Every November, when the Moon is at its darkest, there’s a stirring on riverbeds, lake bottoms and marshlands around Europe. Countless silver serpents respond to an ancient urge and turn towards faster-moving water, beginning a perilous, 4,500-mile journey down deep ocean trenches and across undersea mountain ranges.

Until now nothing has been known about their incredible journey, only that the smallest larvae of the European eel are found in the mid-Atlantic Sargasso Sea. How the spawning adults get there and how long it takes them is one of the animal kingdom’s most enduring mysteries: bar a single specimen recovered from the belly of a sperm whale, not a single silver eel has ever been recovered from the open ocean.

With the help of a tiny floating tag, the first details of their epic journey are being revealed. The device was implanted into the belly of an 87cm (34in) female European eel near Hoganasa on the west coast of Sweden last November and was discovered on a Scottish beach two weeks ago.

“We were astonished,” said Dr David Righton of the Centre for Environment, Fisheries and Aquaculture Science, the leader of the project he has named the Eeliad on account of its epic proportions. (His previous project, involving cod, was called the Codyssey.) “The oceans are vast and there was a high risk that none would be found but we’ve had two turn up already. The first came back very soon after release, on the west coast of Ireland. That eel died and the tag came out. But the second tag has revealed some spectacular information.”

The mystery of where eels — Anguilla anguilla — come from perplexed Aristotle, the world’s first zoologist, back in 243BC. None had ever been found with an egg, so he was forced to speculate that they were spontaneously generated in the bowels of the earth. It wasn’t until 1922 that an expedition to the mid-Atlantic discovered eel larvae in the Sargasso Sea – a windless, weed-filled area of the Atlantic.

Dr Righton’s miniature tag records only temperature and depth, but thanks to research expeditions and networks of ocean sensors this data can be used to trace the eel’s movements. “We can look at the depths and temperatures that the eel visits and we can match those up to oceanographic databases. You know where your tag started and where it ended, and approximately how fast the eel can swim, and so can calculate the route with a fair degree of accuracy,” he said.

Eel number A03486 wound its way out of the Baltic approaches, north up into the Norwegian trench for 200 miles and then west into the Shetland-Faroe trench. She covered tens of miles every day and reached depths of up to 650m (more than 2,100ft) in water only 1C above freezing.

“Now we know that at least some eels go over the top of Scotland rather than through the Channel,” Dr Righton said. “More than that we don’t know.”

Currents could help them along the way, but add thousands of miles to their journey. But, like pilots using the winds, a longer route might end up being more efficient. This is key, for Dr Righton calculates that the silver eel – which can grow to more than 130cm (4ft 3in) – must complete its 4,500-mile feat of endurance on the fuel equivalent of three Big Macs.

The tagged eel had already travelled about 1,200 miles when its journey was interrupted for a second time just west of the Shetlands.

“The temperature was no longer consistent with the tag being inside a free-living eel, but with being inside something else — probably a shark,” Dr Righton said. Two weeks after that the tag passed through the shark and floated to the surface, where it drifted until it was eaten by a seagull.

“The temperature went up to 40C. It could have been a marine mammal, except for the fact it never dived for the eight hours the tag was inside.”

The tag would have continued to function for two years, and float undamaged for 20, but after only another day and a half it was washed up on the beach on the northwest coast of Shetland, where an islander spotted it while walking her dog. Printed on its side was: £50 Reward. If found return to CEFAS, Lowestoft.”

Another 150 tags were implanted in eels released from Galway, the Loire estuary in France and Sweden. By the end of the project more than 500 floating tags will have been deployed by the project’s European partners — including the French Navy, which has released some eels beyond the continental shelf, away from trawlers.

The eel’s journey exceeds the 1,800-mile migration of Atlantic salmon, but is dwarfed by the grey whale’s journey of up to 12,500 miles, from Baja California to the Bering Straits, the world’s longest animal migration.

The early phases of the eel’s life are also dramatic. After riding ocean currents from the Sargasso Sea to European coastal waters, the larval stage metamorphoses into glass eels. The saltwater glass eels throng upstream through river mouths and begin to change into freshwater-dwelling elvers. They remain in this form — coloured yellow, green or brown — until it is time for their return, when they transmute one final time, taking on their silver skin and adapting their bodies for the harshness of a saltwater environment. Some never do: elvers up to 88 years old have been caught. Others reach the open sea only to turn back inland.

No one knows for sure why they make their epic journey, but one possibility is privacy. “The Sargasso Sea is a barren, quiet place,” Dr Righton said. “There are almost no predators – it gives the young a great start in life.”

No one has ever seen what happens when the eels finally arrive, but a rare mating witnessed in an aquarium gives a hint. In a beautiful and complex ritual, the swollen female and her shorter, thinner partner entwine, mixing her eggs with his sperm.

If you find a tag, go to www.eeliad.com or call CEFAS on +44 1502 524526

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