Part of the regional summaries: Chagos Archipelago - Great Chagos Bank West / Brothers / Eagle and Chagos Archipelago
More info
Chagos (NOAA CRW virtual station)A lagoon fringing reef at about 18 m in Three Brothers where large gorgonian sea fans and massive coral bommies shelter dense fish schools, beside open rubble flats with scattered colonies. The reef is Porites-led at 37% cover, with one of the lowest bleaching rates in the archipelago (3.2%). 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.
Eight 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)
13 of these 22 photographs are shown beside the text that discusses them under "Site description" below.
Site description
General benthic features
Porites is the leading genus, carrying over a third of all coral recorded, followed by Acropora, though the balance shifts with depth: the shallow zone is algae-dominated with the two genera nearly tied, while the deeper zone is markedly more coral-abundant and clearly Porites-led. Underwater photographs show large gorgonian sea fans and massive coral bommies sheltering dense fish schools, alongside open rubble flats with only scattered colonies (e.g., Image 1, Image 2, Image 3, Image 5, Image 6, Image 10, Image 14, Image 16, Image 18, Image 21, Image 24, Image 25).
Coral cover
Formal transects recorded 37% live coral cover against 34.8% algae, with a published bleaching rate of just 3.2%, among the lowest measured in the archipelago. 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 9–11 points lower than the transects (CoralNet 27.4%, Coralscapes 25.7%); the three are not interchangeable and should not be averaged (see the full report).
Stressors
One close-up shows a small partially bleached coral colony, a sporadic finding consistent with the site's otherwise low bleaching level (e.g., Image 19).
No crown-of-thorns starfish (COTS) or other corallivores evident.
Comparisons with other data
No independent survey was made at this exact site; the nearest ones are listed in the table below.
Dive record
BITB53 lies on a submerged fringing reef inside the lagoon at Three Brothers, Chagos, surveyed at 18.3 m in March 2015.
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) | 36.7 |
| CoralNet | 35.2 |
| In-house DINOv3 | 30.8 |
| CoralSCOP edited | 27.4 |
| Claude | 23.4 |
Independent surveys nearby
| Agency / program | Year(s) | Stations / distance | Link |
|---|---|---|---|
| Sheppard et al. (2008) | 2006 | 1 station, 3.76 km | Records ↓ |
| Spalding et al. 2025 (Chagos 2024 bleaching report) | 2024 | 1 station, 3.79 km | Records ↓ |
| XL Catlin Seaview Survey | 2015 | 1 station, 3.76 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 (37%) sits 13.3 points below the recent value (50%); 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) | 3.76 km | HeatCRD database |
| XL Catlin Seaview Survey | transect 36004 | 2015 | 21% | 3.76 km | — |
| Spalding et al. 2025 (Chagos 2024 bleaching report) | Great Chagos Bank - Middle Brother, lagoon (E aspect, 1-3 m) | 2024 | 50% | 3.79 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