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 structurally complex forereef off South Brother with 34 coral genera, unusually diverse for Chagos, where Acropora pushes cover to nearly 64% at mid-depth before plate-forming Pachyseris takes over deeper. Fish life is abundant, including two whitetip reef sharks, with little bleaching and no disease or crown-of-thorns. 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.
Fourteen 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 74 photographs are shown beside the text that discusses them under "Site description" below.
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Site description
Setting
BITB54 lies on a fringing forereef off South Brother in the Three Brothers group, surveyed 26 March 2015 at about 19 m under intermediate wave exposure.
General benthic features
Acropora is the leading genus overall, but the reef's depth transects tell a layered story: coral cover builds to nearly 64% at mid-depth under heavy Acropora dominance, then drops sharply at the deepest transect, where Pachyseris takes over instead. The site is unusually diverse for the archipelago, with 34 coral genera recorded, and the photo record shows a structurally complex reef with abundant fish life, including two whitetip reef shark sightings, and no signs of disease, predation, or crown-of-thorns activity (e.g., Image 2, Image 8, Image 9, Image 20, Image 21, Image 26, Image 34, Image 44, Image 15, Image 17, Image 51).
Coral cover
Formal transects recorded 50% coral cover against 32.1% algal cover, dominated by crustose coralline algae rather than turf, and a published image-based bleaching estimate of just 4.3%. The bleaching percentage is a computer vision figure — CoralNet annotation of the informal survey images, not a diver's field estimate — and human-vs-model agreement is weaker for bleaching (r = 0.40) than for coral cover (r = 0.72). Image-based methods read this reef 28–29 points lower than the transects (CoralNet 21.4%, Coralscapes 20.3%); the three are not interchangeable and should not be averaged (see the full report).
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
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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) | 49.5 |
| CoralNet | 34.3 |
| In-house DINOv3 | 20.8 |
| CoralSCOP edited | 30.9 |
| Claude | 29.8 |
Independent surveys nearby
| Agency / program | Year(s) | Stations / distance | Link |
|---|---|---|---|
| Sheppard et al. (2008) | 2006 | 1 station, 1.97 km | Records ↓ |
| Spalding et al. 2025 (Chagos 2024 bleaching report) | 2024 | 1 station, 1.28 km | Records ↓ |
| XL Catlin Seaview Survey | 2015 | 1 station, 1.75 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 0.5-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) | 1.97 km | HeatCRD database |
| XL Catlin Seaview Survey | transect 36004 | 2015 | 21% | 1.75 km | — |
| Spalding et al. 2025 (Chagos 2024 bleaching report) | Great Chagos Bank - Middle Brother, lagoon (E aspect, 1-3 m) | 2024 | 50% | 1.28 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