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Endangered Sea Turtle Migration May Be Linked to Ocean Warming Events

A Harvard research group found that loggerhead sea turtles were crossing a cold Pacific Ocean barrier during times in which the water was unusually warm, bringing scientists closer to understanding how the species migrates from Japan to North America.

(CN) --- Sporadic ocean warming events may be the key to understanding the travels of endangered sea turtles, a Stanford research group said Thursday.

In a study published in the journal Frontiers in Marine Science, a team at the Stanford Woods Institute for the Environment and other institutions detailed its new study into the obscure migration patterns of the North Pacific loggerhead turtle.

“The North Pacific Loggerhead sea turtle (Caretta caretta) undergoes one of the greatest of all animal migrations, nesting exclusively in Japan and re-emerging several years later along important foraging grounds in the eastern North Pacific,” the article said.

Young loggerheads leave Japan to find food, usually hanging around in the Central North Pacific.

Some, however, make it all the way across the ocean to the Baja California peninsula in Mexico. According to the researchers, over 43,000 congregate there and stay for up to 20 years.

Eventually the loggerheads return to Japan to reproduce and live out the remainder of their years.

Just how these turtles were swimming so far has remained largely a mystery, especially since they have to pass through the Eastern Pacific Barrier, a “wall” of cold water and one of Charles Darwin’s “impassable barriers” that usually keeps members of a species on one side or the other.

Sea turtles are ectotherms, meaning unlike humans who regulate their own body temperatures, their core body temperature is tied to the outside environment.

“Thus, sea turtles rely on thermal cues to both select (or avoid) habitats and initiate long-distance movements,” the study pointed out.

Years ago “it was hard to imagine a juvenile sea turtle undertaking a 12,000 km transpacific migration,” the researchers said.

“For decades, our ability to connect the migratory dots for this endangered species has remained elusive,” said study lead author Dana Briscoe, who was a research associate at the Stanford Woods Institute for the Environment during the research and now works at the Cawthron Institute in New Zealand, in a press release.

“We suggest evidence of a thermal corridor that may allow for pulsed recruitment of loggerheads to the North American coast as a function of ocean conditions,” the study hypothesizes.

The research team used a previous study that tracked 231 loggerheads from 1997-2013 through satellite tags. About 30% moved into eastern waters, but only six made it to North America.

The team further investigated the bones of 33 turtles found in North America, using isotopes to identify what they ate and when. This showed them exactly when the turtles moved from the open ocean onto the coast.

The researchers said both the tracking study and the bone isotopes show turtles moving farther east during years in which the ocean was warmer, usually caused by an event like El Niño.

“During warm periods, it appears a transport pathway opened intermittently as a function of the warm sea surface temperatures, thereby creating a thermal corridor that may facilitate successful recruitment across the Eastern Pacific Barrier to coastal foraging grounds,” the study found.

Although they said the data set used is very small and more research is needed, the team said its findings are promising in understanding the movements of this threatened species, which can live up to 80 years.

If these “thermal corridors” are facilitating loggerhead migration, warmer ocean temperatures caused by global warming could open them permanently or more often. This could in turn expose the turtles to even more fishing and habitat dangers.

“Understanding how and why species like the North Pacific loggerhead move among habitats is crucial to helping them navigate threats,” said study senior author Larry Crowder, professor at Stanford’s Hopkins Marine Station. “Emerging technologies and analyses can help illuminate these journeys.”

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