The big fish that prowl the Caribbean reefs—gaping groupers, sharp-toothed barracuda, and gigantic sharks—are completely gone in some places due to overfishing, a new study says.

The problem is worst in the most densely populated Caribbean countries, where fishers have wiped entire reefs clean of large predators.In such places, smaller predators have begun to fill in niches left by the big hunters—sendingcoral reefs into a tailspin.The new research, based on a public database of fish sightings by trained volunteer scuba divers, provides one of the most comprehensive glimpses so far of the decline in large Caribbean predators.

"Healthy and intact coral reefs need large predatory fish in order to continue to provide human societies with food and with beauty," said study author Chris Stallings, a researcher at Florida State University's Coastal and Marine Laboratory.