American eel (Anguilla rostrata) migration, which has been a mystery for more than 100 years, has now become clear. In a new study, scientists were able to detect and track eel migration to the Sargasso Sea for the first time in history.
For many years, it was widely known that these eels spend their lives in freshwater rivers in north eastern North America. However, in a 1904 discovery, they found eel larvae in the Sargasso Sea, an algae-rich area in the middle of the Atlantic Ocean.
The Sargasso Sea is a region in the middle of the North Atlantic Ocean which is the only sea on the planet that has no coastline. It is surrounded by ocean currents at all sides. Experts have suggested that they went there for spawning. The migration route has never been established because no adult eel was ever seen in that area.
However, this study published in the journal Nature Communications is the first to determine how eels reach the Sargasso Sea without being seen by anyone. For the first time, a team of scientists from Canada installed pop-up satellite archival tags to 38 eels captured in the coast of Nova Scotia. They were released in the Scotian coast in the fall of 2012, 2013 and 2014.
Out of the 38 released eels, only 28 transmitters showed up in the Atlantic Ocean, which provided the needed data for the researchers. They found out that eels were measuring the temperature and salinity level of the ocean in order for them to locate high areas.
"Our data nonetheless shows that the eels don't follow the coastline the whole way, they can cover the route in just weeks, and they do go to the Sargasso Sea. 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," said author and scientist, Julian Dodson.
They reiterated that the tags do not cause any harm to these animals but may cause a dragging sensation making it a little harder for them to swim. These tags continuously recorded water temperature, depth, and light levels throughout the whole journey of the eels.
They discovered that the eels go through two distinct migratory phases in which upon release in the Scotian Shelf, the eels went south and slightly east toward the edge of the continental shelf. The first phase mainly dealt with swimming in shallow waters at night and in deep waters during the day.
Then they go east along the Scotian shelf, exiting at the Laurentian Channel and the Gulf of Saint Lawrence that flow into the Atlantic Ocean. The second phase involves their travel to the Sargasso Sea where they rely on the salinity and temperature of the water to reach their spawning site.
"The American eel still has many secrets to reveal. We're continuing to satellite-tag open-ocean-migrating eels to unravel more of their mysteries," the scientists noted.
Mark Chatterley | Flickr
