Expedition report: Little and large: surveying and safeguarding coral reefs & whale sharks in the Maldives (October 2025)

Abstract 

In October 2025, Biosphere Expeditions ran its 13^th annual Reef Check survey expedition to the Maldives. International citizen scientists, supervised by a professional coral reef biologist, performed Reef Check surveys for two weeks in three central atolls (Ari, South Male’ and Vaavu atolls) at exposed and sheltered sites. The expedition was slightly hampered by strong winds in week 2, which affected plans to survey more exposed sites in South Male’ and Vaavu atolls. 

The Maldives escaped a mass coral bleaching event in 2025, following the significant thermal stress event recorded in 2024. Only minor, localised coral bleaching was observed during the 2025 surveys, with standard transect data indicating that generally less than 1% of the observed colonies were affected.

Hard corals at the Holiday Thila inner reef site showed slight signs of disease, likely linked to chronic pollution and sedimentation originating from the nearby developed island of Mamagili. Existing literature connects elevated sedimentation rates to a higher occurrence of white-band disease in branching Acropora. In contrast, surveys did not record any hard coral disease at the more exposed outer reef locations. Predation by corallivorous snails was patchy, but occurred at nearly all surveyed sites, peaking in intensity at the inner reef of Kuda Falhu with 9–18 affected colonies per transect. Other anthropogenic impacts included marine debris, with lost fishing lines entangled at 7 sites and solid waste present at 6 sites; the highest trash accumulation was documented at Farish Faru near active mooring buoys.

The highest hard coral cover was recorded at Beybe’s Bellybutton (64% - shallow transects) and Bathaalaa Maagaa (60% - deep transects), with robust cover also at Rasdhoo (49% deep) and Kuda Falhu (51% shallow). The communities at Kuda Falhu and Beybe’s Bellybutton were predominantly comprised of bleaching-tolerant growth forms, distinct from outer reefs, which remain dominated by more thermal-sensitive branching and digitate Acropora and Pocillopora.

Long-term temporal changes in benthic substrate were evaluated across four key sites over a six-year interval (2019–2025). Hard coral cover increased along the deep transect at Rasdhoo and across both depth zones at Kuda Falhu. Conversely, coral cover at Dega Thila remained critically low (under 5%), as the reef remains locked in an ecological phase shift dominated by the corallimorph Discosoma, particularly at shallow depths of 3-5 m. While hard coral cover at Farish Faru held stable at 25-30%, the substrate faces severe space competition from an unidentified encrusting grey sponge. Unstable coral rubble also increased at both depths for Dega Thila and Farish Faru between 2019 and 2025. Collectively, these multi-year trends reveal a highly fragmented recovery pattern: certain sites demonstrate strong resilience and coral recruitment, while others are succumbing to degraded, low-diversity phase shifts.

Reef fish assessments revealed moderate populations of snappers, driven largely by schools of humpback snapper Lutjanus gibbus at Kuda Falhu and the Guraidhoo outer reef. However, other ecologically important target species that typically occupy outer slopes, such as the black and white snapper Macolor niger and red snapper Lutjanus bohar, were absent on transects, except for a lone Macolor niger noted in deeper waters off-transect at Kuda Fushi. Commercially valuable species were scarce or undersized across all surveyed sites; large emperors and groupers were rarely recorded, with no grouper exceeding 50 cm in length. One humphead wrasse Cheilinus undulatus was encountered off-transect on Guraidhoo Outer reef.

Invertebrate indicator species were generally scarce. Three banded coral shrimps Stenopus hispidus were observed across two outer reefs (Rasdhoo and Bathaalaa Maagaa). Lobsters (Panulirus spp.) were recorded in low numbers at six sites, primarily along shallow profiles, with the exception of the deep transect at Beybe’s Bellybutton, which had the highest density ever recorded by Biosphere Expeditions (six individuals). Commercially targeted sea cucumbers (mostly Holothuria edulis) appeared at six sites, peaking at Bathaalaa Maagaa. Notably, zero Crown-of-Thorns starfish (Acanthaster planci) or triton trumpet shells (Charonia tritonis) were seen on any transect, and two inner reef sites (Farish Faru and Holiday Thila) lacked any invertebrate indicators apart from giant clams (Tridacna spp.). Giant clam densities were higher on shallow transects across ten sites, ranging from 1 to 6 individuals per 100 m² with the majority of individuals measuring under 20 cm in length.

Megafauna sightings off-transect included blacktip (Carcharhinus melanopterus) and whitetip reef sharks (Triaenodon obesus), as well as green (Chelonia mydas) and hawksbill (Eretmochelys imbricata) sea turtles. A giant reef ray (Mobula alfredi) was also recorded at the Guraidhoo Outer reef near a major atoll channel.

Two short effort-based whale shark surveys was carried out at the outer reefs of South Ari Marine Protected Area and south of Guraidhoo on South Malé Atoll. No sharks were sighted.

In conclusion, the primary global and local stressors threatening Maldivian reefs, including climate breakdown, ocean acidification, overfishing, coastal sedimentation and poor waste management, have persisted unabated since monitoring began. Current trajectories indicate a catastrophic outlook aligned with IPCC projections, suggesting that 70-90% of global tropical reefs may succumb by 2050 under a 1.5°C warming scenario. In the Maldives, inner lagoonal reefs face a bleak future within our lifetimes due to high thermal stress and poor water quality, while outer slopes benefit from cooling oceanic upwellings. This spatial variance defines the "inner/outer reef dichotomy," a paradigm first proposed in expedition reports explicitly corroborated by Pancrazi et al. (2025) using long-term data from these expeditions. Their study confirms that post-bleaching recovery on inner reefs is severely stunted, leaving simplified seascapes dominated by small, slow-growing, bleaching-tolerant colonies. To reverse this widespread degradation, immediate, localised enforcement alongside systemic eco-political governance reforms must be deployed across the archipelago.