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 show Cyclone Chido wiped out 45 percent of corals across the island when it struck on 14 December 2024, killing 40 people and leaving 41 missing as it swept through the territory.
Combined with bleaching linked to El Nino earlier in the year, the two events decimated about two-thirds of Mayotte’s corals, representing a 35 percent loss of coral cover across the lagoon.
The report says the combined impacts have caused “very significant degradation of coral populations” across the island’s reefs. It describes the losses as major for a lagoon long regarded as a biodiversity hotspot in the Indian Ocean.
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Reefs already weakened
The destruction varied across sites. The north-east, where the cyclone first struck, was hardest hit, with most corals wiped out. The double barrier reef fared better, losing roughly a quarter of its coral cover.
“When you first put your head underwater, there are areas that are particularly well preserved and others where nothing is left, though before there was richness and significant biodiversity,” Yoan Doucet, head of engineering at the Mayotte Marine Nature Park, told RFI in January.
He said surveys carried out before the cyclone had already measured an average 35 percent mortality from bleaching.
“It is therefore possible that afterwards, with the passage of Cyclone Chido, reefs that were already weakened could not resist Chido’s impact.”
The scale surprised park scientists. The mortality was quite unprecedented, with the last episode of this magnitude in 1998, said Oriane Lepeigneul, marine ecosystems officer at the Mayotte Marine Nature Park.
“They serve as both a habitat and a feeding ground. When you lose that habitat, you potentially lose functionality for these species,” she told RFI. “So either these species will move elsewhere, or perhaps some of them will decline.”
The study, which involved scientific support from consultancies Marex and Creocéan, says the island’s reefs were hit hard by the combined effects of bleaching and storm damage.
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Threats to coastal protection
The collapse of corals threatens marine life and the livelihoods of those who depend on it.
The study warns that damage to the reef barrier also compromises natural protection for Mayotte’s shoreline, leaving the coast more exposed to storms and future cyclones.
Despite the devastation, some areas showed resilience. The cyclone’s force may even help recovery in certain places by clearing dead coral that had blocked regrowth after bleaching.
“If dead corals remain standing, that prevents recolonisation by live corals. But if the rock underneath is bare, that allows new corals to settle,” Lepeigneul told RFI.
The marine park says its priority now is to conserve surviving reefs while reducing human pressures such as pollution, poor water quality and coastal development.
“What will be most important now is to manage to conserve the reefs that have resisted,” Lepeigneul added.
Active restoration measures are being explored, though researchers caution that even with coral propagation techniques, only a fraction of what has been lost could be restored.
