By Jody Bourton
Earth News reporter
Where do they all go?
The European eel's migration to the Sargasso Sea to spawn is one of nature's great unsolved mysteries.
For many years, biologists have puzzled over exactly where they go and what they do after leaving our rivers.
Now scientists using satellite tags have tracked 22 eels, revealing what they do in the first 1,300km of an epic 5,000km migration.
Using this method, biologists hope the whole journey to the Sargasso Sea will soon be revealed.
The results of the tracking study are published in the journal Science.
They provide unique insights into the migratory behaviour of the European eel (Anguilla anguilla), including the direction and depth the eels swim.
"It's exciting data because no-one has managed to do this before," says Dr David Righton from the Centre for Environment, Fisheries and Aquaculture Science (CEFAS) in Lowestoft, UK, a member of the multinational team that undertook the study.
"People have often speculated about which direction eels may take, how they might travel, so what we've been able to show is which of those speculations are actually right," Dr Righton says.
The European eel has a mysterious life cycle.
It spends its early years in our rivers before heading out to sea and across the Atlantic to the Sargasso Sea in the western Atlantic near the Bahamas.
Here, it is assumed they spawn and lay eggs, however this has never been witnessed.
The eggs hatch into transparent larvae called leptocephalus and make the return journey to Europe floating on oceanic currents.
By the time they reach our shores, they have developed into tiny glass eels that swim against the current into Europe's rivers.
While living in our rivers, the eels can grow up to 1m long.
In 2006, the team attached miniaturised pop-up satellite tags to 22 large adult eels which they released from the coast of Galway, western Ireland.
The tags record not only the location of the eel but also the daily activity of the eel, recording speed, depth and direction.
The tags are set to "pop off" after six months and float to the ocean surface, sending data via satellite back to the research team.
"We found the direction the eels take wasn't a straight line from the west coast of Ireland towards the Sargasso Sea, but it was to the south of that, as if they were heading toward the Azores," Dr Righton explains.
"So basically they are heading towards the conveyor belt ocean current which will help propel them toward the Sargasso Sea.
"That implies some navigational ability. It also suggests they are not just going the shortest distance; they are going the most efficient distance."
The researchers also found the eels swim too slowly to get to the Sargasso Sea by the April spawning period.
The researchers suggest this means the eels may gain speed and travel efficiency by entering the ocean currents that begin west of Africa and continue as part of the subtropical gyre system that flows to the Caribbean.
The data also reveals that during the night eels swim in shallow warm water then at dawn they make a steep dive to a depth of 1,000m where they remain for the day before ascending again.
Eels do not feed on their migration and so this behaviour cannot relate to foraging, though it may help them avoid predators.
The researchers speculate that the shallow warm water helps the eels maintain a high metabolism, while the cooler water at depth delays the sexual development of the eels helping them maintain their streamline form.
"What we're publishing here is the first step towards finding out where the ultimate destination is," Dr Righton says.
"What we expect is with improvements in technology and improvements in our techniques and experience, we will see step changes in the data we are getting."
In 1922, the Danish scientist Johannes Schmidt first deduced the Sargasso Sea as the spawning ground of the eel. Dr Righton hopes the final piece of the puzzle will soon be completed.
"Our project runs until 2012," he says, explaining that it would be fantastic to track the adults returning to the Sargasso Sea on the 90th anniversary of Schmidt's discovery.