Regional summary of 8 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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Sheltered lagoon and bank reefs on the northern Great Chagos Bank and isolated Victory Bank, dived in April 2015, include one site that is more than four-fifths blue coral and records of the endemic Ctenella chagius. An active crown-of-thorns outbreak was photographed at one northern-bank site.
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).
8 sites in this region — click a marker to open that site.
Regional setting
Most of this unit lies on the Great Chagos Bank, which Spalding et al. (2001) describe as a largely submerged atoll and, at roughly 13,000 km², the world's largest atoll structure, bigger in area than even the largest Maldivian atolls. Only a few islands break the surface along its rim. Nelson Island, on the northern rim, became a Strict Nature Reserve in 1998, and the atlas map shows Victory Bank as a small, separate bank lying north of the Great Chagos Bank. The archipelago formed over the volcanic trace of the Réunion hotspot and is the youngest, southernmost part of the Chagos–Laccadive Ridge, isolated from the Maldives by about 500 km of open ocean. Before 1998, corals typically covered 50–80% of seaward and lagoon slopes to about 40 m. No one measured bleaching intensity in 1998, but coral loss on seaward reefs is estimated to have averaged 80–85% and was close to 100% in some areas. As of 2001, the authors still counted the Chagos reefs among the most pristine and best protected in the Indian Ocean. Spalding et al. (2001) estimate about 3,770 km² of reef across the archipelago as a whole, and Souter et al. (2021) give 2,731 km² for the Chagos ecoregion from a newer map.
In the GCRMN analysis of Souter et al. (2021), coral cover in the Chagos subregion fell sharply after 1998, rebuilt until about 2015, and then declined again after the 2016 bleaching. Across South Asia, the 2016 event killed more than 42% of living hard coral, cutting average cover to 26%, and localized bleaching from 2017 to 2019 added further losses. Atoll reefs on the Lakshadweep–Maldives–Chagos Ridge have generally recovered better than nearshore mainland reefs, which carry heavier fishing and pollution, but the authors caution that back-to-back disturbances may now outpace recovery, so reefs never fully return to their earlier state before the next event. They name bleaching, rather than local human pressure, as the primary threat to the protected Chagos atolls.
Overview
This unit pairs reefs inside the Nelson Island lagoon on the northern Great Chagos Bank, plus a fourth northern-bank site at BINI80, with the isolated offshore Victory Bank. The eight sites were surveyed between 20 and 30 April 2015. Logged maximum depths ran from 16.4 m at BIVB109 to 31.2 m at BIVB99, with BINI79 and BIVB81 close behind at 30.9 m and 31.1 m; BINI80's logged maximum was 30.4 m, with transects at about 10-20 m. All eight are lagoonal patch reefs or submerged bank and rim reefs, including BIVB109, a submerged bank reef on isolated Victory Bank. Acropora leads at most sites, often giving way to Porites at depth, as at BINI77. BIVB99 is the exception: blue coral (Heliopora) makes up more than four-fifths of the coral there, in dense thickets of upright folded blades. At BIVB109, fire coral (Millepora) leads the coral at over a third, ahead of Acropora and Porites. The Chagos-endemic Ctenella chagius was recorded at BINI77, BIVB108, and BIVB109.
Cover ranged from 22% at BINI78, where non-coral invertebrates cover 23% of the benthos, more than the live coral, to 60% at BIVB99. The substrate varied from clean, coralline-crusted rock at BINI77 to silty turf and macroalgae at BIVB81 (30% sediment-laden turf and 13% macroalgae). Photos at BIVB81 also show a near-continuous stand of whorled plate coral that the transects did not record.
Published bleaching ran from 5.0% at BINI79 to 16% at BINI77, where stark white tables and branching colonies stood beside table framework that appears to be standing dead. At BIVB108 at least five of 15 frames show chalk-white tables beside pigmented neighbors, suggesting its 11% figure is conservative. At BINI78 published bleaching was 12%, and several colonies in the photographs are bleached white, including a dome bleached over most of its surface; a pale blue-gray encrusting sponge there also mimics bleached coral. Anderson's sampling sheet records one of the four colonies sampled there as 15% bleached and two as very pale. At BINI80, an active crown-of-thorns outbreak was photographed: at least eleven starfish in five frames, including about three on freshly killed white skeleton. The expedition's field report counted 194 crown-of-thorns starfish within a 10 × 50 m area on this reef and found about 30% of the tables with recent crown-of-thorns feeding, 10% dead, and 10% with white syndrome (Bruckner 2015, British Indian Ocean Territory (BIOT) Field Report, p. 15). Anderson's sampling sheet records seven of the eight pocilloporid colonies sampled there as 20% to more than 90% bleached. A whitetip reef shark and a dogtooth tuna were seen at BINI79, nurse sharks were photographed several times at BIVB109, and one large shark with the shape of a nurse shark rested among the tables on the first BINI80 dive.
Highlights
- Heliopora blue coral makes up more than four-fifths of the coral at BIVB99, which has 60% cover
- Active crown-of-thorns outbreak at BINI80: at least eleven starfish in five frames
- Chagos-endemic Ctenella chagius recorded at BINI77, BIVB108, and BIVB109
- Cover ran from 22% at BINI78, where non-coral invertebrates cover 23% of the benthos, to 60% at BIVB99
- Nurse sharks photographed several times at BIVB109, and a whitetip reef shark and dogtooth tuna at BINI79
Notable observations
- Chagos Archipelago - Great Chagos Bank North (BINI80): Crown-of-thorns: active outbreak, at least eleven sightings across five frames in one day, including about three animals on freshly exposed white table skeleton. No bleaching figure was published for this station, but Anderson's sampling sheet records seven of the eight pocilloporids sampled as 20% to more than 90% bleached, and the field report records white syndrome on 10% of the tables (Bruckner 2015).
- Chagos Archipelago - Great Chagos Bank North (BINI77): Bleaching 16%; stark white tables and branching colonies, and much of the table-coral framework appears to be standing dead. No crown-of-thorns; no lesion resolves in the photographs, though Anderson's sampling sheet logs two of the colonies sampled as bleached or diseased, and all three Seriatopora sampled were more than 90% bleached.
- Chagos Archipelago - Victory Bank (BIVB108): Bleaching 11%, photos suggest conservative; at least five of 15 frames show chalk-white tables beside pigmented neighbors. No predation or crown-of-thorns in the photographs; Anderson's field notes record most pocilloporids as bleaching, bleached, or diseased, much of the reef above 20 m bleaching, and bleached colonies being smothered by algae and cyanobacteria.
- Chagos Archipelago - Great Chagos Bank North (BINI79): Published bleaching 5.0%, while Anderson's photographs show stark-white or pale colonies in five frames, including a table white across its whole surface with its polyp texture intact (bleached, not dead) (photos not shown). No crown-of-thorns or disease appears in the photographs, although Anderson's field sheet notes “mostly dead coral, COTS outbreak likely, low diversity/cover at 12 m.”
- Chagos Archipelago - Great Chagos Bank North (BINI78): Published bleaching 12%; several colonies in the photographs are bleached white, and a pale blue-gray encrusting sponge mimics bleached coral. Anderson's sampling sheet records one of the four colonies sampled as 15% bleached and two as very pale.
- Chagos Archipelago - Victory Bank (BIVB99): Published bleaching 6.8%; nearly white compact branching colonies beside the dominant Heliopora sheets, and Anderson's notes record nearly all pocilloporids at the site bleaching and every Seriatopora seen but one totally bleached.
- Chagos Archipelago - Victory Bank (BIVB81): Published bleaching 5.1%; photographs show small stark-white fine-branching colonies beside pigmented neighbors, and Anderson's notes record fully bleached Seriatopora as fairly common at 25-30 m (one of the two sampled was 100% bleached).
- Chagos Archipelago - Victory Bank (BIVB109): Bleaching 8.7%; Anderson's field notes record much of the Millepora bleached and diseased Pocillopora colonies as common, and the sampling sheet logs one sampled colony (ChJo283) as 50% diseased. No crown-of-thorns.
Sites in this region
- Chagos Archipelago - Great Chagos Bank North (BINI77) — 42% coral cover
- Chagos Archipelago - Great Chagos Bank North (BINI78) — 22% coral cover
- Chagos Archipelago - Great Chagos Bank North (BINI79) — 34% coral cover
- Chagos Archipelago - Great Chagos Bank North (BINI80) — 49% coral cover
- Chagos Archipelago - Victory Bank (BIVB108) — 47% coral cover
- Chagos Archipelago - Victory Bank (BIVB109) — 48% coral cover
- Chagos Archipelago - Victory Bank (BIVB81) — 43% coral cover
- Chagos Archipelago - Victory Bank (BIVB99) — 60% coral cover
% coral cover by method
Pooled across 8 dive sites
| Method | Mean % coral cover | Std. dev. between dive sites | n dive sites | Note |
|---|---|---|---|---|
| Manual point count (author) | 38%* | — | 1 | 1 of these 1 values rests on 10–19 photographs (*) |
| CoralNet automated point classifier | 26% | ±5.3 | 7 | 6 of these 7 values rest on 10–19 photographs (*) |
| CoralSCOP (SAM-based mask-sum), edited photos | 27% | ±6.0 | 7 | 5 of these 7 values rest on 10–19 photographs (*) |
| Claude (Anthropic), blind point count | 26% | ±8.7 | 7 | 5 of these 7 values rest on 10–19 photographs (*) |
| In-house DINOv3 point classifier | 29% | ±6.7 | 7 | 5 of these 7 values rest on 10–19 photographs (*) |
| In-water diver transect (line/point intercept) | 43% | ±11 | 8 | — |
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 8 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 11.0 pp across 1 dive site, then CoralNet automated point classifier 17.9 pp (n=7), Claude (Anthropic), blind point count 18.3 pp (n=7). 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 |
|---|---|---|---|---|---|---|---|---|
| BINI77 | — | 30%* n=11 | 34%* n=11 | 26%* n=11 | 26%* n=11 | 42% | 38%* n=11 | 41%* n=11 |
| BINI78 | — | 24%* n=16 | 28%* n=16 | 25%* n=16 | 29%* n=16 | 22% | 27%* n=16 | 29%* n=16 |
| BINI79 | — | 19%* n=15 | 21%* n=15 | 18%* n=15 | 22%* n=15 | 34% | 27%* n=15 | 25%* n=15 |
| BINI80 | 38%* n=10 | 27%* n=14 | 35% n=24 | 29% n=24 | 37% n=24 | 49% | 42% n=24 | 46% n=24 |
| BIVB108 | too few photos n=1 | 25%* n=10 | 28%* n=11 | 12%* n=11 | 24%* n=11 | 47% | 23%* n=10 | 18%* n=11 |
| BIVB109 | too few photos n=1 | 35% n=32 | 25% n=33 | 35% n=33 | 39% n=33 | 48% | 67% n=33 | 66% n=33 |
| BIVB81 | — | too few photos n=5 | too few photos n=5 | too few photos n=5 | too few photos n=5 | 43% | too few photos n=5 | too few photos n=5 |
| BIVB99 | — | 21%* n=11 | 19%* n=11 | 35%* n=11 | 25%* n=11 | 60% | 64%* n=11 | 63%* 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
4 independent survey records within 5 km of a site in this region (each record counted once).
- XL Catlin Seaview Survey — transect 36024 (2015): 17% hard coral cover, 1.58 km from Chagos Archipelago - Great Chagos Bank North (BINI79)
- XL Catlin Seaview Survey — transect 36025 (2015): 3.4% hard coral cover, 1.71 km from Chagos Archipelago - Great Chagos Bank North (BINI79)
- XL Catlin Seaview Survey — transect 36026 (2015): 12% hard coral cover, 0.41 km from Chagos Archipelago - Great Chagos Bank North (BINI79)
- XL Catlin Seaview Survey — transect 36027 (2015): 14% hard coral cover, 1.76 km from Chagos Archipelago - Great Chagos Bank North (BINI78)
All images from Coral Reef Diagnostics. Data are from the Khaled bin Sultan Living Oceans Foundation Global Reef Expedition, Coral Reef Diagnostics, and XL Catlin Seaview Survey (University of Queensland).
© 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]
- 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
- Bruckner AW (2015). Global Reef Expedition: British Indian Ocean Territory. Field Report. Khaled bin Sultan Living Oceans Foundation, Annapolis, MD. 36 pp. PDF