Ocean Currents Drive Eel Population Changes: Study
December 29, 2013

Ocean Currents Drive Eel Population Changes: Study

Ranjini Raghunath for redOrbit.com - Your Universe Online

Fluctuations in the European eel population – a long-time holiday delicacy – may be linked to changing ocean currents in the Atlantic, according to new research published in the journal Current Biology.

The European eel gives birth in the Sargasso Sea and the larvae drift towards the European coast 2700 miles away. There, in fresh water and along the coast, they grow and mature into adults, before returning to the Sargasso Sea to reproduce.

European eels are now considered critically endangered species; since the 1970s, their population has fallen by about 90 percent. Breeding eels in captivity is also impossible as they can generally give birth only after swimming for a very long distance.

Scientists have struggled to monitor their life cycle and understand why and how eel numbers drop. Overfishing, parasite infection and destruction of river habitats have been suggested as reasons, but little information has been brought to light about how their numbers fluctuate during their 300-day-long migration in the ocean.

A computer model developed by an interdisciplinary team of researchers from Germany, London, Belfast and Chile now sheds some light on the fate of the larvae during their long trip to Europe.

The researchers seeded the Sargasso Sea with 8 million particles – mimicking the eel larvae – and studied how their migration changed with wind and the weather. The model was run for 45 years, from 1960 to 2005, simulating the weather patterns of each year, and studying how the particles’ drifting changed.

“Only those who reached the European shelf seas within two years were considered viable. This also corresponds to eel life cycle," said Christophe Eizaguirre, biologist at GEOMAR Helmholtz Centre for Ocean Research and senior author of the study, in a statement.

The researchers found that eel migration greatly changed with small changes in ocean currents. In the years when large eel numbers were reported, there were more westward-flowing ocean currents near the Bahamas, providing a “shortcut” of the longer route to the Gulf Stream through the Caribbean Sea, the researchers wrote. In other words, when currents favored shorter trips, higher eel numbers were reported across Europe. A large fraction of young eels possibly reached the European coast within weeks during those years, the researchers found.

When the trip was made longer by unfavorable ocean currents, the numbers dropped. These “flow” changes in ocean currents therefore played a crucial role in controlling the eels’ migration, the researchers believe.

When the researchers combined these findings with genetic studies, they also found that, unlike what biologists generally believe, eels don’t go to random spots in the Sargasso Sea to give birth, but to the same place where they were born – just like how salmon return to their birthplace to spawn.

"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," said Miguel Baltazar-Soares, another biologist at GEOMAR and lead author of the study.

The model was originally developed to study melting glaciers in Greenland, and has ten-times-better resolution than existing ocean and climate models, the researchers reported.

"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," Baltazar-Soares stated.

Although the model was able to predict migration patterns for the most part, there were still some difficulties predicting the eel populations. The numbers predicted by the model matched reported appearances of the eels reaching the coast up until the 1980s. But afterward, eel population appeared to be “disconnected” from climatic changes in the Atlantic, the researchers report.

"Since then fishing pressure, habitat destruction in European rivers and diseases appear to play an increased role," said Baltazar-Soares. Local currents in the Sargasso Sea also likely influenced the sharp decline in eel populations after the 1980s, the researchers believe.

The study shows how climatic factors and genetic conditions also influence eel growth and numbers, apart from biological factors such as predators or availability of resources. Analyzing the eel’s genome may also give insight into how they navigate the ocean currents, the researchers believe.

The study demonstrates the “potential power” of combining ocean models and population genetics, providing a valuable tool to understand the threats that marine animals face, the researchers concluded.