Scientists have unravelled a mystery surrounding one of nature's most incredible journeys.
Every year, eels leave European rivers to travel in an epic migration to the Sargasso Sea in the North Atlantic to breed for a single time, then die. Although this final destination has long been suspected, until now there has been no direct evidence.
By fitting eels with satellite tags, researchers have tracked the creatures on the final leg of the route. And they say the information will help in the conservation of the critically endangered species. "This is the first time we've been able to track eels to the Sargasso Sea and we are delighted we have the first direct evidence of adult European eels reaching their spawning area," said Ros Wright of the Environment Agency, who led the research (published in Scientific Reports).
"Their journey will reveal information about eel migration that has never been known before."
Eels arrive around the European coast as tiny, fragile and transparent glass eels, having drifted across the Atlantic for two or three years from the Sargasso Sea. They adapt to freshwater and mature in rivers - growing up to 1m long - until they are ready to swim all the way back to reproduce once and die.
Until now, it has been very difficult to study their migration across the ocean; previous studies have tracked adult eels all the way to the Azores, but from there the trail went cold. The researchers have now tagged adult eels in the Azores, showing they can swim all the way to the Sargasso Sea.
Unravelling the routes taken and locating where eels spawn is critical for understanding the reasons behind their decline and to inform conservation measures.
The life cycle of the eel has long puzzled scientists. Even the Greek philosopher Aristotle pondered the question of where eels came from, deciding that they sprang up spontaneously from the mud.
Almost 100 years ago, it was assumed that their destination was the Sargasso Sea, in the western Atlantic near the Bahamas, but until now final proof had been lacking.