Aristotle thought they came from earthworms. Others thought they spontaneously generated. And to this day, no one really knows precisely where eels are made.
Yet there are eels – lots of them.
Over the past century, a consensus has formed that American and European eels journey thousands of kilometres across the ocean to spawn in the conducive conditions of the Sargasso sea. In this vast, self-contained gyre of water in the western Atlantic, near Bermuda, the water is warmer and saltier than the surroundings. The newly spawned elvers then make their way home.
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But this extraordinary mission is a matter entirely of inference, first drawn by Danish researcher Johannes Schmidt after a series of expeditions to the Sargasso a century ago. No adult eels have ever been caught spawning there. Until recently, none had even been seen en route. “Their migration remains a complete mystery,” says Melanie Beguer-Pon of Laval University in Quebec. Yet there must be something to the Sargasso tale, says Håkan Wickström of the Institute of Freshwater Research in Drottningholm, Sweden. “They must spawn there because the tiniest eel larvae are found there,” he says.
And so the story goes: on the brink of sexual maturity, eels leave the shores of Europe or North America for the depths of the Sargasso sea, where they engage in panmixia, a wriggling orgy of individuals randomly mating with each other. The resulting larvae mature into transparent “glass” eels as they make the return journey to spend their lives in river estuaries on their respective continents.
This slippery story is a matter of more than academic curiosity. Both European and American eels have been in precipitous decline in recent decades, and are listed as endangered by the International Union for the Conservation of Nature – yet no one knows why. Pollution, dams that block rivers, fishing and ocean warming have all been blamed, but that leaves no room to explain the numbers of eels returning to European rivers since 2009.
In the latest attempt to crack the mystery, scientists on both sides of the Atlantic began to equip mature “silver” eels with tiny microwave trackers that pop off and float to the surface after six months, transmitting their data to passing satellites. Initially there was not an inkling – perhaps because the tagged eels could not make it across the Gulf Stream lugging the 45-gram trackers on their backs, suggests Beguer-Pon.
But then, in 2014, one American eel from Nova Scotia to the northern limit of the Sargasso, tag and all. As evidence goes, it’s not much. No European eels tagged in Sweden have successfully sent back any data from beyond the Azores, less than halfway to the Sargasso. And until scientists track even one eel to the moment of spawning, any claim to understand these slippery customers will remain a fisherman’s tale.
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