The red sea turtle completes the seemingly impossible journey through the hidden "corridor" across the Pacific Ocean

The North Pacific red sea turtle (Caretta Caretta) hatches off the coast of Japan and lives most of the time on the high seas of the Pacific Ocean, but sometimes mysteriously appears mysteriously in Mexico, 9,000 miles (14,500 kilometers) away from their original nesting grounds. This incredible journey requires them to cross potentially deadly icy waters, an environment that should be less suitable for them, as the red sea turtle relies on the warmth of its surroundings to maintain its core body temperature. Now, scientists have new clues as to how turtles survived this epic migration.

"This puzzle has been around for decades, and no one knows how to interpret it." Senior author Larry Crowder said he is a professor of marine ecology and conservation at the Hopkins Marine Station at Stanford University in the United States and a senior fellow at the Stanford Woods Environmental Institute. Professor Claude noted that along the Pacific coast of North America, seasonal winds from the north periodically blow down the coastline, pushing warm surface waters offshore. Then, icy waters from the deep sea rise, replacing warm waters, bringing with them an abundance of nutrients. Professor Claude added that tropical animals, including red sea turtles, rarely venture from the open Pacific Ocean to this cold water. Charles Darwin even described the area as a place where creatures that love warm water "can't get through."

But, according to a new study published in the journal Frontiers in Marine Science, the red sea turtle may have a brief chance to reach the coast of Mexico — during el Niño, el Niño is a climatic cycle that moves warm water in the western tropical Pacific Ocean eastward along the equator, and it is marked by a significant rise in surface water temperatures in the Pacific Ocean near the coast of South America. This changes the movement of heat and water vapor in the atmosphere.

Professor Claude told Life Science: "A warm 'door' to Mexico needs to be opened for these turtles. The study's authors refer to this temporary door as a "hot corridor" — essentially a warm water channel. "During el niño, turtles have the opportunity to cross the Pacific Ocean." Not only does the study shed light on a long-standing puzzle, but it also provides key information for protecting red sea turtles. The International Union for Conservation of Nature considers the red sea turtle "vulnerable". Professor Claude noted that as sea turtles respond to warming seawater caused by climate change, conservation efforts may need to accelerate the implementation of improvements.

The red sea turtle completes the seemingly impossible journey through the hidden "corridor" across the Pacific Ocean. Carolyn Cour, associate professor of biological sciences at the University of California, San Diego, said: "As climate change, ocean surface temperatures are getting hotter and longer the duration of warm water events in the Pacific Ocean is getting longer. She was not involved in the study. If the tropical corridor hypothesis is correct, more red sea turtle pups will migrate to the North American coast over time, Professor Kuhl told Life Science in an email: "This could be good for baby turtles because the nutrients abundant in these coastal waters increase the turtle's food supply." Professor Kuhl also noted that if more baby turtles accidentally wandered into coastal fishing nets, "it would be terrible", especially since these baby turtles would not have the opportunity to return to Japan to mate and breed offspring.

According to the National Oceanic and Atmospheric Administration, red sea turtles are found all over the world, mainly in subtropical and temperate waters. The red sea turtle is divided into nine subpopulations, including the North Pacific subpopulation. Professor Claude noted that in 1996, the North Pacific red sea turtle Adelita was released in Baja California, USA, and immediately headed straight to East Asia, which is the first time scientists have learned about the frequent migration of the North Pacific red sea turtle between Mexico and Japan. According to PBS, marine biologist Wallace Nichols labeled Adelita with satellite tags so he was able to track her entire journey to Japan. Professor Claude and his co-authors used similar tracking data in their new study, but they extracted data from a vast dataset of 231 juvenile red sea turtles whose migration patterns had been monitored for 15 years.

Or do the red sea turtles have a short chance to reach the Mexican coast? They cross the Pacific Ocean through corridors. About 97 percent of the red sea turtles remain on the high seas and do not venture to the north American coast. In fact, when these turtles are close to the edge of coastal ecosystems, they quickly turn around and return to East Asia. At the time, Dana Briscoe, a postdoctoral researcher in Lauder's lab, found that the time they returned appeared to be driven by Earth's magnetic field, as turtles could sense Earth's magnetic field and use it to navigate the oceans. Claude says the ability could help them avoid the onslaught of icy waters. Briscoe notes, however, that not all turtles make u-turns at typical sites — 6 of the 231 turtles continue their happy journey, swimming straight into coastal waters into an area known as the California Current Mega Marine Ecosystem. The team took a closer look at the 6 anomalous sea turtles and found that each of them migrated in the spring, and according to data collected from remote sensors, the stray turtles "experienced an unusually warm environment" compared to other sea turtles. Professor Claude said: "The two asteroids closest to Baja experienced the warmest water environments. Considering this relationship to warm water, the team proposed the tropical corridor hypothesis.

But because only 6 of the 231 sea turtles (about 3 percent) entered the California Ocean Current Mega marine ecosystem area, the team needed more data to support their theory. So they collaborated with Caladra Turner Thomas Zevich, a scientist at the National Oceanic and Atmospheric Administration's Southwest Fisheries Science Center, whose team has been studying turtle bones stranded in Mexico and died on the shore. Turtle bones contain annual rings, similar to trees. Every year a new ring grows on the outside of the bone, and one of the rings corrodes from the center of the bone. Professor Claude noted that the humerus of a turtle, a type of leg bone, has 6 to 8 growth rings at any one time. These rings contain clues about what turtles eat in a given year in the form of "stable isotopes", a chemical element containing different numbers of neutrons. If scientists know when the turtles died, they can use these chemical clues to determine what the turtles ate throughout their lives, thereby determining where the turtles might be located.

Professor Claude said: "The stable isotope ratios in open ocean foods, such as jellyfish, are significantly different from crabs, which are the food of red sea turtles in coastal waters. In this way, the team determined when a particular sea turtle jumped from open water to coastal waters, and then they checked the water temperature that year. Thomas Zevich and her colleagues analyzed the growth periods of 33 red sea turtles in Mexico and found that more than 60 percent of the turtles entered areas close to the coast in warm ocean conditions. Turtles classified by year indicate that far more turtles arrive in Mexico in warmer years than in cooler years. Professor Claude noted: "The skeletal growth layer analysis definitely reinforces their thermal corridor hypothesis, which supports the idea that a brief passage of warm water helps more turtles reach the North American coast." Professor Claude stressed that this hypothesis may also explain why red sea turtles from Japan appeared in San Diego Bay in 2016, which happened to be the year of El Niño. According to the San Diego United Tribune, these turtles rarely appear in Southern California, and even there, they do not appear in herds as they did in the past. The strange event has left some scientists wondering if red sea turtles will come to San Diego more often as climate changes.

Of course, the thermal corridor hypothesis is still just a hypothesis. Ideally, Professor Kuhl revealed, the team would be able to flag more red sea turtles with satellites, track their activity, and observe how many turtles migrate to Mexico during El Niño years and how many turtles migrate to Mexico in non-El Niño years. However, Claude said, given that so few turtles enter California's current large marine ecosystem area, such an effort could be expensive and impractical. Instead, he said he hopes to conduct an experimental study of dozens of sea turtles that release two groups of red sea turtles into the high seas near the boundary of california's Ocean Current mega-marine ecosystem area, one in an El Niño year and one in a non-El Niño year. But for now, even with limited data, current research "will help those seeking to understand and manage one of the most long-migrating vulnerable species in this animal kingdom." Kuhl told Life Science magazine.