IUCN scientists were part of a recent expedition, Tara Oceans, to investigate coral bleaching on the reefs of Mayotte, an island that lies to the north west of Madagascar. The team found that bleaching here, which was first reported in March this year, is the worst seen in the Indian Ocean.
Climate change has emerged as the most significant threat to coral reefs on a global scale with coral bleaching one of the most visible and serious effects. This is of particular concern as coral reefs support the richest marine biodiversity in the world. They also provide food, storm protection, jobs, recreation and other sources of income for more than 500 million people worldwide.
The expedition, organised by Tara Oceans (Tara Foundation and scientific consortium Oceans), assessed 34 sites around Mayotte. On board the Tara boat were scientists from IUCN, L’Institut de recherche pour le développement, University of Milano-Bicoca, Coastal Oceans Research and Development in the Indian Ocean (CORDIO), Mayotte Marine Research Center, University College Dublin, University of La Reunion and the EMBL and CNRS.
“The reefs around Mayotte have experienced the worst bleaching and mortality so far recorded in the Indian Ocean with over 50 % of corals affected by the bleaching overall and up to 30% coral mortality at the worst-affected sites,” says Dr David Obura, Chair of IUCN’s Coral Specialist Group and Director of CORDIO.
The researchers confirmed a pattern that is increasingly reported in other locations, whereby corals accustomed to the inner turbid waters of lagoons showed higher resistance to bleaching than corals on outer clear-water reefs.
“Turbidity or green water during a high temperature stress event appears to protect corals,” explains Dr Obura. “This is in contrast to our prior expectation of corals in clear oceanic waters being generally healthier.”
“The majority of coral species have been affected by the bleaching across all depths”, says Dr Francesca Benzoni, Principal Scientist of the Tara Oceans Expedition. “While the dominant fast-growing table corals (Acropora) were visually dominant due to their abundance and striking bleached white appearance, many other genera were even more susceptible to the bleaching.”
“Recovery of these sites will depend on a number of critical factors including the extent of mortality and recruitment but also on the herbivorous fish communities and populations present at these sites,” says Dr Ameer Abdulla, of IUCN’s Global Marine Programme. “Unfortunately, many areas around Mayotte suffer from extensive fishing to the point of overfishing. A few areas, including the marine protected areas, hold healthy communities of fish. These areas will probably have the highest chance of recovery.”