Eel Migration Influenced Less by Currents and More by Humans in Recent Years

An international team of researchers has identified a previously unknown relationship between ocean currents and variation in eel migration, shedding light on how the animal makes its way from its spawning grounds in the Sargasso Sea to the European shelf - and back again.

Published in the journal Current Biology, the multidisciplinary study is based on an ocean model originally used to measure the effects of melting Greenland glaciers on the North Atlantic where the Sargasso Sea is located.

"The new model allows us to understand even small-scale changes in the ocean, so we came up with the idea of using it for a simulation of eel migrations," Miguel Baltazar-Soares, lead author of the new study and a researcher from Germany's GEOMAR, said in a statement.

Using the model, the researchers simulated the years from 1960-2005, taking into account external factors like wind and weather. The results revealed how the eel larvae might have migrated to Europe during each of these years, suggesting a great deal of fluctuation in eel recruitment over the 45-year-period.

"In the early 1980s, for example, only a fraction of the larvae managed their way to Europe," said Arne Biastoch, a theoretical oceanographer also from GEOMAR.

Especially influential were small, wind-driven currents, which could either lengthen the eels' trip and thus limit the number that made it, or vice versa.

"We had not seen these flow changes in any of the older ocean models. But they seem to play a crucial role in the migration of the eel larvae," Biastoch said.

By combining these results with genetic analyses, the researchers uncovered evidence that when eels return to the Sargasso Sea, they do so to the place where their mother spawned rather than happening upon a random location, as is commonly believed.

"This is a new finding - so far, it was assumed that the mating in the Atlantic takes place completely independent of the area of origin," Biastoch said, adding that future research is needed to corroborate their findings.

The researchers further noted that while the computer simulations matched up for the years 1960-1980, this changed as time went on, with eel populations and climatic influences in the Atlantic becoming increasingly disconnected in more recent years.

"Since then fishing pressure, habitat destruction in European rivers and diseases appear to play an increased role," said Baltazar-Soares.