Canadian eel tracked on 2,400-kilometre migration to Sargasso Sea -1st direct evidence of eels migrating to tropical nursery

By Emily Chung, CBC News

Posted: Oct 27, 2015. Updated: December, 2018 10:50

The team attached satellite tags to 38 eels over three years. During their first attempts to track them, the eels released in the Gulf of St. Lawrence all got eaten by sharks, $4,000 tags and all.
It’s one of the big biological mysteries of our times.- Julian Dodson, Laval University

The team attached satellite tags to 38 eels over three years. During their first attempts to track them, the eels released in the Gulf of St. Lawrence all got eaten by sharks, $4,000 tags and all. (Mélanie Béguer-Pon)

 
A mystery that has puzzled scientists for a century — how swarms of baby eels appear in the Sargasso Sea near Bermuda when adults have only been found in faraway places like Canada’s St. Lawrence River.
 

For the first time, Canadian researchers have tracked an adult female eel from Nova Scotia all the way to the northern edge of the Sargasso Sea with a satellite tracker — a 45-day journey of about 2,400 kilometres, described in a new paper published in Nature Communications.If they can confirm the path taken by that eel is the typical migration route used by Canadian eels, that may help scientists figure out measures that could be taken to conserve the endangered fish.

American eels, known by the scientific name Anguilla rostrata, are found in watersheds from Venezuela in the south to Greenland in the north, says Julian Dodson, a University of Laval researcher who co-authored the new paper.
In Canada, they historically lived throughout the St. Lawrence Seaway and Great Lakes, although their populations have fallen dramatically in the past 20 years, largely because of fishing and hydroelectric dams that they have trouble crossing.
Males typically live further south than females.

Eels with satellite tags
Postdoctoral researcher Mélanie Béguer-Pon, lead author of the paper, used surgery to run plastic threads like fishing line under the eels’ skin behind their heads to attach a harness that could help support the torpedo-shaped satellite tracker. (Martin Castonguay/DFO)

“In the St. Lawrence, it’s 99 per cent female,” said Dodson, adding that adult eels can spend up to 20 years in freshwater before migrating back to the ocean to breed.

The “nursery” for both American and European eels was first discovered in the Sargasso Sea near Bermuda in 1904. That’s where swarms of transparent, leaf-like baby eels appear each March and April.  Gradually, those young eels drift northward, growing less transparent and more eel-like along the way. By the time they reach Canada, they are “yellow eels.”

But no adult eels have ever been found in the Sargasso Sea or anywhere in the ocean along the way.

“It’s a fascinating question – you’ve got a bunch of fish at Point A and you’ve got a bunch of young fish at Point B and you have absolutely no idea what happens between Point A and Point B,” Dodson said. “It’s one of the big biological mysteries of our times.”

The mystery means that scientists have no idea whether migrating eels face threats along their migration routes that may be contributing to their dramatic population crash in recent decades.

For a long time, researchers have wanted to be able to use satellite tracking to follow eels on their migration, but it’s not easy. Satellite trackers are typically large, fitted as collars on larger animals like bears. Eels in the St. Lawrence grow to be just three kilograms and 1.2 metres long and have no necks, wrists or ankles to fit collars on.

Finally, the technology has shrunk satellite trackers down to about 40 grams — a size that a large eel could carry.
A team co-led by Dodson and Fisheries and Oceans Canada researcher Martin Castonguay spent several years trying to use torpedo-shaped satellite trackers to follow eels on their migration.

Postdoctoral researcher Mélanie Béguer-Pon, lead author of the paper, used surgery to run plastic threads under the eels’ skin behind their heads to attach a harness that could help support the trackers. The tags were pre-programmed to pop off after a certain length of time and bob to the surface where they could transmit their data to the researchers.

 
 
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