Mysterious oceanic wriggler continues to defy science
Truly one of the world’s most curious fish, European eels are catadromous – which means they live in fresh water and breed in the sea. However, their migration between freshwater and the sea remains one of the natural world’s great mysteries.
The only known eel spawning ground is in a peculiar area of the western Atlantic Ocean called the Sargasso Sea. A “sea without a shoreline” the Sargasso Sea is a deep, calm and clear 2,000,000 square mile eddy, surrounded by Gulf Stream currents and festooned with seaweed. Encountered by Christopher Columbus on his transatlantic voyage, and later featured in A Hundred Leagues under the Sea and as an inspiration for Jean Rhys’ Wide Sargasso Sea, the area has entered popular culture as a symbol of mystery and uncertainty. The mysterious tale of the eel only heightens the intrigue.
Adult eels can grow to over a metre in length, maturing for up to 20 years in freshwater rivers and lakes. Incredibly, eels have been known to leave the water to move across damp ground in search of watercourses in which to feed or migrate through. This curious behaviour is thought to be especially common on stormy autumn nights under a waning half-moon.
When adult eels leave freshwater to start their 5,500 km migration to the Sargasso Sea, a number of fascinating changes occur to their body. They stop feeding – relying on energy reserves to fuel their journey; their bodies turn silver to aid concealment from ocean predators; and their eyes enlarge and optimise for sight in low oceanic light. Once they reach the Sargasso Sea, they spawn and die.
The hatched eel larvae begin the journey towards freshwater, carried on the Gulf Stream current, taking around 3 years to reach European shores, by which time they measure around 8-10 cm. Often known as ‘glass eels’, the tiny, translucent fish are widely considered a culinary delicacy, and have been historically harvested in their millions. Moving upstream from the river estuary, the eels become known as ‘elvers’, developing a darker pigmentation as they feast on invertebrates and small fish. Once the eel reaches a suitable habitat withsufficient food and shelter, it will settle in to mature and start the cycle again.
Eels were once incredibly common throughout Europe. The town of Ely in Cambridgeshire, UK was named after the rent of 100,000 eels that were paid annually to the lord of the manner. However, in recent years there has been a dramatic decline in eel numbers with some estimates suggesting that only 1% of the number of young eels are returning to freshwater than were counted 20 years ago.
Where to see: Whilst eel numbers have decreased dramatically, you may still be lucky enough to see elvers wriggling up weirs and waterfalls on European rivers throughout the summer months. Herons often stand on these obstructions, waiting for the eels to pass by and provide a meal. However, as this video shows, the eel is likely to put up a fight!
There are many aquariums throughout Europe containing European eels too!
Find out more:
- Tom Fort’s fantastic “Book of Eels” – an exploration of eels in history, science and culture
- Zoological Society of London’s eel research programme
- “Unlocking the mystery of European eel migrations” on Science Daily
- BBC Radio 4 “World on the Move” series on the eel
- The EELIAD research project
- European eel at Fishbase
Videos at ARKive:
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