North Pacific sea turtles are very temperature sensitive, preventing them from swimming across the cold eastern Pacific. However, a small number of them somehow make it to the coast of Baja California. Until now, researchers have been unable to explain the secret of this migration mystery, but new evidence suggests that rare streams of warm water lead them to Baja California.
“I think this is a really major discovery in sea turtle science,” senior study author Larry Crowder, a marine ecologist with two California institutes, Hopkins Marine Station and Stanford Woods Environment Institute, told ABC News. “People have long known that loggerhead sea turtles are found in Baja California and the Pacific coast of Mexico, but they didn’t know where they came from.”
This population of loggerheads breeds and hatches on the coast of Japan. From there, baby turtles travel east, usually to the central Pacific, where they stay until they are large enough to be less vulnerable to predators, and then return to Japan. Most turtles stop traveling further east because, like animals that depend on their environment to maintain their body temperature, they are especially sensitive to cold water. But some flocks manage to cross the ocean to the coast of Baja California. A Stanford University team led by Dana C. Briscoe, including other members, used a variety of methods to “tie” the turtles’ paths to their travel conditions. They published the details in the journal Frontiers in Marine Science.
First, the team noticed that satellite tracking data from 231 turtles identified six that had reached the east coast. All six of these turtles migrated for many years when the water was warmer than usual due to El Niño (fluctuations in surface water temperature in the equatorial Pacific Ocean that have a significant impact on the climate. In a narrower sense, El Niño is the phase of the Southern Oscillation, in which the region of heated near-surface waters shifts to the east, while the trade winds weaken or stop altogether, and the upwelling slows down in the eastern part of the Pacific Ocean, off the coast of Peru. The opposite phase of the oscillation is called La Niña). “There is a door that opens when it gets warm, and if the turtles are in the right place at the right time, they can jump to another hemisphere,” Crowder said.
To further explore their “thermal corridor hypothesis,” they studied the atomic composition of the humerus bones in a turtle. Variants of the same element that differ in weight, such as heavier or lighter types of carbon, are present in different amounts in different foods. And in turtles that travel different paths, their diet causes differences in bones, so-called isotope signatures. These signatures allow researchers to compare the turtles’ diet and determine when they got to the coast.
“For every turtle that had a transition in the stable isotope signature, we could get the year of their arrival,” Crowder said. “In warmer years, more turtles appeared, which is encouraging confirmation of the hypothesis.”
The thermal corridor hypothesis adds an interesting feature to consider when studying tortoise migrations. However, it also raises some questions, said Christine Figgener, a Costa Rican marine biologist who studies the behavioral ecology of sea turtles. One is how they return to their nesting grounds in Japan. “This opens up the opportunity for us to open and close thermal corridors,” she said.
While the discovery is encouraging in itself, Figgener noted, it is also important information about the potential impacts of climate change. “As temperatures change and currents get warmer or colder, this will affect our population,” Figgener said, “but I’m not entirely sure for better or for worse.”
In 2014-2016, several turtles appeared in San Diego, California. “Everyone assumed they were in Mexico, had just moved along the coastline,” Crowder said. A previous study by one of the authors of this article analyzed the isotope signature on skin samples from swamps in San Diego. If the turtles moved up the coastline, they would expect the isotope signatures to reflect the signatures of a boob in Baja California. But they found the opposite. The turtles migrated during the El Niño event and the ensuing heatwave in the ocean. Moreover, they did not come from the lower coastal zone. “The loggerheads came to San Diego, but they didn’t come from Mexico – they came from the central Pacific.” According to Crowder,
As the oceans heat up, this could expand turtles’ habitat to places they’ve never been, and with this, females may have more offspring. “We need to figure out how to keep this environment safe for young turtles so they can grow up and return to Japan,” Crowder said.