Chagos Archipelago - Great Chagos Bank West (BITB43)

Khaled bin Sultan Living Oceans Foundation — Global Reef Expedition

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Part of the regional summaries: Chagos Archipelago - Great Chagos Bank West / Brothers / Eagle and Chagos Archipelago

Photo: Anderson Mayfield/Coral Reef Diagnostics

Country

Chagos Archipelago

Survey date(s)

2015-03-22

Coordinates

-6.1053, 71.4988

Coral cover

29%

Benthic surveys

Coral bleached

0.0%

Computer vision (CoralNet), Mayfield & Dempsey (2026)

Observations

bleaching

Heat stress

Today’s heat load

28.9°C 0.0°C-weeks

16 Sep 2026 · No heat stress

5-year thermal stress load

5-year mean max. degree heating weeks (DHW)

4.4°C-weeksⁱ

2021–2025

NOAA Coral Reef Watch — Chagos virtual station

i 5-year mean max. DHW is the mean of the annual maximum DHWs over the five complete years named above, not conditions today.

More info

Chagos (NOAA CRW virtual station)

Photographs

Site summary

An unusual Three Brothers bank reef where sponges, sea squirts, and sea fans cover more of the bottom than coral does. The published bleaching figure is zero, but photos show bone-white patches on a cauliflower coral and elsewhere, and three gray reef sharks were seen. Surveyed on the Living Oceans Foundation's Global Reef Expedition.

Max. depth dived

30.7 m

dive-log maximum

Water temp. during survey

29.7°C

+0.8°C vs the 2015 mean sea temperature (28.9°C)

Bottom time

46 min

in at 14:33

Four 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)

21 of these 22 photographs are shown beside the text that discusses them under "Site description" below.

Photo credit & attribution

Site description

General benthic features

BITB43 is an intermediate-exposure submerged bank forereef outside the lagoon at Three Brothers, surveyed 22 March 2015 at 29 m, and its defining feature is not its coral: non-coral invertebrates take 32.2% of the benthos, more than the 29% coral cover, mostly sponges, tunicates, and gorgonian sea fans, conspicuous in nine of the 22 frames (e.g., Image 3, Image 5, Image 8, Image 12, Image 16, Image 21, Image 26).

Across 28 genera, 30th of 109, both leaders are non-Acropora: Porites 7.33% of the benthos and Montipora 6.18%, nearly half the coral, with Acropora a distant fourth at 2.46%.

Coral cover

Formal transects recorded 28.7% coral against 26.8% algae — 80th of 109 Chagos sites for cover but 95th for algae, so this is a low-algae reef whose free space went to invertebrates rather than weed. The depth finding is an Acropora collapse rather than a cover collapse — cover barely moves, 30.4% to 27.1%, while Acropora falls twenty-four-fold from 4.72% of the benthos to 0.20%, Porites climbs to a third of all deep coral, and bare substrate triples to 18.7%; at the shallower station invertebrate cover (37.5%) exceeds coral cover outright. Method agreement is unusually tight (CoralNet 22.2%, Coralscapes 22.9%), but the three should still not be averaged.

Stressors

Published bleaching is 0.00% on the canonical % bleaching (2nd training) column, 108th of 110 dives, both runs agreeing — yet the photographs contradict it: one frame shows a bone-white sector sharply demarcated across the upper branches of an otherwise pigmented pocilloporid, with white tissue in two further frames (e.g., Image 10, Image 14, Image 25).

It is not a reef-wide field, but zero is not what the imagery shows. No crown-of-thorns or disease in any of the 22 frames (e.g., Image 1, Image 4, Image 6, Image 9, Image 15, Image 19, Image 22, Image 24).

Fish and other fauna

Three gray reef sharks were photographed (e.g., Image 7, Image 18, Image 20).

Comparisons with other data

No prior survey data from this site. If you would like to share yours, please email us.

CV model comparison

Headline is the transect value; every other row is listed beside it for comparison and none is averaged in. ⚠ The reference here is NOT the same instrument as at Mauritius, Oman, Kenya or the Red Sea.

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)	28.7
CoralNet	35.9
In-house DINOv3	14.7
CoralSCOP edited	36.6
Claude	22.3

Data & links

All images from Coral Reef Diagnostics. Data are from Coral Reef Diagnostics.