Chagos Archipelago - Great Chagos Bank West (BITB40)

Setting

BITB40 lies on the lagoon-channel side of Middle Brother in the Three Brothers group, an intermediate-exposure emergent fringing reef surveyed 21 March 2015 at 27.8 m.

General benthic features

Across 33 genera, 6th-highest of the 109, Acropora leads at 13.7% of the benthos but the second tier is unusually flat, Montipora 6.9% and Porites 6.8% near parity (e.g., Image 2, Image 16).

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The depth profile is the finding: cover declines monotonically 54.3%, 44.2%, 36.2%, while Acropora collapses nine-fold from 30.0% of the benthos to 3.5% and Porites rises to 11.5% to lead the deepest stratum, whose coral shortfall is taken up by bare substrate (23.7%) rather than algae (e.g., Image 13).

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Coral cover

Formal transects recorded 44.9% coral against 32.4% algae — 37th of 109 Chagos sites for cover and 82nd for algae, the lower quartile — with 13.7% bare substrate and 9.1% non-coral invertebrates, visible in the photographs as gorgonian sea fans, wire corals, and encrusting sponges (e.g., Image 6, Image 9, Image 18).

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Two caveats belong on the number: the site has no shallowest stratum at all, the survey starting one depth band deeper than its neighbors, and the two image products disagree wildly with the transects and with each other (CoralNet 12.4%, Coralscapes 30.0%), so the three should not be averaged. Coral cover: 45%. 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 32.4% 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 44.9% · CoralNet points 12.4% · Coralscapes masks 30% (15 frames). These products have different sampling footprints and denominators and should not be averaged.

Stressors

Published bleaching is 0.00% on the canonical % bleaching (2nd training) column, 107th of 110 dives and identical to the first run, and all 15 frames were reviewed without finding any bleaching field — only white Acropora branch tips, read as normal growth. No crown-of-thorns, disease, or predation.

Fish and other fauna

Two gray reef sharks and an embedded giant clam were photographed (e.g., Image 12).

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Comparisons with other data

No independent survey was made at this exact site; the nearest ones are listed in the table below.