Cracking the code of a long-distance swimmer

Not all eels are electric, but some might actually be magnetic.

Born in the Sargasso Sea , that Atlantic Ocean gyre east of Bermuda, baby European eels will travel 4,000 miles to the freshwater rivers of Europe. Now scientists might have answered a century-old question of how these young eels accomplish such vast oceanic migrations.

They use a GPS. But not like the one in your car or smartphone.

European eels have a sort of internal GPS or global positioning system tuned to the Earth's magnetic field, according to research appearing today in the journal Current Biology.

“With this study, we show for the first time how eels actually use Earth's magnetic field as a map to orient themselves during their long ocean journeys,” said Nathan Putman, lead author and scientist at NOAA's Atlantic Oceanographic and Meteorological Laboratory and the Cooperative Institute for Marine and Atmospheric Studies at the University of Miami.

A scientist holds juvenile American eels. Almost indistinguishable from the European eel, the American eel is also born in the Sargasso Sea. It makes a similar long-distance migration to North American fresh water rivers. Researchers of the new study on European eels theorize the American eel might also tune into the magnetic field to assist its migration. (Courtesy of Virginia Institute of Marine Science).

The findings may help improve management of this commercially and culturally important species of eel, as well as similar species, such as the American offsite link and Japanese eel. All these eel populations are considered depleted due to fishing pressure, loss of habitat because of dams that block their passage, pollution and changes in ocean conditions.

'Elvers' have left the building ... for the Gulf Stream

Putman and an international team of scientists tested the European eels' mapping skills by exposing juveniles – known as elvers – in a laboratory to a series of magnetic fields that mirrored the various magnetic conditions found along the eels' migration route. They found that eels' orientation differed depending upon the magnetic conditions, but in each case, the eels headed into what would have been the Gulf Stream – a powerful current that is thought to propel young eels from the Sargasso Sea toward Europe.

European, Japanese, and American eels have been fished for centuries to support valuable commercial, recreational and subsistence fisheries. Juvenile eels just entering freshwater can fetch staggering prices – in some years more than $2,000 per pound for eels caught in Maine. In 2014, total U.S. commercial eel landings were valued at approximately $9.8 million.

Putman's earlier research showed many marine animals, such as Pacific salmon and sea turtles, use Earth's magnetic field as a large-scale map. By learning what environmental cues animals use to guide their movements, scientists can better predict changes in their migratory routes and distribution. More research is likely to also contribute to better management of other valuable fisheries for migratory species such as tunas, sailfish, swordfish and sharks.