Case Study: Coral Bleaching in Mauritius
By Dr. Anusha Devi Nawoor • Dr. Nora von Xylander
Coral reefs are among the most diverse and valuable ecosystems on Earth. These vibrant ecosystems support more than 30 percent of marine biodiversity, providing essential ecosystem services that sustain more than 500 million people worldwide and contribute an estimated global economic value of approximately $10 trillion per year.
These ecosystems face a constant threat from coral bleaching. Coral bleaching occurs when stressed corals expel the symbiotic algae (zooxanthellae) that resides within their tissues. This not only robs them of their vibrant colors but also deprives them of their most essential source of energy, making them susceptible to starvation, disease and mortality.
The primary driver of coral bleaching is marine heatwaves caused by climate change, with high light intensity and rising sea surface temperatures (SST) acting as major stressors. The frequency and intensity of marine heatwaves have increased in recent decades, resulting in more frequent and severe coral bleaching events on a global scale. In 2015, the world experienced a third global coral bleaching event. During this period, maximum heat stress levels reached Alert Levels 1 and 2, indicating prolonged exposure to temperatures ≥ 4 to 8° C above normal per week, a threshold known to trigger mass bleaching.
NOAA reported the fourth global bleaching event at the start of 2024. This unprecedented event comprised record-breaking SSTs, with values exceeding 20° C heating weeks in several locations across the Indo-Pacific. The severity of this event forced NOAA to introduce two new bleaching alert levels (4 and 5), as previous scales were insufficient to capture the extent of coral loss. Alert Level Five signifies near-total mortality.
At present, the full extent of the fourth global coral bleaching event remains uncertain, but what is clear is that inaction is not an option.
Case Study: Mauritius
Coral bleaching is a major concern for Mauritius, an island nation in the Indian Ocean that reies heavily on its coral reefs for coastal protection, marine biodiversity and tourism. The rise in ocean temperatures and increased frequency of coral bleaching events due to climate change has increasingly affected local reefs.
The rising SSTs, particularly during El Niño and positive Indian Ocean Dipole events, have triggered severe bleaching in Mauritius (i.e., 1998, 2006, 2016 and 2024), with even small anomalies (≥1° C) causing mass coral bleaching. Since 2003, SSTs around Mauritius have risen by 0.16° C per decade, surpassing the bleaching threshold of 27.5° C and diminishing coral fitness. Ocean acidification (OA), driven by increasing CO₂ levels, further compromises reef resilience. Declining pH levels at sites such as Bel Ombre, Bambous Virieux, and Trou aux Biches may reduce calcification rates and impair reef formation, affecting not just corals but all calcifying marine species. Rising sea levels in Mauritius, which averaged 3.8 mm per year from 1987 to 2014, further amplify coastal erosion and flooding. These changes impact shallow fringing reefs due to sediment shifts and changes in tidal dynamics. Cyclones present a double-edged sword. While they can reduce thermal stress through water mixing, they also cause mechanical damage, smothering corals with debris and sediments. As cyclone frequency and intensity increase, the damage to already stressed reefs becomes harder to reverse. These stressors create damaging feedback loops. Bleached corals become more susceptible to disease and algal overgrowth, especially in overfished and nutrient-rich areas. Dead coral structures are quickly colonized by algae, further hindering recovery. Sediment and pollution exacerbate these effects, while predator outbreaks such as crown-of-thorns starfish (COTS) delay natural regeneration. The interplay between these climate stressors combined with local human activities, such as coastal development, agricultural runoff, and fisheries pressure, further weakens coral reefs and inhibits their recovery.
Mauritius: Coral Bleaching History
Mauritius experienced its first recorded mass coral bleaching in 1998 during the strongest El Niño on record, when SSTs rose by 1 to 1.5° C, disrupting heat-sensitive zooxanthellae and resulting in bleaching corals. Mortality was generally below 10 percent at most sites, with the highest bleaching (38.6 percent) in the south at Le Bouchon. The comparatively low impact, especially compared to >90 percent mortality in parts of the Seychelles and Maldives, was attributed to cyclonic activity cooling surface waters and reducing solar exposure. Vulnerability was greatest in shallow, poorly flushed lagoons, while deeper or well-circulated lagoons had higher survival.
In January 2005, reef surveys at Ile aux Aigrettes, Flic en Flac, Grand Baie, and Bel Ombre found live coral cover generally under 5 percent at most sites, except Bel Ombre, which had nearly 100 percent cover and high species diversity. Key threats included: nutrient pollution, algal overgrowth, cyanobacterial mats and predation by COTS. Recommendations included: improved wastewater treatment, effluent reuse, coral restoration, shoreline protection and long-term monitoring. By April to May 2006, monitoring showed recovery from the 2005 bleaching, with most sites returning to pre-bleaching conditions, except Totor in northern Rodrigues, where 15 percent standing dead coral, turf algal dominance, and limited coral recruitment indicated impaired recovery.
During the third global bleaching event in 2016, Mauritius was again less severely impacted than some other Western Indian Ocean nations. Thermal stress began in mid-December 2015, peaking at 16° C heating weeks between late March and May 2016. Over 40 percent of corals were partially bleached, with severe impacts (more than 65 percent affected) at Belle Mare, Flic en Flac, and Île aux Bénitiers, while Blue Bay, Bel Ombre, and Mon Choisy experienced less than 15 percent bleaching. Severe bleaching peaked in March to April, with about 35 percent of observations reporting more than 50 percent bleaching, but mortality was low at monitored sites, such as Anse la Raie Lagoon, where coral cover remained stable (approximately 35 percent) from 2013 to 2017. The absence of a national post-event survey limits the accuracy of mortality estimates, particularly for Rodrigues, where losses were reportedly high.
The 2024 global bleaching event, driven by a strong El Niño and positive Indian Ocean Dipole, brought severe thermal stress to much of the Western Indian Ocean. In Mauritius, bleaching was reported as moderate, with some sites showing medium to high severity, though data submissions were fewer compared to neighboring countries. Regionally, 73 percent of observations showed moderate to severe bleaching, and 9.9 percent of reefs experienced high mortality. While site-specific mortality data for Mauritius remain limited, approximately 80 percent of reefs in the region were affected, underscoring the urgent need for ongoing monitoring, targeted conservation and climate adaptation measures.
Outlook for Coral Reefs in Mauritius
The reported coral bleaching events have brought Mauritius’ reefs to a tipping point. While isolated signs of resilience persist, compounded impacts from warming seas, pollution, and coastal development continue to undermine reef recovery. Without sustained intervention, Mauritius risks losing its reefs’ critical ecosystem functions. In response, Mauritius is actively working on a range of coral reef conservation initiatives focused on restoration, community engagement, policy and technology. Key efforts include the Tech4Nature initiative (Huawei and IUCN), which uses nursery-grown coral fragments and real-time monitoring to rehabilitate degraded reefs, and the Adaptation Fund, a program aimed at selecting heat-tolerant corals to restore climate-resilient reefs in Mauritius and the Seychelles. Community-based coral culture projects, led by the Mauritius Oceanography Institute and the Nairobi Convention’s WIOSAP, train locals in reef restoration while supporting sustainable livelihoods. The establishment of marine protected areas (MPAs), such as Anse la Raie, and the use of digital monitoring tools strengthen current efforts. These combined strategies are critical to enhancing the resilience of Mauritius’s reefs in the face of escalating climate pressures.
However, local and global stress factors attacking Mauritius’s coral reef system must be comprehensively assessed and optimally managed for the sustainability of reefs. Natural sources such as cyclonic conditions have occasionally helped to lessen the bleaching to some extent, but constant anthropogenic impacts make recovery challenging. Higher carbon emissions and consequently higher global temperatures require immediate intervention to reduce the effects on and promote the successful adaptation of corals.
Without addressing root causes, the survival of coral reefs worldwide remains at serious risk. To secure long-term reef resilience, local measures must be paired with more global science-based restoration efforts and urgent action on climate change. Mauritius’s reefs reflect a broader global trend: recovery windows are narrowing, and without transformative change, future bleaching events may push these fragile ecosystems beyond their capacity to recover.
Dr. Anusha Devi Nawoor is an environmental scientist at Tunley Environmental.