In a bit of rare good news for the world’s oceans, distressed populations of green sea turtles near Hawaii and other American Pacific island territories are bouncing back.

Between 2002 and 2015, research divers surveyed coral reefs around 53 islands and atolls in U.S. Pacific waters, tallying up all the turtles they found, reports Maanvi Singh at Science News. In total, they found 3,400 turtles and of those, about 90 percent were green sea turtles. The survey shows that around the Hawaiian islands, the turtles have increased about 8 percent per year, and around American Samoa and the Mariana Islands, they’ve increased around 4 percent per year. The research appears in the journal PLOS One.

“From a conservationist’s point of view, that’s pretty phenomenal,” co-author Rusty Brainard, of the United States National Oceanic and Atmospheric Administration tells Singh.

However, other species are not on the rise. Hawksbill sea turtles, which dwell in similar coral reef habitats, only made up about 8.6 percent of the turtles identified and researchers could not gather enough data to estimate population trends, a sign that the critically endangered species is still struggling.

Kashira Gander at Newsweek reports that the new study complements other surveys of turtle nests on beaches. While the numbers of nests has increased over the last few decades, that doesn’t necessarily mean more baby turtles are making it into the ocean and surviving to maturity.

That’s why researchers decided to systematically count the turtles swimming around in coral reefs. Every April for 13 years, pairs of scuba-diving researchers attached themselves to a rope attached to a slow moving boat that dragged them through coral reef ecosystems for a total of 4,660 miles. The dangling researchers kept their eyes open, noting habitat, tallying other wildlife and counting all the turtles they spotted.

“It’s a spectacular way to see the reef system, one hour at a time,” Brainard tells Singh. “[The turtles] are just so graceful. We’d see them sort of gliding along or sleeping in the caves and overhangs of the reefs.”

He saw other, less wonderful things as well, like fishing nets snagged on reefs that continue to trap turtles and fish as well as shipwrecks leaking iron and other contaminants, altering the ecosystems around them.

Green sea turtles were pushed onto the endangered species list primarily by meat and egg hunters; for example, green sea turtle soup was particularly popular 100 years ago. Hawksbill turtles, on the other hand, were extensively hunted for their beautiful shells, which when processed is called tortoiseshell. It was used in things like combs, ornaments, jewelry boxes and other objects. Another recent study found that up to 9 million hawksbills were killed over 150 years for the tortoiseshell trade.

Today, hunting isn’t the main problem for either species. Coastal development and human disturbance interfere with their nesting grounds, while unregulated fishing nets and coral reef destruction are their main threats in the water. Rising ocean temperatures and plastic pollution are also becoming concerns.

“With regard to [plastic pollution], it may be that baby sea turtles are heavily impacted by plastic ingestion in their open ocean juvenile phase which can cause them to be collocated with garbage patches,” Brendan Godley, a conservation biologist at the University of Exeter, who was not involved in the study, tells Gander. “They are omnivorous and pretty unselective in this life stage and eat plastic pieces which really pose a chance of harm. There is a real possibility that population-level effects could be felt from this threat that could hold back population recovery being helped by protection on beaches and inshore waters.”

Dragging scientists through the water is a pretty expensive endeavor, and the researchers don’t believe they can continue the survey long term, even though the data is important in monitoring the health and recovery of the turtle population. In the future, they may continue the surveys using autonomous underwater drones or by probing for environmental DNA.