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BAY OF RANOBE, Madagascar — The coral reef itself was exquisite. Growing about 3 miles offshore in 50 feet of water, it was a rugged terrain of pinks, blues, and oranges, set against a backdrop of deep blue. The coral pieces, each a colony of living animals, took on a range of unusual forms, from cake platters and pencil shavings to antlers and brains.

But there was one obvious thing missing: fish. Like a city without people, the reef was mostly empty — not only of fish, but also of crabs, eels, and other typical marine life on a coral reef.

It was a sunny morning in September, and I was diving on a coral reef in southwest Madagascar, an island nation that sits east of continental Africa. And like many reefs in the region and across much of the world, it’s on the verge of collapse. Overf