By Dr. Becker
European eel larvae have been detected in the Sargasso Sea, which is near Bermuda, but as adults these intriguing creatures live in freshwater rivers in Europe and North Africa. How they make the trek across the ocean has remained a mystery, especially since eels migrate at night, far below the ocean’s surface where observing and tracking them is difficult.
“Larval eels are transported by ocean currents associated with the Gulf Stream System from Sargasso Sea breeding grounds to coastal and freshwater habitats from North Africa to Scandinavia,” researchers wrote in the journal Current Biology.1 Even more interesting, the study revealed that juvenile European eels find their way via information they glean from the Earth’s magnetic field.
Eels Find Their Way Using Earth’s Magnetic Fields
It’s already known that sea turtles, salmon and other marine mammals turn to magnetic fields to help navigate the oceans during long (and short) migrations, so researchers with the National Oceanic and Atmospheric Administration in Miami, Florida decided to see if eels do too. They surrounded a 3,000-liter saltwater tank with copper wires in order to simulate magnetic fields.
“With no electric current, the eels didn’t swim in any particular direction. But when the magnetic field matched what eels would experience in the Sargasso Sea, the fish mostly swam to the southwest corner of their tank,” Science News reported.2 In the wild, southwestward swimming from the Sargasso Sea would put eels into the Antilles Current and move them northwest into the Gulf Stream to their final destination near Europe.
Researchers also used a magnetic field similar to one the eels would experience further along in the North Atlantic Ocean, and this caused the eels to swim northeast. According to the study, swimming northeastward while in this region would lessen the chances of the eels remaining near North America and would instead facilitate movement to the east so they could join up with the Gulf Stream.
“Thus, the magnetic map in juvenile eels appears tuned to promote transport out of the western Atlantic via the Gulf Stream System, which provides an energetically efficient route toward Europe,” they noted, continuing:3
“Simulations using an ocean circulation model revealed that even weakly swimming in the experimentally observed directions at the locations corresponding to the magnetic displacements would increase entrainment of juvenile eels into the Gulf Stream System.”
Adult Eels Take a Meandering Route Back to the Sargasso Sea
After living for an average of 20 (and possibly up to 90) years in their freshwater habitats, European eels (like their American eel cousins) travel back to the Sargasso Sea to spawn, and die soon after. Adult eels, which are known as “silver eels” in this stage of their life, take a meandering route back across the ocean, one that may take a year or more to complete.4
The spawning migration of the European eel typically begins between August and December (known as their escapement), and it was previously thought that eels would return en masse, reproducing the springtime following their migration. When researchers tagged more than 700 eels, however, this assumption was challenged. The migration routes of about 80 of the eels were able to be reconstructed in detail, showing that the eels traveled anywhere from 2 to 29 miles a day.
What’s more, while some of the eels rapidly migrated to the Sargasso Sea, others did not arrive until the following year’s spawning season.5 Whether or not the adult eels also rely on magnetic fields to find their way, as the young eels do, remains to be seen but is a topic of current research. No eel had ever been observed traveling across the open ocean to reach the theorized spawning area — until a study was published in 2015.6
Researchers attached satellite-transmitting tags to 38 American eels collected from Nova Scotia, Canada, then tracked their movements after they were released. Previous attempts have seen the tags fall off prematurely or be consumed by predators, but in this case eight of the eels traveled through the open ocean, with their tags still attached.
The eels were recorded swimming at a depth of 1.2 miles, and one eel, which the researchers named “Star,” swam all the way to the Sargasso Sea — a nearly 1,500-mile journey — before the tag was lost.
European Eels Are Critically Endangered
Learning more about eels’ migration routes and methods is extremely important, as the species is critically endangered. Although precise data is limited, the International Union for Conservation of Nature (IUCN) estimates there’s been a 50 percent to 60 percent decline in silver eel escapement over the last 45 years, along with a 90 percent to 95 percent declines in European eel recruitment (when the young eels migrate to European waters) during the same period.7 IUCN cited a “suite of threats” facing the European eel, which may be responsible for their decline. Among them:
Damage from hydropower turbines
Poor body condition
Changes in oceanic currents
Diseases and parasites
Exploitation and trade
As more is uncovered about the different life stages of eels and how they move about through the ocean, it’s hoped that increasing steps can be taken to protect this rapidly declining species.