Chagos Archipelago - Great Chagos Bank West (BITB55)

Setting

BITB55 is a coral-abundant fringing forereef off South Brother in the Three Brothers group, windward on Anderson's field sheet, surveyed 26 March 2015 on a dive to 30.5 m.

General benthic features

The reef is a mix of Porites and Acropora, with Porites edging out overall; shallower ground is dominated by tabular and corymbose Acropora, while the deepest stratum, a single transect, is led by massive Porites (e.g., Image 3, Image 4, Image 15, Image 17, Image 24, Image 33).

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Photographs show a structurally complex reef (e.g., Image 1, Image 5, Image 8, Image 10, Image 12, Image 30, Image 32).

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Coral cover

• Reference: Mayfield & Dempsey (2026)
• CoralNet submission package
• Published transects record 51% coral cover against 30% algae.
• Transect mean 51 ± 3.1% SE (n = 10 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 48% (Mayfield & Dempsey 2026, Oceans, suppl. S1).

Stressors

Published bleaching is 8.1%. The bleaching percentage is a computer vision figure: CoralNet annotation of the informal survey images, not a diver's field estimate (Mayfield & Dempsey 2026, suppl. S1). A crown-of-thorns starfish was photographed beside a gloved hand holding forceps (e.g., Image 21); Anderson's sampling sheet notes it as the only one seen at the site.

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One of the colonies sampled here was 35% sickly or diseased (see Dive record below). The Living Oceans Foundation transects here recorded no cyanobacteria and no recently dead coral.

Fish and other fauna

An octopus was photographed (e.g., Image 11, Image 13, Image 29); Anderson's field notes for this dive record a “mating octopus.”

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Comparisons with other data

Sannassy Pilly et al. (2024) surveyed a nearby station (South Brother), 0.20 km from the pin. Cover values from that work are not among the survey records shown on this page. Within 5 km, two independent stations recorded station-year means of 21–41% hard coral cover between 2006 and 2015; the nearest of these, XL Catlin Seaview Survey at transect 36004, 3.08 km away, read 21% in 2015.

Dive record

Anderson's afternoon dive on 26 March 2015 entered the water at 13:48 and lasted 58 minutes, to a maximum depth of 30.5 m (the field sheet), on a near-vertical wall with current and surge; the notes call the coral at 5–10 m spectacular. Anderson sampled two Pocillopora damicornis colonies: CHPd48 (collected 14:22 at 14.6 m; a small colony one to two years old, identity not certain in the field, on a survey transect next to the only crown-of-thorns seen at the site; confirmed as P. damicornis by RFLP) and CHPd49 (about 14:00, 15 m; collected by Sam). Anderson also sampled three pocilloporids for a collaborator: ChJo147 (13:59, 29.2 m; likely P. verrucosa, 35% sickly or diseased; tissue next to the diseased part was sampled), ChJo148 (14:05, 20.3 m; Pocillopora sp., healthy, with 50% old mortality), and ChJo149 (14:30, 8.5 m; Pocillopora sp., a huge, healthy colony of candelabrum form, common at the site). The notes record P. damicornis as present but very rare, and no P. acuta or Seriatopora. 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', 'windward'); the expedition's 933-site table classes the station as 'intermediate' (fringing 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 south-southwest side of South Brother, facing south-southwest (about 200°): side-on (close to the leeward cutoff) for the northwest monsoon winds of the March survey and side-on (close to the windward cutoff) for the southeast trades of April to November, so the imagery reads this station as side-on rather than windward; the aspect shown is the sheet's label.