France, Sept. 2 -- Nearly half of Mayotte's coral reefs were wiped out when Cyclone Chido hit the French territory in December, a new report has found. The study is the first assessment of marine damage since the storm, and warns the losses come on top of bleaching that had already weakened the lagoon.

The Mayotte lagoon- one of the world's largest at 1,100 square kilometres- once teemed with coral that sheltered fish and crustaceans.

Enclosed by an outer barrier reef and fringed by an inner reef, it normally protects the island from ocean swells and cyclones. But the ecosystem is under strain from population growth, with poor sewage treatment and waste management threatening water quality.

Surveys by the Mayotte Marine Nature Park sho...