Sea turtles’ trans-Pacific journey

Until 25 years ago, marine biologists studying loggerhead turtles off the coast of Baja Mexico presumed the reptiles came from some yet-unknown hatchery in the northeastern Pacific Ocean. Discovering that the Mexican turtles were actually hatched in Japan opened a new question: How could a cold-blooded reptile cross from the warm Western Pacific into the cold Eastern Pacific without freezing to death?

“This mystery had been around for decades, and nobody had a clue how to explain it,” Stanford marine ecologist Larry Crowder told Live Science.

But now, the longtime turtle researcher says he might have an answer. Crowder and his team of international scientists theorize in research published in Frontiers of Marine Science that the loggerheads cross the Pacific by taking advantage of El Niño’s periodic warming of the waters, giving the endangered turtles a small window of opportunity to reach the other side of the world.

Originally, marine biologists weren’t certain whether the loggerheads found off the Mexican coast were a separate population of turtles, or if they actually migrated across the ocean. Then marine biologist Wallace Nichols attached a satellite tracker to a North Pacific loggerhead sea turtle he found off the coast of Mexico in 1996. Nichols tracked the animal as it swam continuously for 368 days on a 9,000-mile journey from Mexican waters across the Pacific to Japan. Since then, scientists have been trying to figure out how the turtles make the marathon journey.

Loggerhead hatchlings can fit in the palm of a human hand, but the reptiles eventually grow quite large. The omnivorous loggerheads gorge on crustaceans, jellyfish, and whatever else they can find en route to becoming the world’s largest hard-shell turtle. Adults typically weigh about 250 pounds and measure 3 feet long.

Crowder and his research team spent 15 years tracking 231 turtles as the creatures swirled around the central North Pacific. According to Crowder’s research, almost all loggerheads turn back when they venture too far East into colder waters. According to the researchers, the cold waters in the Eastern Pacific create a barrier for a number of species, including the cold-blooded turtles. But six of the turtles made a break for the North American coast during the 15-year tracking period. “A warm ‘door’ needs to open for these turtles to get to Mexico,” Crowder told Live Science.

Trade winds moving across the Pacific Ocean from east to west tend to push warm surface water toward the Solomon Islands in the southwest. While warm water piles up in the western Pacific, cold water upwelling from below makes the eastern Pacific off the coast of South America and Central America cooler. The temperature differences tend to create a barrier in the ocean. Species that thrive in warm water rarely travel east to the cooler waters around the Americas.

But sometimes those easterly trade winds die out, resulting in the return of warm water to the surface of the typically cold eastern Pacific Ocean. Peruvian fishermen in the Pacific in the 16th century called the phenomenon El Niño. The National Oceanic Service says the phenomenon occurs every two to seven years. Crowder’s team theorizes this gives the loggerheads enough warm water to transit the ocean. “During El Niño, the turtles get a shot at going across,” Crowder said.

If Crowder’s hypothesis bears out, it could help conservationists arrest the declining population of the distinct group of turtles. NOAA’s five-year report on the north Pacific loggerhead released last year said that turtle populations had declined between 50 and 90 percent over the past 60 years. By knowing more about how and where some loggerheads cross the ocean, conservationists can more precisely direct their rescue efforts.

“It is imperative that endangered animals be studied while they can, so that the most informed conservation and management decisions can be made,” the study’s authors wrote.