The long-distance migrations of American eel have puzzled and fascinated scientists for more than a century. An adult eel has never been caught in the open ocean or at their putative spawning grounds in the Sargasso Sea. This project aimed to establish migration routes of eel, and identify living and non-living factors encountered along the way. Investigations of the movement patterns of juvenile (yellow) and adult (silver) eels from the St. Lawrence River to the Sargasso Sea began in 2010.

Multiple complementary approaches were used, including acoustic and satellite telemetry, numerical modelling (OTN project 4.1) and chemical analysis. Acoustic receivers were deployed from the upper St. Lawrence River to the mouth of the Gulf of St. Lawrence, where 604 eels were acoustically tagged to determine the timing and pathways of this migration segment. While detection rates were unexpectedly low, data revealed that the eel migrated downstream more than 200 kilometres over 13 to 67 days using nocturnal ebb-tide transport to leave the Gulf.

The oceanic migrations of adult eels were documented using pop-up satellite archival tags, which recorded eels’ positions over a three-month period before detaching and transmitting their archived data via satellite. Data from 60 satellite-tagged adult eels revealed substantial predation by porbeagle sharks before exiting the Gulf.

At-sea trajectories were reconstructed for 27 eels, including five tracked all the way to the Sargasso Sea, providing the first formal observation of adult eels entering their spawning grounds. These tags recorded epic oceanic migrations of more than 2,700 kilometres using similar paths and against ocean currents, indicating the complexity in the orientation and navigation mechanisms used by eels throughout the migration.