Unlocking the mystery of European eel migrations

Pop up satellite tags
To coax these secrets from the eels, scientists fitted 22 European eels with a prototype Pop up Satellite Tag (PSAT) in 2006. The tags register and store light, depth and temperature.

New & surprising knowledge
Eels are difficult to follow once they leave European shores, so their behaviour as they migrate to their spawning grounds is almost a complete mystery. The study has added to the understanding of the migration and surprised the scientists in several ways.

Surprising results from tagging project
September 2009. The spawning migration of European eels from rivers and brackish waters in Europe to the Sargasso Sea is one of the most impressive feats of animal migration and orientation. Yet after over 2000 years of study it is still a mystery to be solved. But, thanks to research ongoing since 2006, we now know a lot more!

What happens when European eels stir in dark autumn nights and leave European rivers and coasts?
An important part of the eel mystery has been solved by Dr. Kim Aarestrup, National Institute of Aquatic Resources and Professor David Righton, Centre for Environment Fisheries and Aquaculture Science, working together with colleagues and by the use of modern satellite tracking technology.

Scientists searched among more than 100.000 live eels caught by Irish fishermen to find 22 large eels for tagging. Eels must weigh about 2 kg or more not to be hampered by the tag. Photo credit: Kim Aarestrup, DTU Aqua.

First of all, the eels don't swim directly towards the spawning grounds in the Atlantic Ocean south of Bermuda.
Secondly, the data shows that the eel dives down every day in daylight, only to ascend back towards the surface at night, a recurring depth change of several hundred meters. Thirdly, the results show that eel on this first stage of the migration swims too slow to reach the hypothesized spawning place at the assumed spawning time in April."

Dr Kim Aarestrup, who is a Senior scientist at the Technical University of Denmark, National Institute of Aquatic Resources (DTU Aqua), and leader of the tagging work in the EU EELIAD project says: "This is a brilliant result in many different ways.

Heading for the Azores
The eels that had migrated the furthest had swum more than 1000 kilometres from the Irish coast. Kim Aarestrups says:"It is very interesting that eels do not migrate directly towards the Sargasso Sea, but instead takes a more southern route towards the Azores. Researchers had previously speculated that eels should migrate south of the Azores to catch a ride on the south and west going currents and this way speed up their migration. Perhaps they were right!"

"This is entirely new knowledge and we can only speculate on why eels do this. We hypothesize that the observed pattern may reflect thermoregulation. Eels may move into warm water at night to maintain sufficiently high metabolic rates and swimming speeds and then descend into deeper and colder waters to delay the maturation of their gonads. This would mean that they maintain their hydrodynamic profile for most of their long migration, rather than becoming bloated with eggs which would then increase the energy needed to swim," says David Righton, who is a Professor at Principal Scientific Officer at the Centre for Environment Fisheries and Aquaculture Science (Cefas) and Coordinator of the project. 

Eel numbers dropped drastically in 30 years
The need to completely understand the eel lifecycle is more necessary than ever, because the eel population has dropped precipitously in the last three decades and the reasons are unknown. Furthermore, the information may be important for the current work of reproducing the eels artificially. Currently, the maturation is 'forced' by injecting hormones into adult eels, but maybe this new data can help to find a way to mature the eels without injecting hormones.