That’s the problem being addressed
by a series of research projects conducted by an oceanographer at the Northeast Fisheries Science Center in Woods Hole, Mass., and a doctoral student at the
University of Rhode Island. The scientists used satellite-tracked oceanographic
instruments called drifters to determine where Kemp’s ridley turtles that are
late to return south in the fall are most likely to float ashore near death.
The critically endangered turtles
lay their eggs on beaches on the Gulf Coast of Mexico
in a mass nesting event
called an arribada. After spending their first couple years far offshore in the
Sargasso Sea near Bermuda, many of the young turtles visit the waters of the
Gulf of Maine to feed on crabs and other small marine creatures that live on
the seafloor.
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| Kemp's ridley turtle release on Cape Cod. (Todd McLeish) |
“There are little warm water bridges
from the Gulf Stream that come up here, and we think some of the turtles are
riding those into the area,” said URI student Felicia Page. “The problem comes
when those little bridges close off and the water in Cape Cod Bay and the Gulf
of Maine stay warm, which keeps the turtles here longer than they should
instead of heading south in September.”
Not realizing that they must swim
north to escape Cape Cod Bay before migrating south, the turtles “hit a wall of
cold water” and become hypothermic or cold-stunned as the water temperature
rapidly cools, slowing their heart rate, respiration and metabolism, according
to Page. Unable to swim, the animals drift on the currents and eventually land
on the shores of Cape Cod Bay.
It’s a problem the turtles have
faced for many years, but because the climate crisis has caused the Gulf of
Maine to warm faster than almost any other body of water, the turtles are
increasingly finding themselves trapped in New England waters.
“It can kill them if they can’t
recover, and they can only recover if the water warms up,” Page said. “They
have to be rescued in order to survive.”
In the 1970s and 80s, fewer than 100
Kemp’s ridley turtles were found stranded on Cape Cod beaches each year, but by
2014 the number reached 1,100. Volunteers with Massachusetts Audubon wander the
coastline each fall to rescue as many as they can. Many are brought to New
England Aquarium and other agencies to be rehabilitated before being released
the following summer.
To help the volunteers identify the
most likely beaches to search from day to day, Page and oceanographer James
Manning deployed drifters built by local students to see if they could forecast
where the turtles would land based on winds and currents. They also deployed
sensors on commercial fishing gear to identify how the water temperature
changes at different depths.
“What physical processes are causing
the turtles to suddenly come ashore at certain days and places?” asked Manning.
“It’s a combination of currents and water temperature, we think.”
The challenge is that the speed and
direction of the current at the surface is often different from that on the
seafloor, he said, and no one knows at what depth the turtles spend most of
their time. The scientists have used surface drifters, underwater drifters, and
even drifters shaped like sea turtles, and they all travel a different route.
“We’ve shown how complicated it is,”
Manning added. “We can’t yet make predictions about stranding locations yet.”
So far, they have confirmed that the
turtles do not begin to strand on Cape Cod beaches until the water drops below
52 degrees. In most years, that means the stranding season begins in late
October and continues through the end of December, when most turtles have
either escaped south, washed ashore alive or died.
Page is continuing her research this
winter to examine how underwater currents affect the stranding locations of
Kemp’s ridley turtles.
“A lot of the turtles don’t float at
the surface; they’ll swim to deeper waters trying to escape the cold or they’ll
sink to the bottom if they get cold-stunned at the surface,” she said. “So
we’re looking at different levels of the water column to see how the current at
different levels affects stranding locations.”
By next fall, when the stranding
season begins again, Page expects to have an app or software program developed
so volunteers can plug in data about wind direction, wind speed and water
temperature and know the best area to search for cold-stunned turtles.
This article first appeared on EcoRI.org on January 8, 2020.
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