After generations of speculation, scientists have finally been able to track European eels all the way to their breeding grounds in the Sargasso Sea – thousands of kilometers across what is considered one of nature’s most impressive animal migrations.
Scientists are buzzing with excitement because this is the first direct evidence of a long-suspected part of the eel life cycle, proposed nearly 100 years ago.
Until now, no eggs or eels have been found in the Sargasso Sea of the North Atlantic Ocean.
“The eel has intrigued scientists for millennia,” said study author and fish biologist Kim Aarestrup from the Technical University of Denmark. He said on Twitter.
“For the first time, we followed eels to their spawning grounds.”
Back in the early 1920s, Danish biologist Johannes Schmidt discovered eel larvae in the Sargasso Sea, far from freshwater, river and coastal habitats in Europe and Africa.
Schmidt, who published his results in 1923, foresaw the next century of research to understand how snails reproduce, describing his efforts as “periods of rapid progress interspersed with disappointment, promising discoveries, and others in which the solution of the problem is shrouded in ever-deeper obscurity.” .”
For decades since then, scientists have been trying to trace the eels back to their breeding grounds—a challenge made difficult by the many obstacles in their path: dams, dams, pollution, habitat loss, and overfishing. Sharp decline of eel in Europe (Anguilla anguillaSince the 1980s, the numbers have made this task more difficult and more urgent.
But don’t underestimate these mysterious creatures. European fish migrate 5,000 to 10,000 kilometers (3,100 to 6,210 mi) to spawn in the sea, after which their larvae drift to land and the relative safety of rivers.
Using satellite tags, the researchers behind this latest discovery obtained tracking data from 21 female European eels navigating the last leg of their epic journey southwest of the Azores, a volcanic archipelago in the North Atlantic Ocean, far west of Portugal.
🕵️♀️ We help solve the mystery that has intrigued scientists for centuries!
❓ Where do European fish spawn and how do they get there?
😮 We have the first direct evidence that European eels reach the Sargasso Sea to spawn.
📰 More: https://t.co/TebziSaaFI pic.twitter.com/m7fr3FTzvp
— Environment Agency (@EnvAgency) October 15, 2022
Past studies tracking eel migration have shown that eels from all over Europe converge around the Azores before heading out into the Sargasso Sea, which is surrounded by four gyrating ocean currents and named for its vast forests of Sargassum seaweed.
The eel was caught, tagged with removable satellite trackers, weighed down for DNA testing and released back into the Atlantic Ocean from the Azores in 2018 and 2019.
Six eels arrived at the Sargasso breeding grounds months later, still with satellite trackers; data was collected from 15 other eels along the way. The longest recorded straight-line distance was 2,275 kilometers (1,410 mi).
“Their journey will reveal never-before-known information about eel migration,” says Ros Wright, a fisheries biologist from the UK’s Environment Agency, who led the research.
It is still not clear how the eels found their way to the Sargasso Sea or how long their spawning season lasts.
The swimming speed of the eel in this study, which averaged 6.8 kilometers (4.2 miles) per day, and the length of the marathon journey, which lasted more than a year, suggest that these long-haulers must have made a very calculated migration.
“Instead of making fast migrations to spawn at an early opportunity, European eels can make long, slow spawning migrations at depth, which conserves their energy and reduces the risk of mortality,” Wright and team wrote in their published paper.
“This time will allow them to complete their reproductive maturity before they reach the spawning grounds.”
“It’s also incredible to know that they go deeper than 1,000 meters on the way!” James Maclaine, Chief Curator, National History Museum, Great Britain, he tweeted. It plunges into darkness at over 3,280 feet.
But questions remain about the timing and navigation of eels to travel thousands of kilometers across the open ocean to reach the Sargasso Sea.
Perhaps they sense Earth’s magnetic fields like Chinook salmon (Oncorhynchus tshawytscha), return to the exact stream they hatched from. Sniffing the sense of smell or tracking ocean currents or temperature fronts are other possibilities.
While it may take some time to unravel these threads, for now, this new study completes the map of eel migration, placing the Azores at the center of conservation efforts to combat eel decline.
“This discovery highlights the role of the Azores eel in the life cycle,” says study author and fish ecologist José Manuel N. Azevedo of the University of the Azores.
“This will help scientists and conservationists take action to restore eel habitats in the archipelago.”
The study was published Scientific Reports.