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, scientists are getting closer to a full understanding.
Dr. Kim Aarestrup, National Institute of Aquatic Resources and Professor David Righton, Centre for Environment Fisheries and Aquaculture Science are happy. Together with colleagues and by the use of modern satellite tracking technology, they have solved an important part of the eel mystery: What happens when European eels stir in dark autumn nights and leave European rivers and coasts.
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 were recently published in the journal Science.
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!".
Goes deep at day
Data from the tags reveals a distinct diel migration pattern. At night the eels swim in relatively warm water at 200-300 m depth, but at dawn they dive down into colder deep water at up to 1000 m depth. This caused astonishment.
"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.
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.
About European Eels
European eel (Anguilla anguilla) is an important fish species. Eel fisheries generate a total annual income of ~€200 million Euro, and employ over 25,000 European citizens in 15 countries. Eel catches have been halved – from 40,000 tonnes in the last three decades to less than 20,000 tonnes today and the recruitment of elvers has dropped even more. The cause of this decline is not known, and more information is needed about eel biology and ecology to help conserve and recover European stocks.
The tagging project
The eels detailed in the Science study were tagged as part of the Danish Galathea project but the data collected has been analysed and the tagging continued in the EU and national government-funded EELIAD (an acronym of European Eels in the Atlantic: Assessment of Their Decline) project. EELIAD is a four-year, €4million collaborative scientific research project, between 12 different institutes in Europe, aiming to resolve some of the mysteries of eel biology. The information it gleans will help conserve European eel stocks.
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