At 2,000 pounds and six and a half feet in length, leatherback turtles are the largest living reptiles. Their size, however, belies their fragility: among the leatherbacks that live in the Pacific Ocean, populations have dropped by 90 percent in the past 20 years. Biologists already knew that fishing gear posed a problem for the endangered turtles, which can get entangled in trawlers' nets, but they were not sure exactly where and when they were running into trouble.

“These animals travel thousands of miles across the Pacific, so there's no way we can track them from land or boat,” says marine biologist Helen Bailey of the University of Maryland Center for Environmental Science. So Bailey and her colleagues set out to follow them by satellite. The scientists positioned harnesses with tracking devices on the leatherbacks' soft shells; the devices transmitted a signal each time the turtles surfaced. The study, published in the April issue of Ecological Applications, pinpoints danger zones where turtles and trawlers meet. These data will help regulatory agencies decide the times and places they might limit fishing to protect the species.

The researchers followed 135 females, some from the eastern Pacific and some from the western Pacific, over 15 years as they crisscrossed the ocean hunting for jellyfish. The study found that the migration patterns for the two Pacific populations were different. Western Pacific leatherbacks leave Indonesian nesting sites to feed in the South China Sea, Indonesian seas and southeastern Australia and along the U.S. West Coast, making them vulnerable to fishing nets in many different areas.

The eastern Pacific leatherbacks traveled from nesting sites in Mexico and Costa Rica to the southeastern Pacific, with many getting snagged in fishing gear along the coast of South America. Because the eastern population is more concentrated in range, its risk of extinction is greater, Bailey says.

The new findings could help decision makers plan short-term fishery closures. Bailey credits a recent decision to close a swordfish and thresher shark fishery in California from mid-August to mid-November each year with dramatically reducing leatherback bycatches. (In 2010 no turtles were caught.) The satellite tracks can help refine the time and area of this closure and guide closures off the coast of Oregon and Washington. In the Galápagos Islands, leatherbacks go through a very specific migration corridor from February to April, so a timely closure in that area could reduce bycatch by 100 percent.

“We had some inkling that fisheries were the problem,” Bailey says, “but now we know where to target our efforts.”