Amid the gloom and tears, Cinner and his colleagues went searching for bright spots. Mimicking Sternin, they looked for reefs that house more fish than expected, relative to other reefs facing comparable pressures. And their surprising results are upending traditional assumptions about what makes a healthy reef.

Contrary to what you might think, the bright spots weren’t all remote reefs, where humans were absent or fishing was banned. Instead, most were home to lots of people, who rely heavily on the corals and who frequently fished. They weren’t leaving the corals and fish alone; instead, they had developed social norms and institutions that allowed them to manage the reefs responsibly.

“Reefs are hugely threatened. I saw my own field site melt down and completely die,” says Julia Baum from the University of Victoria. “The danger is that we lose hope, or we feel like there’s nothing to be done. That’s why this study is so important. It shows that the end state of people relying on and using coral reefs doesn’t have to be reef degradation.”

“It’s really nice when you can shine a powerful analytical light on what’s going well,” says Nancy Knowlton, a self-described ocean optimist based at the Smithsonian Institution. “We want to start talking about things that are going well in conservation.”

Cinner, a social scientist by training, began by asking his marine biologist colleagues for their data. “I asked 30 reef scientists to give me their life’s work, and if you’ve ever met a scientist, you know that can be like pulling teeth,” he says. “But my dad’s a dentist, so I’m okay with that.”

He ended up with data on 2,514 reefs from 46 nations. His team then created a mathematical model that predicts how many fish a site should have given 18 factors, including water depth, population size and growth, tourism, fishing, whether the reefs were protected, and more. Finally, they identified 15 bright spots and 35 dark spots, which departed substantially from those predictions. These were not the reefs in best or worst condition, but the ones that most defy expectations.

The results were surprising. “The Great Barrier Reef is widely heralded as the most wisely managed coral reef in the world, but it performed exactly as well as we expected it too,” says Sinner. “The Caribbean is widely considered the basket case of the world’s coral reefs, and had a few dark spots but performed largely as we expected.”

Some remote sites like parts of the north-west Hawaiian islands, which have long been textbook examples of how pristine reefs can be when fishing is rare, emerged as dark spots. Meanwhile, most of the 15 bright spots were in fished and populated areas, and near both rich and poor countries.

“Conservationists typically look for the highest absolute biomass and the places that are most untouched. These are the gems, so let’s stop people from going there,” says Cinner. “We looked for places that had more fish than they should, given the condition. Some had biomass below the global mean, so they weren’t pristine, but they were doing better than they should be.”