After analyzing data from coral reef systems in the western Indian Ocean, an international team of scientists has documented an eight-step process — much of it linked to overfishing — that leads to reef collapse. The group, which includes researchers from the New York-based Wildlife Conservation Society, said the health of the coral reefs was directly tied to the density of fish in those ecosystems. The results, published in the Proceedings of the National Academy of Sciences, said that thriving, well-protected reefs typically have 1,000 to 1,500 kilograms of fish per hectare, and that as the density drops below 1,000 kilograms, early warning signs appear, including seaweed growth and sea urchin activity. Below a density of 300 kilograms of fish per hectare, reef ecosystems face collapse, the scientists said. Fish are vital to reef ecosystems because they crop back the algae that would otherwise smother the reefs. The study found that reefs where fishing was strictly prohibited were the most healthy and that unprotected reefs fared the worse. But the study concluded even modest restrictions on fishing around reefs can help maintain healthy ecosystems.