Regional summary of 9 dive sites. Figures are averages across the sites that report them (n = sites with a value); each site's own page has its individual values. Part of the Chagos Archipelago overview.
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Egmont atoll's reefs, two Danger Island stations, and outlying Cauvin Bank were dived in March 2015, while bleaching was still building. One Egmont dive captured the whole sequence on tabular corals, from healthy tissue through bleaching to freshly dead, algae-covered skeleton.
Heat stress
Long-term record: 3 years from 1985 through 2025 reached 8 or more °C-weeks at NOAA’s Chagos Archipelago, UK station. Download the annual DHW data (CSV).
9 sites in this region — click a marker to open that site.
Regional setting
Egmont is one of the five true atolls of the Chagos Archipelago (Spalding et al. 2001). The atlas map places it just southwest of the Great Chagos Bank, with Danger Island on the bank's western rim and Cauvin Bank at its southeastern edge. The Great Chagos Bank itself is a mostly submerged atoll of about 13,000 km², and Danger Island was declared a Strict Nature Reserve in 1998. The archipelago formed over the volcanic trace of the Réunion hotspot and is the newest, southernmost section of the Chagos–Laccadive Ridge. It is also remote, some 500 km from the Maldives and more than 1,500 km from Sri Lanka. With about 220 scleractinian species, its reefs are highly diverse. They include Ctenella chagius, the only living member of the family Meandrinidae in the Indo-Pacific, which may be restricted to the Chagos. Spalding et al. (2001) report coral cover of typically 50–80% on seaward and lagoon slopes before 1998, and an estimated average loss of 80–85% on seaward reefs during that year's bleaching. They estimate about 3,770 km² of reef across the whole archipelago, and Souter et al. (2021) give 2,731 km² for the Chagos ecoregion.
The earliest monitoring data in the South Asia analysis of Souter et al. (2021) come from the Chagos in 1978, and the subregion's record shows coral cover recovering from the 1998 losses until about 2015, then falling after the 2016 bleaching. Across South Asia, that event killed more than 42% of living hard coral, and localized bleaching from 2017 to 2019 extended the decline. The authors warn that repeated bleaching may leave reefs too little time to recover between events, although atoll reefs along the ridge have rebounded better than mainland ones. With restricted access and a very large marine protected area (MPA), the Chagos face little local human pressure, and bleaching is the main threat. Earlier, Wilkinson (2006), summarizing the 2004 status report, had noted encouraging recovery in the Chagos after 1998.
Overview
This unit covers Egmont atoll, two Danger Island stations, and the outlying Cauvin Bank, surveyed between 9 and 14 March 2015 on the earliest leg of the expedition. Logged maximum depths ran from 4.8 m at BIDI16.5 to 30.5 m at BIEG04 (BIDI11 has a station CTD cast only). The six Egmont sites carry 33% (BIEG04) to 49% (BIEG06) coral cover. Acropora is the leading genus at BIEG04, with a 38% share of the live coral on the transects. BIEG05 (44%) is led by Porites and Acropora (33% and 29% of its live coral). BIEG07 holds a richly mixed small-scale community, including a huge, fully pigmented brain-coral boulder.
The two Danger Island stations differ sharply. BIDI11, a forereef outside the lagoon (station CTD cast to 23 m), had only four coral genera on its single short transect, with Porites making up three-quarters of its 40% coral, against 21% algae. At BIDI16.5, several Pocillopora colonies were strongly mottled (20% bleached, from point counts on the photographs). Cauvin Bank is a separate, outlying bank about 114 km from the other sites in the unit, not part of Egmont's reef system. Its single site, BICA01, has 4.6% coral cover. Its corals are a thin, evenly mixed veneer of 11 genera on open ground, with a third of the benthos bare and no bleaching recorded.
The Egmont dives came before the 2015 bleaching had fully developed. Station CTD-cast temperatures (cast midpoints, not dive readings) were 28.9 to 30.0°C, and published bleaching values ran from 0.5% at BIEG04, a clean pre-event baseline, to 18% at BIEG07. Branching Acropora and Pocillopora whitened first while massive corals stayed pigmented. A single dive at BIEG05 recorded the full sequence on tabular Acropora, from healthy tissue through partial and full bleaching to freshly dead, algae-overgrown skeleton. At BIEG06 new coral recruits were settling on dead framework. A crown-of-thorns starfish was photographed beside a collection tool at BIEG02, along with two cushion stars. Silvertip and gray reef sharks were seen at several Egmont sites, and BIEG04 held a bait ball of thousands of small silver fish.
Highlights
- BIEG05 recorded the full bleaching sequence on tabular Acropora in a single dive
- Egmont station CTD-cast temperatures ran 28.9-30.0°C, and published bleaching from 0.5% (BIEG04) to 18% (BIEG07)
- New coral recruits settling on dead branching framework beside pale Acropora plains (BIEG06)
- BIDI11: only four coral genera, with Porites making up three-quarters of its 40% coral
- Cauvin Bank (BICA01), an outlying bank about 114 km away, has 4.6% cover
- Silvertip and gray reef sharks at Egmont, and a bait ball of thousands of fish at BIEG04
Notable observations
- Chagos Archipelago - Egmont (BIEG02): Crown-of-thorns starfish unambiguously present, photographed alongside a stainless steel bone-rongeur laid beside it; two cushion stars. Bleaching 3.6% as a tight cluster of fully white Acropora; one large Porites with older, coralline-crusted partial mortality on its upper face.
- Chagos Archipelago - Egmont (BIEG03): Four to five fully bleached branching Acropora colonies in one frame (site figure 4.0%).
- Chagos Archipelago - Egmont (BIEG05): Bleaching 6.5%: a completely bleached Pocillopora, and tabular Acropora showing healthy, partially bleached, wholly bleached, freshly dead, and CCA-overgrown stages on one dive.
- Chagos Archipelago - Egmont (BIEG06): Bleaching 10%: pale, washed-out Acropora plains, colonies with bone-white branch tips, and standing dead branching framework colonized by algae; new recruits on dead substrate.
- Chagos Archipelago - Egmont (BIEG07): Bleaching 18%; bleaching or paling on branching Acropora and Pocillopora in three of eight frames.
- Chagos Archipelago - Danger Island (BIDI16.5): Bleaching 20%: several Pocillopora colonies strongly mottled with bone-white branch clusters; pale patches on massive corals more likely partial mortality or older die-off.
Sites in this region
- Chagos Archipelago - Cauvin Banks (BICA01) — 4.6% coral cover
- Chagos Archipelago - Danger Island (BIDI11) — 40% coral cover
- Chagos Archipelago - Danger Island (BIDI16.5) — 46%* coral cover (informal station, no transect; not in the regional mean)
- Chagos Archipelago - Egmont (BIEG02) — 37% coral cover
- Chagos Archipelago - Egmont (BIEG03) — 40% coral cover
- Chagos Archipelago - Egmont (BIEG04) — 33% coral cover
- Chagos Archipelago - Egmont (BIEG05) — 44% coral cover
- Chagos Archipelago - Egmont (BIEG06) — 49% coral cover
- Chagos Archipelago - Egmont (BIEG07) — 48% coral cover
* Based on only 10–19 reef-habitat photos; this estimate is less certain.
% coral cover by method
Pooled across 7 dive sites
| Method | Mean % coral cover | Std. dev. between dive sites | n dive sites | Note |
|---|---|---|---|---|
| Manual point count (author) | 31% | ±7.8 | 4 | 1 of these 4 values rests on 10–19 photographs (*) |
| CoralNet automated point classifier | 51% | ±9.6 | 2 | — |
| CoralSCOP (SAM-based mask-sum), edited photos | 28% | ±7.6 | 5 | 1 of these 5 values rests on 10–19 photographs (*) |
| Claude (Anthropic), blind point count | 26% | ±9.4 | 5 | 1 of these 5 values rests on 10–19 photographs (*) |
| In-house DINOv3 point classifier | 30% | ±8.9 | 5 | 1 of these 5 values rests on 10–19 photographs (*) |
| In-water diver transect (line/point intercept) | 36% | ±15 | 7 | — |
The mean is an UNWEIGHTED mean of the dive site values — each dive site counts once, whatever its photo count — which is the convention of the published tables and of every other figure on this page. Each value is the one printed on that dive site's own page; a page that prints no value for a method (fewer than 10 photographs) is left out of that method's mean.
This table pools only the 7 dive sites in this unit that have a method comparison; the table for all 101 dive sites is on the Chagos Archipelago page.
Closest to the reference
Over the dive sites where an independent reference exists, Manual point count (author) sits closest to it — a mean absolute deviation of 8.0 pp across 4 dive sites, then CoralNet automated point classifier 9.5 pp (n=2), Claude (Anthropic), blind point count 14.2 pp (n=5). Not in this ranking: Coralscapes v1, Coralscapes v2, and CoralSCOP (edited), which measure coral area rather than points; DINOv3, which counts points over a different denominator. A mean absolute deviation is not an accuracy score: a model with a consistent offset is correctable, while one with the same deviation scattered in both directions is not.
By dive site
| Dive site | Manual point count | CoralNet | CoralSCOP (edited) | Claude | DINOv3 | Diver transect | Coralscapes v1 | Coralscapes v2 |
|---|---|---|---|---|---|---|---|---|
| BIEG02 | 25% n=21 | — | 24% n=21 | 18% n=21 | 24% n=21 | 37% | 31% n=21 | 35% n=21 |
| BIEG03 | 24% n=36 | — | 26% n=36 | 17% n=36 | 18% n=36 | 40% | 32% n=36 | 34% n=36 |
| BIEG04 | 34%* n=19 | — | 28%* n=19 | 26%* n=19 | 35%* n=19 | 33% | 36%* n=19 | 39%* n=19 |
| BIEG05 | 41% n=35 | 58% n=24 | 41% n=59 | 39% n=59 | 40% n=59 | 44% | 61% n=59 | 64% n=59 |
| BIEG06 | — | 44% n=30 | 23% n=30 | 32% n=30 | 34% n=30 | 49% | 46% n=30 | 46% n=30 |
| BIEG07 | too few photos n=8 | — | too few photos n=8 | too few photos n=8 | too few photos n=8 | 48% | too few photos n=8 | too few photos n=8 |
| BICA01 | too few photos n=1 | — | too few photos n=1 | too few photos n=1 | too few photos n=1 | 4.6% | too few photos n=1 | too few photos n=1 |
| BIDI16.5 (not in the regional mean) | 46%* n=13 | 39%* n=11 | 30%* n=11 | 26%* n=11 | 42%* n=11 | — | 37%* n=11 | 39%* n=11 |
n under each value is the number of photographs that method scored at that dive site. ⚠ marks a cell that is not an independent check. * marks a value resting on only 10–19 photographs, which is less certain.
Reference: IN-WATER DIVER TRANSECT, independent of the imagery (Mayfield & Dempsey 2026, Oceans 7:1). Denominator: line/point intercept along the transect; not a photo-point denominator. Source: the mean of all of each station's transects (Living Oceans Foundation per-transect benthic data); the published station values, which weight each depth equally, are in Mayfield & Dempsey 2026, Oceans 7:1, suppl. S1 (formal 38.7% cover / 30.5% bleaching, tourist-diver 35.1% / 31.8%) For all 104 dive sites in the comparison: ⚠ The reference here is NOT the same instrument as at Mauritius, Oman, Kenya, or the Red Sea. Those are point counts on the photographs; this is a diver swimming a transect. The models and the reference never saw the same thing. ⚠ SITE-LEVEL ONLY. The 2,500 frames were not associated with any transect taken within 200 m of them, so transect-vs-image comparisons are site means, never frames. The transect value is the mean of all of each station's own transects in the expedition's per-transect benthic data.
Nearby external survey stations
6 independent survey records within 5 km of a site in this region (each record counted once; 3 lines, and a line that pools several records says how many).
- Sheppard et al. (2008) — Egmont (2006): 19% hard coral cover, 1.01 km from Chagos Archipelago - Egmont (BIEG05) (4 records)
- XL Catlin Seaview Survey — transect 36028 (2015): 11% hard coral cover, 2.17 km from Chagos Archipelago - Egmont (BIEG03)
- XL Catlin Seaview Survey — transect 36029 (2015): 28% hard coral cover, 1.94 km from Chagos Archipelago - Egmont (BIEG07)
All images from Coral Reef Diagnostics. Data are from the Khaled bin Sultan Living Oceans Foundation Global Reef Expedition, Coral Reef Diagnostics, XL Catlin Seaview Survey (University of Queensland), and Sheppard et al. (2008).
© Coral Reef Diagnostics.
References
- Souter D, et al. (eds) (2021) Status of Coral Reefs of the World: 2020. International Coral Reef Initiative / Global Coral Reef Monitoring Network. [PDF]
- Spalding MD, et al. (2001) World Atlas of Coral Reefs. University of California Press, Berkeley. [PDF]
- Wilkinson C (2006) Status of coral reefs of the world: summary of threats and remedial action. In: Côté IM, Reynolds JD (eds) Coral Reef Conservation. Cambridge University Press, Cambridge. [PDF]
- Mayfield AB, Dempsey AC (2026). Using tourist diver images to estimate coral cover and bleaching prevalence in a remote Indian Ocean coral reef system. Oceans 7(1):1. PDF
- Sheppard C, Harris A, Sheppard A (2008). Archipelago-wide coral recovery patterns since 1998 in the Chagos Archipelago, central Indian Ocean. Marine Ecology Progress Series 362:109-117. PDF