* Numbers have fallen 78 percent in last three decades
* Turtles nest in New Guinea and forage across the Pacific
* The Atlantic leatherback is in better shape
By Verna Gates
BIRMINGHAM, Ala., Feb 27 (Reuters) - The giant Pacific leatherback turtle, known for its arduous 6,000-mile (10,000 km) ocean trek from the U.S. West Coast to breeding grounds in Indonesia, could go extinct within 20 years as its population continues to plummet, scientists say.
"Sea turtles have been around about 100 million years and survived the extinction of the dinosaurs but are struggling to survive the impact of humans," said reproductive biologist Thane Wibbels of the University of Alabama at Birmingham (UAB), a member of a research team studying the fate of these reptiles.
The leatherback - the world's largest turtle - can grow to six feet (1.8 meters) long and weigh as much as 2,000 pounds (900 kg).
A study published this week in the Ecological Society of America's scientific journal Ecosphere estimates that only about 500 leatherbacks now nest at their last large nesting site in the Pacific, down from thousands previously. The study tracked the turtle's ongoing population decline since the 1980s.
"If the decline continues, leatherback turtles will become extinct in the Pacific Ocean within 20 years," Wibbels said.
The Pacific leatherback braves a transpacific journey that is one of the longest migrations in nature. Experts say its continued existence is imperiled by threats like climate change, plastic pollution, fishing methods, predation and human hunting.
In the past 27 years, the numbers of western Pacific leatherback turtles have dropped by 78 percent, making it critically endangered, said Ricardo Tapilatu, a turtle researcher at UAB and the State University of Papua in Indonesia. He has studied the turtles at their last remaining refuge, the remote Bird's Head Peninsula on New Guinea.
More than 75 percent of all western Pacific leatherback nesting occurs there, numbering 489 turtles in the last breeding season, the researchers said. The turtles forage across the Pacific as far away as the U.S. coast of California, Oregon, and Washington state.
The research team also included scientists from the U.S. National Oceanic and Atmospheric Administration, National Marine Fisheries Service and the World Wildlife Fund Indonesia.
The turtles can dive as deep as 4,000 feet (1.2 km). To survive the cold depths, the leatherback can control its temperature, staying warmer than surrounding waters. They feed on jellyfish, eating hundreds a day.
The leathery shell feels like tire tread and it is distinctively different from hard-shell sea turtles.
Their exact lifespan is unknown, but is believed to be up to 80 years. It is difficult to determine since males never return to the beaches, living their lives in the sea.
Of the four primary Pacific nesting places of the past century, the Malaysian population is extinct, and the Mexico and Central American populations have fallen 95 percent.
The leatherback is the only sea turtle that lives in open ocean, negotiating numerous dangers along the way.
"They migrate 6,000 miles in seven months, and then back, going through the territorial waters of at least 20 countries. There is constant danger of being caught and killed," said Tapilatu, a native of New Guinea.
For example, fishermen's drift nets and long-lines can snag the air-breathing turtle, drowning it. Humans also introduced wild hogs and dogs to the remote beaches where they nest. The hogs are especially voracious predators of turtle eggs.
Near the nesting site, local fishermen still capture and slaughter leatherbacks to consume the meat. A local tribe has historically harvested about 100 turtles per year, as well as eggs.
On some beaches, as few as 20 percent of the eggs hatch due to increased beach temperatures, which could worsen with climate change, Tapilatu said. Sand temperature determines the gender of hatchlings, with higher temperatures favoring females.
There is hope of restoring the population of the endangered reptiles, the researchers said. The Atlantic leatherback, which is genetically different from the Pacific turtles, has made a comeback through mutual country agreements to ban harvesting adults or eggs on beaches.
Tapilatu said he plans to return to New Guinea to help replicate that success story with the leatherback turtles struggling to survive in his Pacific home. (Editing by David Adams; Editing by Will Dunham)