARO - Vjosa river storymap

• European red list of freshwater fish

• Fishbase

• OSPAR

• National Geographic

• FAO