Part of the regional summaries: Chagos Archipelago - Great Chagos Bank West / Brothers / Eagle and Chagos Archipelago
2015-03-28
More info
Chagos (NOAA CRW virtual station)A lagoon channel reef in the Three Brothers region of Chagos, where a thin, algae-covered shallow flat deepens into a high-cover bank of table Acropora and columnar Turbinaria. Giant clams, anemonefish, an eagle ray, and a sea turtle were seen, alongside a few bleached colonies. 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)
24 of these 75 photographs are shown beside the text that discusses them under "Site description" below.
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Site description
Setting
This protected, submerged channel reef sits inside a lagoon passage of the Three Brothers, Chagos Archipelago.
General benthic features
Coral cover builds strongly with depth, from a thin, algae-covered shallow flat to a high-cover deep bank where tabular Acropora and columnar Turbinaria together make up the great majority of the coral present (e.g., Image 22, Image 26, Image 33, Image 38, Image 40).
Underwater photographs show mostly healthy, well-pigmented coral along with giant clams, anemonefish, an eagle ray, and a sea turtle, though a few isolated bleached colonies were also documented (e.g., Image 2, Image 5, Image 9, Image 15, Image 20, Image 24).
Overall the reef presents as coral-abundant and structurally complex, particularly at depth (e.g., Image 3, Image 4, Image 6, Image 7, Image 10, Image 11, Image 13, Image 17, Image 21, Image 29, Image 31, Image 32, Image 39).
Coral cover
Formal surveys recorded 40% live coral cover against 46.8% algae, dominated by crustose coralline algae rather than fleshy macroalgae; per-dive computer-vision bleaching estimates (Mayfield & Dempsey 2026) are listed in the Dive history table below. 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 6–8 points lower than the transects (CoralNet 32.3%, Coralscapes 34.1%); the three are not interchangeable and should not be averaged (see the full report).
Stressors
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.
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) | 40.1 |
| CoralNet | 44.0 |
| In-house DINOv3 | 33.2 |
| CoralSCOP edited | 36.4 |
| Claude | 43.4 |
Dive history
Coral bleached: computer-vision (CoralNet) estimate from the survey images, per dive, as published in Mayfield & Dempsey (2026), supplemental file S1.
| Date | Sub-site | Time in | Max depth (m) | Bottom time (min) | Coral bleached (%) |
|---|---|---|---|---|---|
| 2015-03-27 | BITB57A | 13:07 | 29.0 | 52 | 1.2 |
| 2015-03-27 | BITB57B | 16:17 | 30.1 | 58 | 12.2 |
| 2015-03-28 | BITB57C | 08:18 | 28.9 | 65 | 4.6 |
Independent surveys nearby
| Agency / program | Year(s) | Stations / distance | Link |
|---|---|---|---|
| Sheppard et al. (2008) | 2006 | 1 station, 1.89 km | Records ↓ |
| Spalding et al. (2024) | 2024 | 1 station, 0.80 km | Records ↓ |
| Spalding et al. 2025 (Chagos 2024 bleaching report) | 2024 | 1 station, 0.80 km | Records ↓ |
| XL Catlin Seaview Survey | 2015 | 1 station, 1.06 km | Records ↓ |
Dr. Coral (CRD)Year-average (no detectable trend)Within 5 km, 4 independent survey stations recorded hard coral cover in 8 records between 2006 and 2024. Surveys since 2021 average 50% (range 50%, n=2). This site's own estimate (40%) sits 9.9 points below the recent range (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) | 1.89 km | HeatCRD database |
| XL Catlin Seaview Survey | transect 36004 | 2015 | 21% | 1.06 km | — |
| Spalding et al. 2025 (Chagos 2024 bleaching report) | Great Chagos Bank - Middle Brother, lagoon (E aspect, 1-3 m) | 2024 | 50% | 0.80 km | — |
| Spalding et al. (2024) | Great Chagos Bank - Middle Brother, lagoon (E aspect, 1-3 m) | 2024 | 50% | 0.80 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