The mystery of the eel's marathon swim

It hardly looks like the smartest or most athletic of animals, but the humble eel pulls off one of the most mysterious feats of animal migration, navigating all the way from the coasts of Europe to the Sargasso Sea, near Bermuda.

Nobody knows how or why. "It's one of the great riddles of biology," said researcher Kim Aarestrup of the University of Denmark.

European eels are born in the Sargasso Sea and drift, as larvae, all the way across the Atlantic, where they spread out from North Africa to Norway. Then, sometime around its 30th birthday, each eel takes off on a one-way journey back to its birthplace - logging about 3,100 miles.

No one knows how they find their way, but Aarestrup and colleagues tracked the first part of their route using tags outfitted with miniaturized satellite transmitters.

So far, the researchers have managed to tag and track 21 eels over the first 800 miles, said Aarestrup, who led the work. He and his colleagues published their findings last week in the journal Science.

To his surprise, the migrating eels plunge into the depths of the ocean during the day, perhaps to avoid being eaten, then move back near the surface by night. They also traveled a little slower and farther south than he'd expected, which may allow them to take advantage of westward-moving currents that will speed them up later.

There aren't many eels left to study, Aarestrup said. By some estimates, their population is down to about 2 percent of what it was 30 or 40 years ago. Some of that drop is likely the result of overfishing, he said, since so many Europeans like to eat them. "We have thousands of recipes for eels." In Denmark they fry or smoke them, and in France they catch them while they're still small and grill them, he said.

Unless that appetite for eels changes, it's possible they will disappear before their migrations are ever understood.