They arrived at this conclusion after studying the bones of dead turtles that had washed up on Mexican beaches, in a bid to find out more about the animal's migratory patterns.

“These turtles are born in Japan, then migrate to the central and eastern north Pacific Ocean for some part of their juvenile lives before returning to Japan to breed and live out the remainder of their lives,” said Carolyn Kurle, an assistant professor of biology at the university. “But nobody knew how long they were spending in each distinct ocean region.”

Due to this lack of information, it was difficult to develop a conservation plan for the turtles. The study, which involved Kurle, Calandra “Cali” Turner Tomaszewicz (her then doctoral student), and Jeffrey Seminoff (NOAA's National Marine Fisheries Service Southwest Fisheries Science Center in La Jolla) - was designed to fill in the missing pieces.

For the study, Turner Tomaszewicz analysed the stable nitrogen isotopes laid down during bone formation in the concentric growth rings of the humerus bones of dead turtles found on Mexican beaches. When compared to the distinct nitrogen isotope profiles of different parts of the Pacific Ocean, the powder from these rings of bone allowed her to determine where the turtles had travelled during each year of their lives.

“From a conservation point of view, especially for a migratory species, the most essential goal should be to prioritise conservation efforts where they can have the biggest impact in protecting the population and facilitating recovery, in the most efficient way possible,” said Turner Tomaszewicz, now a postdoctoral fellow.

One of those places appears to be off the Baja coast, where the age of loggerhead turtles ranges from three to 24 years of age.

“Cali demonstrated that there is a bimodal distribution to loggerhead settlement into these waters off Baja from the central North Pacific, with one group of turtles recruiting to that area at around an average of seven years of age and a second group recruiting at an average of 16 years of age,” said Kurle. “She also determined that loggerheads mature at around 25 years of age, meaning they leave this area when they are about 24 years old to migrate back to Japan so they can breed.”

This means that the turtles live from 10 to more than 20 years off the Baja coast, a region where a separate study had estimated that about 1,000 loggerheads a year end up as bycatch in small-scale “artisanal” fishing operations.

From those estimates, Turner Tomaszewicz calculated that loggerhead turtles that spend 10 years in Baja have a 30 percent of living long enough to travel back to Japan, while those that spend 20 years in Baja have only a 10 percent chance to do so.

The findings from the study is being used to develop better management techniques for the Mexican artisanal fishery to prevent the high rate of turtle bycatch.

“Our study shows that this foraging hotspot is an incredibly important habitat for the reproductively-valuable juvenile life stage for North Pacific loggerheads and that this area needs to be a high-priority conservation habitat for this population,” said Turner Tomaszewicz.