After it hatches and scurries to the sea, a young loggerhead sea turtle embarks on a maritime odyssey.
Some will complete an epic journey from their birthplace on the beaches of Japan to the coast of Baja California, Mexico. This voyage can take decades and has been shrouded in mystery, leading researchers to call this portion of a turtle’s life “the lost years”.
The first study of its kind, 15 years in the making, has shed new light on how these animals, who thrive only in warm water, are able to navigate a bitterly cold stretch of ocean once considered impassable.
The findings could help in the design of conservation measures to protect sea turtles and other migratory sea creatures, as changes to the climate alter their movements. “For decades, our ability to connect the migratory dots for this endangered species has remained elusive,” Dana Briscoe of the Cawthron Institute in New Zealand, who led the study, said.
“For the first time ever we are excited to provide evidence of a ‘thermal corridor’ to explain a longstanding mystery of one of the ocean’s greatest migrants.”
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The researchers set out to understand how some loggerhead turtles can travel to the west coast of North America while others remain in the central Pacific Ocean. To make it to Baja California, the turtles, which are highly sensitive to temperature, must cross a frigid 4,000-mile stretch of ocean called the Eastern Pacific Barrier.
During the nineteenth century, Charles Darwin called the barrier “impassable”. Since then scientists have confirmed that many species of marine animal cannot cross it.
The researchers reviewed 15 years of data on more than 200 turtles that had been tagged with satellite tracking devices. Half a dozen stood out for having ventured towards North America.
This was combined with an analysis of dead turtles found washed up on the beaches of Baja California. Chemical signatures found in their bones could be used to trace where the turtles had been feeding, and when they had moved from the open sea towards the coast.
The researchers also looked at weather patterns. The data suggests the creation of temporary “thermal corridors” through the ocean — unusually warm sea surface temperatures — by El Niño, a climate feature seen in the Pacific Ocean that can affect weather worldwide. These appear to allow the turtles to broach the Eastern Pacific Barrier.
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The researchers suggest that global warming is causing obstacles such as the barrier to break down. “This, in turn, is shifting the distributions and migratory pathways of creatures ranging from sea birds to white sharks and presenting new conservation challenges,” they add.
For the North Pacific loggerhead turtle, this may mean a higher risk of being caught by commercial fishing off the Baja California coast.
The International Union for Conservation of Nature lists six of the seven sea turtle species, including the loggerhead, as critically endangered, endangered or vulnerable.
Efforts to protect them have been hampered, researchers say, by a limited understanding of how they travel the ocean.
“Understanding how and why species like the North Pacific loggerhead move among habitats is crucial to helping them navigate threats,” said Professor Larry Crowder of Stanford University, a co-author of the paper. “Emerging technologies and analyses can help illuminate these journeys.”
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The study was published yesterday in the journal Frontiers in Marine Science.
