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. The eels were tagged as part of the Danish Galathea project, but the data from the tags has been analysed, and the tagging work continued in the EU and national government-funded EELIAD project. The first results are published in Science this week.

New surprising knowledge in several ways
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 (see below) says: "This is a brilliant result in many different ways. 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. First of all, the eels don’t swim directly towards the spawning grounds in the Atlantic Ocean south of Bermuda. Secondly, our data shows that the eel every day dives down at daytime, 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.”

The eels that migrated longest had swum more than 1000 km 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!".