The waters of the Northeast were once chock-full of American eels, which lived in freshwater rivers and estuaries before swimming hundreds of miles each year to spawn in the Sargasso Sea. However, thanks to development and hydroelectric dam construction, the eels have lost 80 percent of their habitat, Newsweek reports. Now, the once-plentiful fish is dwindling in population—making it all the more important for conservationists to ensure they make it safely to their only known breeding ground. 

Between 1904 to 1922, researchers found eel larvae in the Sargasso Sea, an algae-rich patch of ocean in the North Atlantic that serves as home to a vast array of diverse marine species. For more than a century, scientists have assumed that the sea was the eels' main reproductive hub. However, they had never witnessed the fish actually breeding there, nor did they have scientific data to support that they actually travel so far away to spawn. So Canadian researchers attached satellite transmitters to 38 American eels, and tracked their journey to see where they ended up.

While not all of the eels made it to their final destination, academics were able to follow one fish for 1500 milesall the way from the Scotian Shelf off Nova Scotia to the Sargasso Sea. The experiment provided researchers with concrete proof that the annual migration exists, the study’s authors explain in the journal Nature Communications.

Not only might this knowledge help protect American eels—which are listed as endangered on the International Union for Conservation of Nature (IUCN) Red List and assessed as "threatened" in Canada—it also sheds new light on the life cycle of an otherwise elusive species.

“We knew that millions of American eels migrated to reproduce, but no one had yet observed adults in the open ocean or the Sargasso Sea. For a scientist this was a fascinating mystery,” one of the study’s authors, Professor Julian Dodson of Université Laval in Québec, said in a press statement

[h/t Newsweek]