According to foreign media reports, the mystery of the North Pacific loggerhead's years-long journey to the sea is still little known. Using data from satellite tracking and other technologies, scientists have revealed a unique phenomenon that may explain the path of this endangered species. This lesser-known journey of sea turtles is known as the "Lost Years," and it can last up to twenty years or more. Now, a study led by Stanford University has shed light on the secrets of the epic migration between the North Pacific tortoise's birthplace on a Beach in Japan and its reappearance in a foraging ground off the coast of Baja California years later.
The study, published April 8 in Frontiers in Marine Science, provides evidence of intermittent passages of warm water that allow turtles to cross otherwise desolate cold ocean barriers. The findings could help design conservation measures to protect turtles and other migratory marine life as climate change is altering their actions.
Lead author Dana Briscoe of the study said: "For decades, our ability to link migration points for this endangered species has remained elusive," said Dana Briscoe, a research associate at Stanford's Woods Institute for the Environment during the study and now working at the Cawthron Institute, New Zealand's largest independent marine science organization. "This work builds on special studies of these 'vanishing years,' and for the first time ever we excitedly provide evidence of 'thermal corridors' to explain the long-standing mystery of one of the oceans' greatest settlers."
Endangered "immigrants"
Wildlife seekers are excited about the sight of sea turtles, but boat traffic, fishing nets and other dangers are less friendly. The International Union for Conservation of Nature lists six of the seven turtle species as critically endangered, endangered or vulnerable. Despite scientific advances in core habitat use, researchers still know little about the movement of turtles and other long-lived marine life between different sites. This knowledge gap makes it impossible for us to effectively assess and protect these species.
The researchers wondered how and why some tortoises traveled to the western coastline of North America, while others remained in the central Pacific. Why are some sea turtles — creatures that are highly sensitive to temperature — able to traverse a cold zone between two marine areas called the Eastern Pacific Barrier, which usually stalls most organisms.
To unravel the mystery, the researchers created the largest ever dataset of satellite-labeled loggerhead turtles, employed sophisticated remote sensing oceanographic techniques, and collected the first detailed records of sea turtle aging and stabilization isotope testing — a skeletal analysis that could be used to provide information about animal life. This work relies on decades of research by an international team of scientists.
They first looked at a 15-year study that tracked the movements of more than 200 sea turtles labeled as satellite tracking devices. Six of the turtles caught the researchers' attention because— unlike their companions — they made a distinct movement toward the North American coast. More curiously, the researchers said the "sentinels" were traveling during the early spring months. Studies of remotely sensed ocean conditions during this period show that the water that the farthest "sentinels" swam through was much warmer than the water encountered by their companions during their travels.
A larger analysis involved determining the year the turtles arrived in Baja California by measuring stable isotope "fingerprints" in the skeletons of sea turtles stranded on beaches. These stable isotopic signatures can reveal when sea turtles transition from the high seas to the coast. The analysis shows that under warm ocean conditions, the number of sea turtles moving eastward each year increases significantly.
The researchers believe that the possible cause is that the unusually warm sea surface temperatures due to El Niño and other intermittent warming conditions create a "thermal corridor" that allows sea turtles to cross the eastern Pacific barrier to reach coastal feeding grounds.
The "corridor" appears in late spring and summer, and in the months leading up to its opening, the temperature also warms up early. This anomaly, especially if it lasts for several months, could provide critical environmental clues for sea turtles and other animals concentrated on the eastern edge of the central Pacific Ocean, indicating that the "thermal corridor" is opening. Studies that combine data from aerial surveys, sea observations, stranded records, and tissue samples of turtles support this hypothesis.
A dangerous trend
This phenomenon may be part of a trend. As the planet experiences unprecedented climate change, places like the Eastern Pacific Barrier, once considered an insurmountable obstacle to species movement, are being redefined. This, in turn, changes the distribution and migration paths of organisms from seabirds to white sharks, and creates new conservation challenges.
For the North Pacific loggerhead, this trend could mean more by-catch — unintentional fishing — off the coast of Baja California and other potentially important North American forage sites, including the Southern California Gulf. The study provides important insights, such as understanding the relationship between animal movement and climate change, which can help predict when turtles and other protected species will be threatened by this.
The researchers caution that their multi-year dataset represents only one snapshot of the turtle's important developmental period. The small number of sea turtles entering the eastern North Pacific limits the ability to fully test the study hypothesis under different conditions. To do this, the researchers called for more satellite labeling and stable isotope studies of turtle bones in the region.
Larry Crowder, a senior author at the Hopkins Marine Station at Stanford University, said: "Understanding how and why species like the North Pacific loggerhead move between habitats is critical to helping them navigate the threat. Emerging technologies and analytics can help illuminate these journeys. ”