CORAL BLEACHING IN MAURITIUS

Coral bleaching in Mauritius.

Authors:

Anusha Devi Nawoor, PhD – Environmental Scientist, Tunley Environmental

Nora von Xylander, PhD

Coral reefs, often referred to as the ‘Oases in the Desert Ocean’, are among the most diverse and valuable ecosystems on Earth. These vibrant ecosystems support over 30% of marine biodiversity¹, providing essential ecosystem services that sustain over 500 million people worldwide^2,3 and contribute an estimated global economic value of approximately $10 trillion USD per year⁴. However, these ecosystems are now facing a constant threat from a phenomenon known as coral bleaching. Coral bleaching occurs when stressed corals expel the symbiotic algae (zooxanthellae) that resides within their tissues⁵.  This not only robs them of their vibrant colours but also deprives them of their most essential source of energy, making them susceptible to starvation, disease, and mortality⁶.

Various threats are causing the global decline of coral reefs. These include ocean acidification, pollution, and destructive ocean-based activities⁷. However, 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 stressors⁸. 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 (Figure 1). In 2015, the world experienced a 3^rd Global Coral Bleaching Event. During this period, maximum heat stress levels reached Alert Levels 1 & 2, indicating prolonged exposure to temperatures ≥ 4–8 °C above normal per week, a threshold known to trigger mass bleaching. Now, the world is facing an even more alarming crisis. The National Oceanic and Atmospheric Administration (NOAA) reported the 4^th Global Bleaching Event at the start of 2024. This unprecedented event experienced 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 & 5), as previous scales were insufficient to capture the extent of coral loss. Alert Level 5 now signifies near-total mortality, underscoring the grave reality that reefs are facing. At present, the full extent of the 4^th global coral bleaching event remains uncertain⁹. However, what is clear is that inaction is not an option. As stated by the International Coral Reef Initiative (ICRI) ‘The realisation that “doing nothing changes nothing” must serve as a call to action, inspiring hope, driving innovation, and reinforcing the urgent need for global efforts to safeguard the future of coral reefs’.

Impacts of Coral Bleaching in Mauritius and Historical Context

Coral bleaching is a major concern for Mauritius, an island nation located in the Indian Ocean, which is heavily reliant 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 impacted local reefs^10,11,12. The impacts of coral bleaching in Mauritius extend beyond the environment¹³. The decline of coral health in the region directly affects local economies by threatening fishing industries and tourism, crucial to the livelihood of Mauritians^14,15. Coral loss also undermines natural coastal barriers, increasing vulnerability to erosion and storm surges¹⁶.

The rising SSTs, particularly during El Niño and positive Indian Ocean Dipole events, have triggered severe bleaching in Mauritius (e.g., 1998, 2006, 2016, and 2024), with even small anomalies (≥1 °C) causing mass coral bleaching^17,18. 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 colonised 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.

Bleaching over the years in Mauritius

Mauritius experienced its first recorded mass coral bleaching in 1998 during the strongest El Niño on record, when SSTs rose by 1-1.5 °C, disrupting heat-sensitive zooxanthellae and bleaching corals^20,21. Mortality was generally below 10% at most sites, with the highest bleaching (38.6%) in the south at Le Bouchon²². The comparatively low impact, especially compared to >90% mortality in parts of the Seychelles and Maldives was attributed to cyclonic activity cooling surface waters and reducing solar exposure²³. 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% at most sites except Bel Ombre, which had nearly 100% cover and high species diversity²⁴. Key threats included nutrient pollution, algal overgrowth, cyanobacterial mats, and predation by COTS²⁵. Recommendations included improved wastewater treatment, effluent reuse, coral restoration, shoreline protection, and long-term monitoring^26,27. 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% standing dead coral, turf algal dominance, and limited coral recruitment indicated impaired recovery^28,29,30.

During the 3^rd 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 ³¹. Over 40% of corals were partially bleached, with severe impacts (>65% affected) at Belle Mare, Flic en Flac, and Île aux Bénitiers, while Blue Bay, Bel Ombre, and Mon Choisy experienced <15% bleaching. Severe bleaching peaked in March–April, with about 35% of observations reporting >50% bleaching, but mortality was low at monitored sites such as Anse La Raie Lagoon, where coral cover remained stable (~35%) from 2013-2017 ^32,33. The absence of a national post-event survey limits the accuracy of mortality estimates, particularly for Rodrigues, where losses were reportedly high³⁴.

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 Ocean³⁵. In Mauritius, bleaching was reported as moderate, with some sites showing medium to high severity, though data submissions were fewer compared to neighbouring countries. Regionally, 73% of observations showed moderate to severe bleaching, and 9.9% of reefs experienced high mortality³⁶. While site-specific mortality data for Mauritius remain limited, approximately 80% 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 & IUCN), which uses nursery-grown coral fragments and real-time monitoring to rehabilitate degraded reefs³⁸, and the Adaptation Fund Project, a $9 million program aimed at selecting heat tolerant corals to restore climate-resilient reefs in Mauritius and Seychelles³⁷. Community-based coral culture projects, led by the Mauritius Oceanography Institute and the Nairobi Convention’s WIOSAP³⁹, 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 further strengthen current efforts^40,41. These combined strategies are critical to enhancing the resilience of Mauritius’s reefs in the face of escalating climate pressures. However, the country’s reefs can only stand the test if local and global stress factors attacking Mauritius’ coral reef system are comprehensively assessed and optimally managed. 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 these root causes, the survival of coral reefs worldwide, including those in Mauritius, remains at serious risk.

Conclusion

Mauritius is in a unique situation to tackle coral bleaching. The island can source from the wealth of historical lessons learnt. From the resilience of specific coral taxa to its unique marine ecosystem and its geographical location, several factors need to be exploited to emerge victorious from the constant interplay between global climate dynamics and local anthropogenic pressures. Mauritius’ multi-faceted approach to coral reef conservation reflects a strong commitment to mitigating the impacts of bleaching, restoring degraded reefs, and enhancing ecosystem resilience. Conservation efforts such as coral nurseries, MPAs, community-based restoration, and the use of real-time monitoring technologies are vital steps forward. However, to secure long-term reef resilience, these local measures must be paired with increased 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.

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42. Figure 1 on coral bleaching and Ramsar sites designed by Sophie Morris, Tunley Environmental Ltd., 2025.