Year: 2026
Type: preprint
Abstract: Western Indian Ocean (WIO) coral reefs extend across the East African coastline from Somalia to South Africa, and eastward to include the Îles Éparses and island states of Madagascar, the Comoros, Seychelles and Mauritius, and the French territories of Mayotte and Réunion. The total mapped area of photic coral reef ecosystems is approximately 8090 km², representing approximately 4.5 % of the global ecosystem extent, and for this assessment they were grouped into 13 ecoregions representing distinct coral reef types across the WIO. The region hosts 350-400 scleractinian coral species and approximately 2,400 reef fish species (~850 endemic to the Indian Ocean), the second most diverse region after the Coral Triangle. These ecosystems face increasing pressure from marine heatwaves causing mass bleaching, overfishing and destructive fishing, land-based pollution and sedimentation, and unsustainable coastal development and tourism practices. This 2025 IUCN Red List of Ecosystems assessment found that coral reef ecosystems across all evaluated ecoregions of the Western Indian Ocean are at a very high risk of collapse. This is principally due to the severity of climate change induced warming predicted to occur over the next 50 years. Heat stress already places all ecoregions at a high risk having experienced widespread impacts from bleaching in 1998, 2016 and 2024. These impacts, compounded by local stressors such as pollution and overfishing, have led to significant declines in the biotic community in nine ecoregions over the past 50 years. Ten ecoregions, making up 89.7 % of WIO coral reefs, are classified as Endangered or Critically Endangered based on observed declines. The results highlight that countries must urgently act to increase holistic, inclusive and equitable approaches to prevent these precious ecosystems from collapsing. This involves immediate and drastic action on climate change alongside implementing solutions to tackle local threats which will not only offer the best chance for coral reefs to recover after climate-driven disturbances, but also provide wide-ranging ecological, environmental and societal benefits beyond coral reefs.
DOI: 10.32942/x2z69s
Authors: Mishal Gudka, Abigail Mwalimu, Karin Moejes, Nimrod Ishmael, Diana Karan, Melita Samoilys, David Obura, Said Ahamada, Léo Barret, Lionel Bigot, Bruce Cauvin, Gilberte Gendron, Pagu Julius, Juliet Karisa, Ewout Knoester, Alex O’Brien, Sean Porter, Ronan Roche, Franz Smith, Gildas Todinanahary, Julien Wickel, Francisco Zivane
Institutions: The University of Melbourne, Coastal Oceans Research and Development in the Indian Ocean, International Union for Conservation of Nature (United States), International Union for Conservation of Nature, University of Reunion Island, Réserve Naturelle Marine de La Réunion, Wildlife Conservation Society, Kenya Marine and Fisheries Research Institute, Wageningen University & Research, Bangor University, University of Toliara
Language: English
Subfield: Ecology
Field: Environmental Science
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W7223179090