In order to determine both the migration routes and the spawning sites of eels, various tagging technologies were used in EELIAD. These devices consist of electronic tags that were attached externally to adult silver eels so as to obtain information about the habitat selection and the migration routes during the escapement.
Tags consist of satellite tags (PSATs) and archival tags. The size of most electronic tags limited the number of species they can be used on so far, especially with regard to European eels (Anguilla anguilla). But, at the time of the project launch, novel miniaturized electronic devices would be small enough to be adapted to European eels.
Satellite tracking has previously shown to be a robust research tool for studying the movement of marine animals whereas archival tags represented a more cost-effective system (roughly 400 € each versus 2000 € for a satellite-communicating tag). Satellite tag data was recovered through the worldwide ARGOS satellite service. All tags were clearly labelled with contact details (i.e. a telephone number and a web-address) in order to be returned if necessary.
The study of the spawning migration of eels throughout the Atlantic Ocean (i.e. from Europe to the Sargasso Sea) was carried out. Electronic tags provided research staff with transmissions from 14 of 22 tagged silver eels that were released from the west coast of Ireland. Researchers were able to determine that eels migrated southwest towards the Canary and Azores current systems with a migration speed varying from 5 to 25 km per day.
As eels journeyed to the Sargasso Sea, electronic tags revealed that they occupied shallow warm water in the night (average: 282 ± 138 m, 11.68° ± 0.48°C). At dawn, eels made an abrupt dive into the cool zone (average: 564 ± 125 m, 10.12 ± 089 °C). At night, they rose steeply back into the upper layer and so on. These daily migrations in different water layers are called diel vertical migrations (DVM).
A first option for explaining DVM of eels consisted in associating their pattern to predator avoidance. DVM allowed pelagic organisms to avoid diurnal exposure to predators as to maximize their feeding at night. But eels do not feed during their migration.
It is now assumed that DVM allowed eels to maintain a sufficient high metabolism and swimming activity whereas descent to deeper and cooler waters enable them to keep their temperature below 11°C, delaying in that way gonadal development until they reach the Sargasso Sea.
The ensemble of electronic tags allowed researchers to track eels up to 1300 km from the release spot whereas the spawning migration of eels represents a 5000 km journey throughout the Atlantic Ocean. Further technical improvements in electronic tags should enable researchers to track eels during their entire migration to the Sargasso Sea.
Oceanic Spawning Migration of the European Eel – 25 September 2009- Vol. 325, Science