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.
|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.