Eels may not take most direct route in epic ocean-crossing spawning runs

Storied spawning runs of European eels may not be an en masse push to a mating site. Roundabout routes may delay many eels so much that they miss the big event and have to wait to mate until next season.

The most extensive reconstructions of individual eel journeys challenge an assumption that Europe’s freshwater eels (Anguilla anguilla) migrate and spawn as a group, says behavioral ecologist David Righton of the Centre for Environment, Fisheries and Aquaculture Science in Lowestoft, England. Eels tagged for the study took so long on their various journeys that tags scheduled to pop off on April 1 — the expected peak of the spawning — found that none of 33 ocean-going eels had yet made it all the way to the Sargasso Sea mating grounds.

Calculating routes and speeds indicates that many eels were already too far off-track to reach the mass gathering that season without unrealistic jumps in swimming speed, Righton and an international team argue October 5 in Science Advances. The eel that came the closest to the Sargasso Sea before its tag detached had already traveled almost 10 months and meandered more than 6,900 kilometers, farther than the distance of a direct route.

Freshwater eels’ migration and spawning have confounded biologists for centuries. A. anguilla can live for 90 years in rivers, but they don’t reproduce there. Each animal spawns once in its life, swimming out to salt water and largely disappearing from scientific view into the Atlantic Ocean.

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About a century ago, a researcher deduced from abundant catches of larval eels that freshwater species must migrate to spawn in the Sargasso Sea, a region within the gyre of great swirling currents and named for its abundant Sargassum seaweed.  This mysterious spawning season may actually begin in December and peak February 14, earlier than previously thought, researchers concluded from a fresh look at previous data on larval size. But even now, no scientist has succeeded in collecting adults there, Righton says.

In a project named the eeliad, an international collaboration fitted tags on 707 large eels and loosed the fish along the coast of Europe. Many were caught by predators, sometimes human ones. In 2009, researchers reported the basics of tracking the first 1,300 kilometers of the estimated 5,000-kilometer journey (SN Online: 9/24/09). Now, further analysis from temperature and water depth recorded by tags has let researchers plot routes for the 33 eels that made it across the continental shelf and into deeper water.

Looking at the end points of the tracking and the distances yet to cover, researchers calculated that for even half the tracked eels to arrive by the peak of spawning, they would have to pick up the pace by an additional 15 to 53 kilometers per day. And that’s taking a direct route, something the eels — contending with ocean currents and navigating by mysterious eel means — had yet to do. With speedups unlikely, the researchers propose that it’s time to rethink the idea that leaving freshwater together in a given autumn means spawning together that same breeding season.

North American freshwater eels likewise leave their rivers and swim to the Sargasso Sea but have a shorter oceanic trip, says Mélanie Béguer-Pon of Laval University in Quebec City. A tag study she and colleagues published indicates these eels take a more direct route and should arrive in time for the current spawning season, she says. But she doesn’t rule out the possibility of some untagged slow travelers on her side of the Atlantic, too. A mixed strategy for the European counterparts covering such a great distance “sounds totally reasonable to me,” she says.

Julian Dodson, at Laval and a collaborator on the North American tagging project, says it’s difficult for him to understand why evolutionary forces would not have favored more efficiency in slow-swimming European eels. Extra months of migration mean more exposure to predators and more energy depletion as migrating eels don’t feed. But “that’s the thing about eels, always more questions than answers,” he says.