Beleaguered populations of green sea turtles living in and around Hawaii and American Pacific island territories are increasing in number.
From 2002 to 2015, scuba diving researchers circumnavigated 53 islands, atolls and coral reefs throughout the U.S. Pacific, conducting the first comprehensive survey in that region of the turtles’ ocean habitats. Over the 13 years, the divers counted more than 3,400 sea turtles. The vast majority — 90.1 percent — were green sea turtles; only 8.3 percent were hawksbills and 1.6 percent were unidentified.
The number of green sea turtles spotted around Hawaii increased by an average of 8 percent each year, the team reports April 24 in PLOS ONE. Around American Samoa and the Mariana Islands, the turtles’ numbers increased by an average of 4 percent per year.
“From a conservationist’s point of view, that’s pretty phenomenal,” says study coauthor Rusty Brainard, an oceanographer based in Honolulu who supervises the U.S. National Oceanic and Atmospheric Administration’s coral reef ecosystem program.
Green sea turtles (Chelonia mydas) are considered endangered by the International Union for Conservation of Nature. The picture is less rosy for hawksbill turtles (Eretmochelys imbricata), which the IUCN lists as critically endangered. In the United States, both species are protected by the Endangered Species Act.
“We didn’t spot enough of hawksbills to be able to analyze their population trends over time. It’s a sign that their population is really struggling,” says ecologist Sarah Becker of the Monterey Bay Aquarium in California. She coauthored the paper with Brainard and aquarium colleague and conservation ecologist Kyle Van Houtan.
The number of green sea turtles nesting in Hawaii and some Pacific island regions has been slowly increasing over the last two decades. But until now, scientists have had little information on how hatchlings fare once they leave their sandy cradles and venture out into the ocean.
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To track turtles and other reef-dwelling organisms, researchers were attached by a line to slow-moving boats that dragged them — in pairs — across stretches of coral reefs. “It’s a spectacular way to see the reef system, one hour at a time,” Brainard says. He saw turtles young and old, curious and shy. “They’re just so graceful,” Brainard says. “We’d see them sort of gliding along or sleeping in the caves and overhangs of the reefs.”
He also floated past shipwrecks that had leached iron, markedly altering local ecosystems, and “large fishing nets — some of which had probably traveled thousands of miles,” Brainard says. “They’d catch and snag on the reefs, breaking off bits of coral in this kind of path of destruction.” Turtles sometimes get caught in them, too, he says.
The reptiles face other perils, including warming global temperatures and habitat loss. Last year, Hawaii’s East Island — an important green sea turtle nesting site — was all but submerged, at least temporarily, in the wake of Hurricane Walaka.
“But at least — thanks to protections under the Endangered Species Act — they are no longer harassed or harvested for consumption,” Brainard says. He suspects that the legal protections help explain why turtle populations appear to be increasing despite the remaining threats.
The new study gives researchers a better sense of how the turtles are doing out in the ocean, says James Spotila, a biologist at Drexel University in Philadelphia who studies sea turtles. “We know the most about the Hawaiian populations,” Spotila says. “But some of the remote island areas looked at in this study have really been black boxes.”
Those results may be the only information scientists have about in-ocean populations of turtles in and around U.S. Pacific territories for a while, Spotila says. NOAA stopped conducting towed-diver surveys of Hawaii and other Pacific islands in 2017 due to a reallocation of funding.