Over the course of seven months in 2012 and 2013, more than 70 scientists collaborated to study and map the coral reefs around the islands of French Polynesia in the South Pacific.

This was a huge undertaking. Researchers covered over 8,000 kilometers (5,000 miles), including places never studied by scientists before. They conducted nearly 4,000 coral reef and fish surveys in 264 dive sites across 29 islands, and mapped over 9,300 square kilometers (3,590 square miles) using satellite imagery. No other coral reef survey in French Polynesia has been conducted at this scale, said Renée Carlton, a marine ecologist with the Khaled bin Sultan Living Oceans Foundation.

Findings from this massive expedition, published in a new report, offer hope. At the time of the surveys, French Polynesia had one of the world’s healthiest coral covers, and some of the highest diversity and density of reef fish on the planet.

The surveys were part of the Global Reef Expedition (GRE), a global mission supported by the Khaled bin Sultan Living Oceans Foundation that aimed to map coral reefs around the world.

“The goal of the Global Reef Expedition was to collect as much data as possible on the status of coral reefs, given the rapid changes we are seeing to reefs around the world,” Carlton told Mongabay. “Having a baseline understanding of the status of coral reefs is important for scientists and managers alike.”

The French Polynesia surveys threw some surprises. The live coral cover on Gambier archipelago, for example, was “extraordinary”, the researchers write, averaging 58 percent and reaching nearly 70 percent in some parts. These numbers are high not just for French Polynesia but for the whole of South Pacific, the researchers add.

Similarly, Tuamotu archipelago, which also had very high coral cover, showcased some of the highest diversity and density of reef fish species compared to many other countries surveyed on the GRE, with an average of about 300 individuals per 100 square meters.

“To date, these are still some of the highest observations of these metrics in the South Pacific,” Carlton said. “The coral cover we saw in Gambier was unheard of for the region, and was some of the highest coral cover we observed on the entire Global Reef Expedition.”

“It’s nice to see that places like that still exist,” Nancy Knowlton, a coral reef biologist emeritus with the Smithsonian National Museum of Natural History in Washington, D.C., told Science Magazine.

The survey researchers are yet to pinpoint the factors that have allowed Gambier and Tuamotu archipelagoes to have healthier corals and fish communities compared to other places. But they suspect that both environmental conditions and human pressure could have something to do with the observed patterns.

“With regards to the fish populations in Tuamotu, lower fishing pressure is likely contributing to the healthier reef fish communities,” Carlton said. “The area is also known as a particularly important breeding site for groupers so there are likely environmental factors contributing to this as well.”

In the Gambier archipelago, fewer people live, she added, so factors like sedimentation and localized runoff that can otherwise lead to reef degradation, are not big problems.

“There are environmental conditions that may be contributing to the higher coral cover as well — factors such as food availability, nutrients, presence of other benthic organisms, fish and invertebrate populations all contribute to a healthy reef,” Carlton said. “This is one thing we’re still trying to understand and are using data from the Global Reef Expedition to parse out possible areas of refuge and identifying reefs that might have higher resilience to environmental changes and Gambier is certainly one we’re going to be focusing on.”

Not all coral reefs in French Polynesia were doing well at the time of the surveys, though.

Some parts of Society and Austral archipelagoes, for instance, had been severely damaged in the early 2000s by tropical cyclones and outbreaks of Crown-of-Thorns Starfish (COTS), a large seastar that feeds on coral polyps. These regions had very low coral cover of around five to eight percent.

“While COTS outbreaks aren’t unheard of, seeing the damage first-hand was initially shocking,” Carlton said.

It’s been six years since the surveys, but the threats remain. In fact, with human-induced climate change, cyclones are expected to be bigger and stronger, Carlton said. Rising sea temperatures have been causing widespread coral reef bleaching, and outbreaks of crown-of-thorns starfish continue to occur.

However, during the surveys, the team observed new corals coming up at some of the damaged sites, which the researchers say is an encouraging finding.

“Our research shows that there may be pockets of resilience in French Polynesia’s reefs,” Carlton said in a statement. “I am hopeful that in the face of continued natural and anthropogenic pressures, the coral reefs of French Polynesia will continue to flourish.”

Since the expedition, local researchers and partners are continuing to monitor the survey sites, Carlton added.

“We have shared our findings with managers and ministers in French Polynesia, as well as partners on the ground that are working toward this currently, so we’re hopeful they can use the data we provide to further protect French Polynesia’s coral reefs,” she said.