Exuma Cays Land and Sea Park in the Bahamas has existed since 1959 and been protected from fishing since 1986—but it took until now to prove that such fisheries management could actually help corals rebound.

One of the Caribbean's largest marine reserves at 171 square miles (442 square kilometers), the park has been studied as part of the Bahamas Biocomplexity Project—an effort to learn about the interaction of water, wildlife and human activity in this subtropical archipelago. As part of that project, researchers proved that grouper—a highly valuable commercial fish species—rebound in areas that are protected as marine reserves.

But "we wondered what was the net consequence of this big number of groupers inside the park," says Daniel Brumbaugh, senior conservation scientist at the Center for Biodiversity and Conservation at the American Museum of Natural History in New York City. "They could suppress parrot fish populations, allow algae to bloom more and further its competition with coral."

In fact, the smaller species of parrot fish were experiencing such detrimental predation, according to a paper in the current Proceedings of the National Academy of Sciences USA. "Their sizes were smaller inside the park than outside," Brumbaugh says. "But the larger species of parrot fish actually were the same size inside and outside, but much more numerous within the park."

This allowed the more numerous variety of parrot fish to graze away the algae blooms and give the underlying coral reef an opportunity to regenerate. Video documentation of the seafloor proves that more baby coral established themselves inside the park—under the protection of the larger parrot fish grazing population—than outside. "They can't make it through this lawn of algae otherwise," Brumbaugh says. "They'll get smothered."

Although this finding does not apply to all marine reserves throughout the world—for example, coral reefs in the Pacific support a more complex food web—it is broadly true of any Caribbean marine reserve. And it may provide these reefs with the kind of resilience necessary to survive the host of threats that face them, from human fishing practices to climate change. "If we're going to have recovery of reefs in the Caribbean and elsewhere, we need baby corals to come back," Brumbaugh adds. "This is an approach that helps baby corals come back."