It's 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 today in Nature Communications.

It's one of the big biological mysteries of our times. - Julian Dodson, Laval University

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.

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.

Shark problems

The team satellite tagged 38 eels over three years. During their first attempts, eels released in the Gulf of St. Lawrence all got eaten by sharks, $4,000 satellite tags and all, Dodson said. The researchers decided it was best to release them in Nova Scotia, farther away from predators, but the eels in that region were half the size of those in the St. Lawrence and too small to carry the satellite trackers. Eventually, the researchers caught a bunch of eels in the St. Lawrence and trucked them around the predators to Nova Scotia for release.

Environmentalist Philippe Cousteau and his fiancee Ashlan Gorse dive in the waters off Bermuda in August 2013. The Sargasso Sea is a major habitat for species such as sharks and the nursery for both American and European eels. (Chris Burville/Pew Charitable Trusts/Associated Press)

In that case, most of the tags successfully transmitted data, although most popped off early. Eight of them stayed on long enough to track the eels as they high-tailed it straight from shore to the edge of the continental shelf, where the depth drops off to 2,000 metres. There, they split off in different directions and most lost their tags, but one managed to keep her tag as she headed south to the northern limit of the Sargasso Sea, covering 2,400 kilometres over 45 days.

The results suggest eels spend very little time in shallow waters, where they can be more easily observed, before effectively disappearing into the open ocean.

"Our results represent the first direct evidence of adult Anguilla migrating to the Sargasso Sea," the paper reported.

Dodson said his team has just tagged and released another 14 fish, and hope to reproduce their result, as the sample size, often referred to as "n," is too small to draw any firm conclusions.

"It's a great observation," he said. "But it's still n equals one."