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Few species — other than the rat, the cockroach and the human being — are able to traverse the globe, adapting to ever-shifting conditions in climate and habitat.
Leatherback sea turtles are one of those species, populating every ocean save for the Arctic and the Antarctic. Yet, one of the first studies of wild leatherback turtle behavior found that they don’t care for active construction sites.
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A recent federally funded study found that loud noises associated with offshore wind development could impact leatherback sea turtles’ habits, causing them to temporarily avoid certain areas and even spend less time looking for food. The findings come on the heels of an earlier study that found that while leatherback turtles overlap very little with offshore wind lease areas, they spend more time near the coasts where they’re vulnerable to deadly boat strikes.
The world’s largest turtle at a whopping 1,000 pounds on average, leatherback sea turtles get their name from their tough, rubbery skin that might resemble a cool jacket. With an Atlantic migratory zone that extends from the Caribbean to Nova Scotia and beyond, the endangered species can travel up to 10,000 miles a year in search of their main food source: jellyfish. But although they can travel far for food, their nesting habitat — where they breed and lay their eggs — has become scarcer and scarcer over the past three generations, leaving the sturdy turtle vulnerable to extinction.
To study the effects of loud noises on leatherback sea turtles, Samir Patel had to get up close and personal with his subject.
“You’re kind of laying on this surfboard or diving board above the water and the turtle is swimming below you, and you have to stick the tag on the turtle while it’s coming up for air,” said Patel, the lead author of the paper on noise impacts published this March. Patel is a senior research biologist at the Coonamessett Farm Foundation, a marine research group based in Falmouth.
Patel applied the tags harmlessly with a suction cup, which can be popped off the turtles remotely when the experiment is done.
“Luckily, leatherback sea turtles have very smooth, wet skin that makes it easy for a suction cup to stay on,” Patel said.
The tag monitors the turtle’s location and depth while a research boat simulates the kind of jackhammering noise often associated with offshore wind drilling. Using both tag data and video monitoring, Patel’s team found that leatherback sea turtles often swim away from loud noises — although not in much of a hurry — and dive to depths where the sound may be less audible. Some turtles also foraged less for jellyfish while the noise played, perhaps distracted by the din.
The study marks one of the first observations of leatherback sea turtle behavior in the wild, Patel said. The suction-cup tagging system, though somewhat ridiculous-looking, allowed Patel to tag the turtles without capturing them, which can often stun or traumatize the turtles and lead to erratic or unusual behaviors.
The result is a more true-to-life slice of leatherback sea turtle behavior. But Patel acknowledged that he would need to learn a lot more about how the loud noises impacted turtles before his information could be used for, say, offshore wind regulation or other regulations on ocean noise. For one, the study only played the drill-like noises for an hour at most, and did not explore the impacts of long-term exposure.
“Are we blowing out their eardrums, or are we just causing them to be like, ‘I’m going to leave now?’” Patel said. “It’s two different scales of impact, but in management, an impact is an impact is an impact.”
Leatherback turtle research methods
Scroll from left to right to view diagram ↓
A) Prior to deployment, researchers search for turtles using boat and aerial-based observers. B) Tags are deployed directly from the vessel without capture of the turtle. C) Turtles are given time to acclimate to the tag. D) Turtles are exposed to the sparker towed by a separate vessel. E) Turtles are allowed to re-acclimate after exposure during the remaining time for the tag release to activate. F) Tags are recovered manually from the tagging vessel.
Credit: nature.com
Pseudo-turtles
Not every study has the privilege of getting so close to a leatherback. Some, like another paper Patel recently co-authored and funded, have to rely on technology instead.
Using machine learning modeling, lead scientist Mitchell Rider from the Cooperative Institute for Marine and Atmospheric Studies at the University of Miami estimated where leatherback sea turtles might be, based on where a small group of tagged turtles have been.
“People usually think of leatherbacks as an oceanic species — and they are,” Rider said. “They’ll go out in the Gulf Stream. They’ll go all over to Europe if they want to, whatever the currents might bring. But we find that they’re spending a lot of time along the coast, especially in the Mid-Atlantic Bight and southern New England.”
It’s the search for sea jellies that’s likely leading leatherbacks closer to the coasts than scientists previously imagined, Rider said. That makes them more susceptible to manmade hazards like vessel strikes and fishing gear entanglements.
In New England, however, leatherbacks appear to venture where few boats can follow.
The study found that leatherback turtles spend a considerable amount of their time around Nantucket Shoals, an area of shallow, ever-shifting ocean bottom off the southeastern edge of Nantucket. The area is infamously difficult to navigate among New England boat captains, although some experienced surfclam fishermen brave the waters looking for catch.
Relatively untouched by humans, Nantucket Shoals is brimming with biodiversity — including plenty of the gelatinous zooplankton that leatherback turtles love to eat.
“The Shoals have that unique aspect of being complicated, which is good for ecology, but also good for avoiding humans,” Patel said.
In spending time near Nantucket Shoals, some of these turtles may travel through the offshore wind lease areas off the coast of Martha’s Vineyard, Rider said. But at least according to this study, they don’t appear to stick around. The conditions around offshore wind projects like Vineyard Wind don’t appear conducive to jellyfish, and therefore leatherbacks likely move on quickly.
The trouble, Rider said, is that he can’t say for certain where leatherbacks aren’t. Rider’s modeling technology, the most advanced in the field thus far, relies on a limited number of tagged turtles, advanced environmental modeling, and a tool called pseudo-absences to determine leatherback turtles’ movement patterns. Pseudo-absences, as the name suggests, are places where Rider’s model has determined turtles are unlikely to be, based on the information at hand. But there’s always a chance that turtles could show up where they aren’t expected.
“It’s one of the big assumptions we’re working with,” Rider said. “That’s why they’re called pseudo-absences — they’re fake absences.”
Ten years from now, Patel said that researchers could have more complete data to fill in these pseudo-absences with real absences. Until then, Rider, Patel, and his colleagues must adapt to the challenges of researching an animal whose numbers are already dwindling.
Working with protected species is highly regulated and expensive, Patel said, meaning most studies will never reach the sample size typical of, say, a medical study.
“Surprisingly, Mitch’s study is one of the largest studies on leatherbacks that’s existed,” Patel said.
As of 2020, there were fewer than 1,700 known leatherback turtle nesting sites within the U.S., most of them in Puerto Rico. The leatherback turtle’s total population has been estimated to have decreased by 40% over the past three generations.
The greatest threat to the species is the loss of nesting habitat in tropical climates. But boat strikes and fishing gear also pose a risk to the slick, black giant. Leatherback turtles can end up as bycatch on fishing boats or stranded on beaches after being hit by a vessel.
Both of these studies, although breakthroughs in leatherback sea turtle research, are further proof of the need to keep studying leatherbacks, Patel said.
“Sea turtles have been around for 150 million years, so they’re a pretty robust set of animals that have the ability to be pretty accommodating to environmental change,” Patel said. But “when that environmental change happens too quickly,” he says, sea turtles “could have significant problems.”
Email Brooke Kushwaha at bkushwaha@newbedfordlight.org.
More stories by Brooke Kushwaha
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