Chagos Archipelago - Great Chagos Bank West (BITB57)

Setting

BITB57 is a channel reef off the southeast side of Middle Brother in the Three Brothers group, leeward on Anderson's field sheet; it was dived three times, on 27 March (57A and 57B) and 28 March 2015 (57C).

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).

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Most of the coral in the photographs is well pigmented; the frames also show a giant clam (e.g., Image 20), a sea turtle (e.g., Image 61, Image 62), and anemonefish (e.g., Image 69), and an eagle ray was photographed on the 57C dive (photo not shown).

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A few isolated bleached colonies were photographed: small stark-white branching colonies beside pigmented ones of the same form on coralline pavement (e.g., Image 8), and a bright-white branching colony beside a pigmented neighbor (e.g., Image 68). Image 52 shows a large table colony with about twenty small stark-white spots on its upper surface, possibly feeding scars; no corallivore is visible, and the cause is unclear.

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Anderson's field notes contrast the dives: 57A and 57C crossed Acropora table meadows, Echinopora meadows, and blue coral, while the 57B dive was “unspectacular ... with tons of dead coral,” mostly dead Acropora table framework with lots of recruitment. 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).

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Anderson's sampling sheet records 24 colonies sampled here at 8.7–29.7 m: six Pocillopora acuta by RFLP; 11 Seriatopora hystrix and one P. acuta by field call; two P. verrucosa, two P. eydouxi and two Pocillopora sp. sampled for a collaborator (field calls); the condition notes run from healthy to 100% bleached, with some diseased or partly dead colonies. 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).

Coral cover

• Reference: Mayfield & Dempsey (2026)
• CoralNet submission package
• Formal surveys recorded 43% live coral cover against 47% 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.
• Transect mean 43 ± 6.8% SE (n = 18 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 40% (Mayfield & Dempsey 2026, Oceans, suppl. S1).
• Published bleaching is 1.2% on the transect dive (57A), with 12% (57B) and 4.6% (57C) on the two other sub-dives (Mayfield & Dempsey 2026, Oceans, suppl. S1). The bleaching percentage is a computer vision figure: CoralNet annotation of the informal survey images, not a diver's field estimate.

Stressors

The Living Oceans Foundation transects here recorded 0.1% cyanobacteria and no recently dead coral.

Comparisons with other data

Apart from the Living Oceans Foundation survey shown here, we found no other surveys at this site as of 2026. 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, 1.06 km away, read 21% in 2015.

Other notes

Aspect follows Anderson's field sheet (LOF fellowship 2016 workbook: 'channel reef', 'leeward'); the expedition's 933-site table classes the station as 'protected' (channel reef). Esri World Imagery (no LOF habitat map covers this station; the LOF Chagos habitat map covers only Peros Banhos) puts the pin off the southeast corner of Middle Brother's reef flat, in the channel toward the small islet to its southeast, facing east-southeast (about 110°): leeward for the northwest monsoon winds of the March survey and windward for the southeast trades of April to November; the sheet's 'leeward' matches the monsoon reading.