American eels are the original mystery animal. These eels spend most of their lives in rivers along the Atlantic coast, but we're aware--because of science and a 1904 discovery of larva in the North Atlantic's Sargasso Sea--that they migrate there to spawn. However, no adult eel has ever been seen in migration in the open ocean or in their presumed spawning area. Slippery fellows, indeed.

A recent study conducted by Canada's Université Laval, Dalhousie University, and Fisheries and Oceans Canada tracked 28 maturing eels' movements using satellite archival tags. The eels were tracked from Canada's Scotian Shelf into the open ocean, and one of them migrated 2,400 km to the far northern end of the Sargasso Sea spawning site, according to a release.

From this, the scientists say that the eels' mystery migration is likely taking place in two phases: One goes over the continental shelf, then into shallow waters; the other heads into deep waters, south (from Canada) toward the area where spawning takes place, the release confirmed.

The study report was recently published in the journal Nature Communications.

Before this, many expeditions to the spawning site have failed to find adult eels there. Researchers say that advanced satellite transmitters made the difference this time. In all, 22 eels starting in Nova Scotia and 16 originating in the St. Lawrence Estuary were tracked. Of those, the 28 transmitters that later showed up in the Atlantic provided the study's data, said the release.

Eels in the study had the same technique: They seemed to gauge temperature and salinity level near the Atlantic coastline in order to locate the high seas. One eel sent back data from its turn south at the continental shelf, then its bee-line for the Sargasso Sea. Because of the direct route, it is likely that eels navigate based on magnetic field detection, said Professor Julian Dodson of Université Laval in the release.

While the researchers are careful about assuming too much from 30 eels, Prof. Dodson noted in the release, "Our data nonetheless shows that the eels don't follow the coastline the whole way, they can cover the route in just weeks, and they do go to the Sargasso Sea. We knew that millions of American eels migrated to reproduce, but no one had yet observed adults in the open ocean or the Sargasso Sea. For a scientist this was a fascinating mystery."

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