Beginning in 1904, larvae of American eels (Anguilla rostrata) have been discovered in the Sargasso Sea near Bermuda – thousands of kilometers away from the inland rivers of North America where adults are found. This led researchers to assume that mature eels migrate as many as 4,000 kilometers (2,485 miles) from continental waters to their spawning area. But in all this time, not a single adult has ever been caught in the open ocean. And pretty much nothing is known about their spawning site or how they get there. 

Now, for the first time, researchers have tracked an adult American eel traveling 2,400 kilometers (1,490 miles) from the east coast of Canada, over the continental shelf, and into the warmer waters of the Sargasso Sea. The findings, published in Nature Communications this week, provide the first direct evidence of the oceanic migration of adult American eels and the first observation of any eel approaching spawning grounds in the Sargasso Sea.

To follow the animals en route to their spawning site, a team led by Dalhousie University’s Mélanie Béguer-Pon attached pop-up satellite archival tags (PSATs) to 38 maturing (called silver) eels caught by commercial fisherman in Nova Scotia and St. Lawrence Estuary in Canada. They’re listed as endangered in the U.S. and threatened in Canada. The two types of tags used, which weighed either 45 grams or 29 grams (1.6 or 1 ounce), were attached at the eels’ center of mass or near the head; large animals were selected to minimize drag caused by the tags. The eels were then released on the Scotian Shelf off Nova Scotia in the fall of 2012, 2013, and 2014.

Of the 38 PSATs released over three years, 28 successfully transmitted their data after popping off between half a day and 57.2 days after deployment. The team obtained a total of 431 days of eel tracks. Two of the tags spent a few days in the stomach of predators, and based on the recorded temperature, depth, and light data, these eels were likely eaten by a porbeagle shark and a bluefin tuna.

All of the eels immediately headed south and slightly to the east from the Scotian Shelf. Eight eels were successfully tracked to the open ocean off the continental shelf. And one of them (eel #28) migrated 2,400 kilometers to the northern limit of the spawning site in the Sargasso Sea in 45 days. It crossed the Gulf Stream 29 days after its release, and it was traveling at an average speed of 49 kilometers (30 miles) per day. The eels experienced a huge temperature range during their migration: from 2.5 degrees Celsius (36.5 degrees Fahrenheit) over the Scotian Shelf to 25.1 degrees Celsius (77.2 degrees Fahrenheit) in the Sargasso Sea.

The team reconstructed the routes of the eels at sea based on their daily locations. It seems that migration occurred in two distinct phases: one over the continental shelf and in the shallow water along its edge, another in deeper waters straight south towards the spawning area. This suggests that salinity and temperature help guide them, since both factors increase from the coast to open waters.

“To fill in the blanks is a dream project for any biologist,” study co-author Julian Dodson of Université Laval told IFLScience. Though, to be clear, “we have only just begun to understand what is going on.”

Here’s a very cool video of the eels being released off Nova Scotia: 

Image in the text: A silver American eel equipped with pop-up satellite archival tag and released off Nova Scotia in Canada. Mélanie Béguer-Pon
Video credit: Mélanie Béguer-Pon