Stanford marine scientists unlock enigma of endangered sea turtles’ migratory patterns

Every several years, small groups of hatchling North Pacific loggerhead turtles, an endangered species, embark on years-long journeys spanning thousands of miles from Japan to the West Coast of North America. 

Possible explanations for the turtles’ olympic feat — a voyage traversing the near-freezing, virtually impenetrable deep ocean-zone to the foraging grounds of Baja California — have eluded scientists for decades. While it is widely understood that loggerheads depend on the Earth’s magnetic fields for navigation, researchers have so far been unsuccessful in elucidating how these tropical sea turtles overcome such extreme temperatures. 

In 2021, however, Larry Crowder, a marine ecology and conservation professor, Dana Briscoe, senior data scientist at the Doerr School of Sustainability, and several of their colleagues published the hypothesis that El Niño southern oscillation (ENSO) and other phenomena intertwined with global warming create warm water channels enabling the turtles to migrate through otherwise impassable regions.

With a team of international scientists, Crowder, Briscoe and other Stanford researchers spearheaded a five-year project called Loggerhead STRETCH (Sea Turtle Research Experiment on the Thermal Corridor Hypothesis) starting in 2022 to prove the thermal corridor hypothesis through satellite telemetry. By equipping turtles with satellite tags, the scientists can monitor their movements in relation to oceanographic conditions. 

The team released a second cohort of 25 loggerhead turtles with tracking devices this July during the onset of the summer’s El Niño conditions. The team aims to release two more cohorts for a total of 100 turtles over the next several years. 

“This year’s turtles in particular are doing something so unique and so novel that every day it has been really exciting to log on to the website and to look at the map and see where they’re going,” Briscoe said.

The project represents a novel approach with the use of experimental oceanography, a new concept within the field. While scientists have used satellite tagging on marine animals for decades, the STRETCH project is the first time that scientists have deployed satellite tagging to test a prior hypothesis. 

“We have a very thorough case from the descriptive data, but we can actually test the hypothesis and let the turtles tell us what we’ve got right and what we’ve got wrong,” Crowder said. “Every day, we’re learning new things from the turtles… It’s more exciting if you’re discovering new things that show where you’re wrong, where your assumptions are wrong, where you can do something better.” 

The unpredictability of the turtles’ movements challenges expectations based on decades’ worth of datasets, Briscoe said. “One of the highlights of this project has been seeing these turtles do something completely different than we ever would have expected.” 

While Briscoe often sits behind a computer to analyze sea turtle data, she also appreciates the opportunity to work directly with turtles at the Port of Nagoya aquarium. “To be able to contribute to seeing these turtles released into the ocean and track their movements has been a real highlight for me,” she said. 

Catherine Lee Hing, a first-year Ph.D. student studying with Crowder and a contributor to the project, described the difficult yet rewarding process of tagging and transporting turtles from Japan to Panama. 

“Even if you think you are a hundred-percent prepared, there are a whole range of other things that come up, so you have to be able to adapt freely,” she said. “When you’re doing an experiment, anything can happen.” 

The project will also yield insights that can inform the development of dynamic conservation and management strategies, according to the researchers. Given increasing ocean temperatures and other warm-water phenomena due to global warming, the project will also allow the scientists to anticipate the changing migration patterns of the turtles and other species within an evolving marine environment. 

“As the North Pacific region continues to undergo significant changes, understanding how these sea turtles may respond and adapt to climate variability is critical for the long-term sustainability of this endangered species,” said Bianca Santos Ph.D. ’24, a contributor to the STRETCH project. 

Between climate change and marine heat waves, Briscoe said that the ocean is undergoing an “almost unprecedented transformation.” What the researchers assume to be areas of established habitat for turtles and other migratory species are going to shift, expand and contract accordingly as the ocean changes.

“It will be even more important for us to create flexible and dynamic management approaches to be able to identify where we might see new habitats establish,” Briscoe said.

The project also offers an opportunity to educate the world about turtles, the ocean and climate change, said Briscoe. “Our biggest hope is that this contributes to conservation of this species, and hopefully other species that we can’t necessarily track, but use the same part of the ocean. Turtles serve as a proxy, or a sentinel, for other vulnerable animals that we may not be able to track as easily.” 

For Crowder, what began as a ‘crazy passion project’ has demonstrated the possibility of pushing the frontiers of knowledge.

“I hope this project opens the door to all the work that’s going on with animal movement, data, terrestrial and marine, to say we can step beyond the descriptive stage to the experimental stage, at least in some cases,” he said. 

Eight years ago, a researcher entrusted Crowder with the legacy dataset of satellite and sea turtles at the Sea Turtle Conference in Baltimore. After the publication of the 2021 paper, Crowder shared his idea for the STRETCH project.

“[The researcher] said to me ‘You’re crazy.’ And over my career, I’ve come to accept that as a compliment,” Crowder said. 

The Stanford scientists aim for the Department of Oceans and the Doerr School to begin incorporating experimental oceanography in future projects. Reflecting on the value of out-of-the-box thinking, Crowder said, “Don’t ask if your dreams are crazy. Ask if they’re crazy enough. I’ve been really lucky throughout my career to do crazy things and actually have them work out sometimes.” 

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