An Eel's Journey to the Sargasso Sea

How do eels travel from their river to the southwestern Sargasso Sea? This is quite a long way to travel, 2,400 km to be exact, for a fish that, once it arrives, will reproduce, and then die. A team of university and government researchers, to which Fisheries and Oceans Canada is party, managed to solve the mystery using satellite tags.

The team of researchers fitted eels with satellite tags that continuously record water temperature, depth and light. These tags are programmed to detach after some time, floating to the surface where they begin to send their data to the Argos satellites in orbit.

With the data it collected, the team was able to trace the eels' migration routes. Five of the tagged eels managed to make it out to the open sea, beyond the continental shelf, in the Laurentian Fan area, more than 500 km off Cabot Strait. One of the eels veered due south and migrated to the northern boundary of the spawning site over a distance of 2,400 km.

The researchers identified two distinct migratory phases. During the first phase on the continental shelf and slope, the eels migrate vertically on a daily basis between depths of 50 m at night and about 240 m during the day. Once they are off the continental slope, their vertical migration pattern changes; they migrate northeast along the slope and then southeast, in the open sea, to the Laurentian Fan. One eel suddenly changed its route (second phase) and migrated due south, while making significant daily vertical migrations (140 m at night and 600 m during the day). It crossed the Gulf Stream 29 days after being released and reached the northern boundary of the spawning site at 30° N latitude, 45 days post‑release.

Considering the speed and the directed migration of this eel, it seems likely that eels use the Earth's magnetic field for guidance. It has long been known that eels can detect the Earth's magnetic field and use it to guide them. This work has shed some light on this mysterious migration.

Note: Fisheries and Oceans Canada researcher Martin Castonguay is part of the research team that tagged the eels with satellite tags.

At the beginning of the 20^th century, a Danish scientist discovered that the southwestern Sargasso Sea, more than 500 km south of Bermuda, was the only location where small larvae of the American eel and European eel (two distinct species) could be found.

Therefore, we have indirectly known about this eel spawning site for a long time. However, no adult American eels were ever found in the open sea or at the spawning site. Furthermore, scientists had no idea of the migration routes and guidance mechanisms of these fish, which embark on one of the most spectacular migrations of the animal kingdom.

Martin Castonguay
Science