An adult American eel has been tracked travelling 2,400km from the east coast of Canada, over the continental shelf and into the Sargasso Sea, reports a study in Nature Communications. This provides the first direct evidence that eels are capable of making the long migratory journey to spawning grounds that previously had only been inferred by the presence of eel larvae.
More than a century ago, eel larvae were found in the Sargasso Sea near Bermuda, thousands of kilometres from the inland continental rivers of North America and Europe where adult eels are found. Since then, no adult eel has ever been recorded in the open ocean, and almost nothing is known about the precise location of their ocean spawning grounds, or the migratory routes they take to get there.
Melanie Beguer-Pon and colleagues fit satellite transmitting tags to 38 adult eels collected from rivers and estuaries in Nova Scotia, Canada, and track their movements after release from the coast. They find that, among the 28 tags that successfully transmitted data, eight were tracked moving over the continental shelf and into the open ocean (at a depth of 2,000m), with a single eel tracked to the northern limit of the Sargasso Sea.
Tracking eels over such large distances is particularly challenging, since not only can satellite tags impede the swimming performance of eels, but they are also designed to be released from the animals after a certain period of time to enable the data they hold to be retrieved. Although two of the tags were eaten by predators during the journey, most of the eels provided tracking data over around 25 days, while the single eel that travelled to the Sargasso Sea held onto its device for 45 days before the tag was released.