Part of the regional summaries: Chagos Archipelago - Great Chagos Bank West / Brothers / Eagle and Chagos Archipelago
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Chagos (NOAA CRW virtual station)A forereef off South Brother in the Three Brothers group with 50% coral cover and 34 coral genera, among the highest diversity in Chagos, including the endemic brain coral Ctenella. The shallows are a zone of massive Porites with Acropora dominating below, and fish are the standout: fusilier and baitfish schools, bluefin trevally, a moray, and a giant clam. Surveyed on the Living Oceans Foundation's Global Reef Expedition.
Three independent survey stations shown with the program’s logo — click one to see how far it is from this site.
Thirteen other CRD dive sites within 5 km shown in orange — click a marker to open its page.
🗺 Detailed map of the Global Reef Expedition dive sites, Three Brothers, Chagos (Khaled bin Sultan Living Oceans Foundation; opens in a new window)
11 of these 64 photographs are shown beside the text that discusses them under "Site description" below.
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Site description
Setting
BITB39 sits on the emergent fringing forereef off South Brother in the Three Brothers group, intermediate exposure, surveyed 21 March 2015 at 28.6 m.
General benthic features
Genus richness is the other headline: 34 genera, equal-2nd of the 109, including the Chagos-endemic brain coral Ctenella at trace level. Acropora leads at 19.7% of the benthos, 39% of all coral, ahead of Porites 8.6% (e.g., Image 2, Image 11).
Coral cover
Formal transects recorded 50.3% coral against 36.3% algae — 24th of 109 Chagos sites for cover, 66th for algae — with coralline crust at 18.8% of the benthos and a substantial fleshy turf component at 12.2%, unusual for a Chagos forereef of this cover (e.g., Image 9, Image 23).
The depth finding is a genus handover at the top of the reef rather than the bottom: cover runs 42.3%, 54.1%, 54.6%, 50.1% down the profile, but the shallowest stratum is a massive-Porites zone on hard coralline pavement (Porites 17.7% against Acropora 11.2%), while every deeper stratum is Acropora-led, peaking at 30.4% of the benthos. Image methods read far lower (CoralNet 29.1%, Coralscapes 26.7%) and should not be averaged with the transects. Coral cover: 50%. Live hard coral measured on the in situ benthic transects of the LOF BIOT-1 expedition (Mayfield & Dempsey 2026, Oceans, supplemental S1), against algal cover of 36.3% of the benthos. The transect figure is this site's authoritative cover value; no image-derived product overrides it. Across methods the same reef reads: transects 50.3% · CoralNet points 29.1% · Coralscapes masks 26.7% (61 frames). These products have different sampling footprints and denominators and should not be averaged.
Stressors
Bleaching on the canonical % bleaching (2nd training) column is 4.19%, 84th of 110 dives, the lower quartile and identical to the first run; 38 of the 61 frames were reviewed without corroborating it, most colonies holding normal pigmentation and the conspicuous white areas being exposed framework and rubble rather than pale tissue, though three pallor observations are left unresolved (e.g., Image 6, Image 24).
No crown-of-thorns, disease front, or predation scarring.
Fish and other fauna
Fish are the standout, with fusilier and baitfish schools, two bluefin trevally, a giant clam, and a moray (e.g., Image 5, Image 19, Image 22, Image 31, Image 38).
Comparisons with other data
No independent survey was made at this exact site; the nearest ones are listed in the table below.
CV model comparison
More info
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 already in the pin `cover` field (109 of 112 pins, sourced to the paper's suppl. S1) and is read from there, never re-imported. The transect is THE coral-cover value for this site and stands alone. The rows below it are a methods comparison only: no computer-vision value is averaged with it, feeds the headline, or carries over to the site's cover figure (Anderson, 2026-09-22).| Method | Hard-coral cover (%) |
|---|---|
| Transect (published) | 50.3 |
| CoralNet | 40.1 |
| In-house DINOv3 | 31.7 |
| CoralSCOP edited | 44.4 |
| Claude | 36.7 |
Independent surveys nearby
| Agency / program | Year(s) | Stations / distance | Link |
|---|---|---|---|
| Sheppard et al. (2008) | 2006 | 1 station, 3.04 km | Records ↓ |
| Spalding et al. 2025 (Chagos 2024 bleaching report) | 2024 | 1 station, 2.31 km | Records ↓ |
| XL Catlin Seaview Survey | 2015 | 1 station, 2.48 km | Records ↓ |
Dr. Coral (CRD)Year-average (no detectable trend)Within 5 km, 3 independent survey stations recorded hard coral cover in 7 records between 2006 and 2024. One survey since 2021 recorded 50%. This site's own estimate (50%) is indistinguishable from the recent value (50%): the 0.3-point gap is inside the ±7 percentage-point noise between methods; the methods differ, so the comparison is context, not a like-for-like test.
Collected by other programs using different methods (the XL Catlin Seaview Survey uses photo-transects with automated image annotation), so these are context for the figures above, not a like-for-like comparison. Where a station was surveyed more than once the range is shown. Station names are not dive-site names.
| Agency | Site / location | Date | Hard coral cover (%) | Distance | Link |
|---|---|---|---|---|---|
| Sheppard et al. (2008) | Great Chagos Bank | 2006 | 41% (5 surveys) | 3.04 km | HeatCRD database |
| XL Catlin Seaview Survey | transect 36004 | 2015 | 21% | 2.48 km | — |
| Spalding et al. 2025 (Chagos 2024 bleaching report) | Great Chagos Bank - Middle Brother, lagoon (E aspect, 1-3 m) | 2024 | 50% | 2.31 km | — |
Data & links
- Read the full site report ↗
- Download benthic survey data (CSV) ↗
- Download all LOF-GRE data (933 sites) ↗
- CoralNet image repository for this region ↗
- HeatCRD database ↗
- (reference) ↗
- Photo portfolio ↗
- Coral Reef Diagnostics page for this area ↗
All images from Coral Reef Diagnostics. Data are from Coral Reef Diagnostics and the organizations named in the data-sources table above.
© Coral Reef Diagnostics.
References
- 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
- Spalding M, Bourasseau M, et al. (2025). Coral bleaching and mortality in the Chagos Archipelago, 2024. British Indian Ocean Territory Administration report (gray literature). PDF