An adult American eel was just tracked traveling nearly 1500 miles from the east coast of Canada, over the rocky continental shelf, into the deep ocean and, finally, at his destination: the Sargasso Sea near Bermuda.
The lengthy and perilous migration, documented in the journal Nature Communications, provides the first direct evidence that eels are capable of making such a journey. The migration was previously only theorized, based on sightings of adult American eels in North American inland rivers and surprising discoveries of their larvae far away in the Sargasso Sea.
The study "sheds light on an often under-appreciated species, which performs one of the longest and most amazing migrations in the animal kingdom," co-author Martin Castonguay of Fisheries and Oceans Canada told Discovery News.
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Lead author Mélanie Béguer-Pon of Dalhousie University, Castonguay and their colleagues fitted satellite-transmitting tags to 38 adult eels collected from rivers and estuaries in Nova Scotia, Canada. The researchers then tracked the eels' movements upon release from the coast.
Predators swallowed some of the tags, but eight were tracked moving over the continental shelf and into the open ocean at a depth of 1.2 miles. A single eel, nicknamed "Star" by the researchers, was tracked to the northern limit of the Sargasso Sea.
Castonguay said that the eels may use temperature and salinity cues for orientation as they migrate on the continental shelf. It is not an easy trip.
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"As eels leave freshwater, they sometimes have to pass hydropower dams and will often be killed going through turbines," he explained. "Moreover, evidence from a previous study of ours indicates that eels can be preyed upon by sharks and tunas as they try to exit the Gulf of St. Lawrence."
The eels do not eat during the journey, but instead rely upon fat accumulated during their time in rivers.
Each day, the eels also move up and down the water column, staying at deeper levels during the day, likely for predator avoidance. They also save energy by remaining in cooler waters throughout the day.
As for why they migrate so far in the first place, Castonguay said, "You can think of the Sargasso Sea as the Eel Mecca. There are over 30 species of eels spawning in the Sargasso Sea, but only two species invade continental waters during their growth phase: the American eel and the European eel."
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He added that all other eel species spawning in the Sargasso complete their entire life cycle in the ocean.
Once at the Sargasso, the eels gather by species into select areas. The American eel spawning area extends more to the west, for example, while the European eels head to the east.
Defining the migration route to and from this site indicates areas that warrant protection to help ensure that the epic eel migration is not impeded. American eels are already classified as endangered and are on the IUCN Red List of Threatened Species.
John Casselman, a professor of biology at Queen's University, told Discovery News that any direct evidence on eel migrations is "sadly wanting, and is a pleasure to see." He said that the findings "help to unravel the mysteries of this iconic species. The more we learn, the more we will respect and appreciate this mysterious and ancient species."
Since the study's completion, the researchers have begun yet another investigation. They are using "an improved tag attachment method," according to Castonguay, and are hopeful that they "will have more spectacular results to report on in a few months, when the tags from this new experiment will float up to the surface and start sending their archived temperature and depth data to satellites."