the eels is hard because the tags are
cumbersome for the eels. They’re also
expensive, so scientists can only buy
so many.NO. 28
One eel from the 2014 tracking
attempt did make the team’s efforts
worthwhile. Eel No. 28 swam from
Nova Scotia and reached the far
northern limit of the suspected
Sargasso Sea spawning site — a first.
But that’s where No. 28’s tag popped up
to the surface, as designed, leaving the
team to guess at its fate.
“We don’t know what happened to
the eel and why it went much farther
than the others,” Beguer-Pon says. “We
just hope that eel No. 28, as the other
tagged eels, all made it to the spawning
area in the Sargasso Sea.”
No. 28 also taught Beguer-Pon’s
team about its migration route. The fish
commuted in two phases. First, it swam
north along the continental shelf in
shallow waters about 100 miles from the
Canadian coast. Then it made a beeline
south for the Sargasso, vanishing east
of Bermuda and confirming Schmidt’s
theories nearly a century earlier.
And late last year, a large group of
European scientists published their
results of tracking eels from thatcontinent. They tagged some 700 eels,
but only 87 made it from coastal waters
into the open ocean. Then predators ate
most of the fish not long after they left
the coast. None made it to the Sargasso.
“How they get there is not known,”
says behavioral ecologist David Righton
of the Centre for Environment,
Fisheries and Aquaculture Science, “or
why the journey has become so long
over evolutionary time.”
Other adult fish, like salmon, live
their lives at sea and return to the rivers
and streams of their birth to spawn.
Eels do the opposite: They’re born in
the Sargasso, and then ocean-spanning
currents carry them to their home
waterways, whether that’s in Maine
or Maryland. That ride determines
where they end up. But on their return
trip, eels aren’t heading for some set
location. The Sargasso Sea’s boundariesshift because it’s surrounded by ocean
currents; it’s the only sea not bounded
by land. So how do adult eels know
where to go?
“Navigation is likely helped by a
number of cues and clues like odor,
magnetism and other oceanographic
features,” says Righton, the lead author
on the 2016 European study published
in Science Advances.A GREAT MIXING
Recent genetic studies are also
uncovering clues about eels’ hidden
spawning. Lab studies reveal that
European and American eels each
belong to a single population. So an
eel that lives in a Norwegian fjord
is no more cold-adapted than one
living in the Mediterranean. It implies
that these eels, like their kin in other
oceans, breed in massive orgies, with
European eels mating somewhere
east of American eels. Scientists call
it panmixia. “It takes place deep in
the water column, not on the surface,”
says ecologist José Martin Pujolar of
Denmark’s Aarhus University. “That’s
why no one has seen it.”
To picture this secret spawning,
imagine swarms of eels writhing about
in the dark. The males cast their sperm
into the water. The females are fertilizedThe Sargasso Sea’s
boundaries shift
because it’s surrounded
by ocean currents.
So how do adult eels
know where to go?
Satellite
tracking
tagsFROM LEFT: MARTIN CASTONGUAY (2); SHILIANG SHAN
October 2017^ DISCOVER^23Scientists recently released
satellite-tagged eels along the
Nova Scotia coast and tracked
one all the way to the Sargasso
Sea — a first. Researchers (from
left) Julian Dodson, Martin
Castonguay and Melanie Beguer-
Pon hold an eel that will soon be
swimming to the Sargasso.