Chagos Archipelago - Great Chagos Bank West (BITB40)

Setting

BITB40 lies off the northeast side of Middle Brother in the Three Brothers group, a fringing reef, leeward on Anderson's field sheet, surveyed 21 March 2015 on a dive to 25.2 m.

General benthic features

Across 33 genera, Acropora leads at 14% of the benthos followed by Montipora (7.0%) and Porites (6.3%) near parity (e.g., Image 2, Image 16).

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The depth profile is the finding: cover declines monotonically 54%, 44%, 36%, while Acropora collapses nine-fold from 30% of the benthos to 3.5% and Porites rises to 12% to lead the deepest stratum, whose coral shortfall is taken up by bare substrate (24%) rather than algae (e.g., Image 13).

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Coral cover

• Reference: Mayfield & Dempsey (2026)
• CoralNet submission package
• Formal transects recorded 45% coral against 33% algae, with 13% bare substrate and 8.9% 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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• Transect mean 45 ± 3.6% SE (n = 15 transects; Living Oceans Foundation per-transect benthic data); the headline above is this all-transect mean, and the published station value, which weights each depth equally, is 45% (Mayfield & Dempsey 2026, Oceans, supplemental S1).
• Two caveats belong on the number: the survey has no D1 stratum (its transects start at D2), and only five photographs were scored by the image methods, too few to compare with the transect.
• Live hard coral measured on the in situ benthic transects of the LOF BIOT-1 expedition (the mean of all of this station's transects), against total algal cover (all algal groups) of 33% of the benthos.
• Image-based estimates (CoralNet, DINOv3, CoralSCOP, and Claude) are listed in the method table for comparison only; the transect figure is this site's cover value.

Stressors

Published bleaching is 0.00%, 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 is visible in the photographs; of the colonies Anderson sampled here, one was small and pale, though likely healthy (see Dive record below). The Living Oceans Foundation transects here recorded under 0.1% cyanobacteria and 0.1% recently dead coral.

Fish and other fauna

A large nurse shark was photographed (e.g., Image 12); Anderson's field notes for this dive record a “huge, pregnant nurse shark.”

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Comparisons with other data

The XL Catlin Seaview Survey covered a nearby station (transect 36004) in 2015, 0.26 km from the pin; we could not confirm that it is the same reef. Within 5 km, two independent stations recorded station-year means of 21–41% hard coral cover between 2006 and 2015; the nearest, XL Catlin Seaview Survey at transect 36004, 0.26 km away, read 21% in 2015.

Dive record

Anderson's afternoon dive on 21 March 2015 entered the water at 14:19 and lasted 44 minutes, to a maximum depth of 25.2 m (the field sheet); the notes record a current so intense that the dive was ended early, and “stunning coral at 10 m.” the sampling sheet lists seven Seriatopora hystrix colonies from this dive, five sampled by Anderson and two collected by Andy: ChSe23 (collected 14:22 at 21.0 m; fairly healthy, with many brittle stars inside), ChSe24 (14:30, 24.2 m; huge and healthy, covered in brittle stars), ChSe25 (14:34, 20.9 m; healthy, with relatively thick branches, apparently toppled down the slope recently), ChSe26 (14:37, 21.0 m; healthy, among soft corals), ChSe27 (14:45, 20.9 m; small and pale, though likely healthy), ChSe28 (about 14:45, 20 m; collected by Andy), and ChSe29 (about 14:45, 25 m; collected by Andy). Anderson also sampled one pocilloporid for a collaborator: ChJo109 (Pocillopora sp.; 14:40, 20.7 m; a small colony of the local morphology, common at the site; no bleaching recorded). Pocillopora acuta colonies from this archipelago were analyzed in Thomas et al. (2026, Global Ecology and Biogeography 35:e70305, https://doi.org/10.1111/geb.70305).

Other notes

Aspect follows Anderson's field sheet (LOF fellowship 2016 workbook: 'fringing reef', 'leeward'; one entry leaves the aspect blank); the expedition's 933-site table classes the station as 'intermediate' (channel reef). Esri World Imagery (no LOF habitat map covers this station; the LOF Chagos habitat map covers only Peros Banhos) puts the pin about 300 m off the northeast corner of Middle Brother's reef flat, in open water beside a small submerged patch, facing northeast (about 45°): side-on for the northwest monsoon winds of the March survey and side-on for the southeast trades of April to November, so the imagery reads this station as side-on rather than leeward; the aspect shown is the sheet's label.