Chagos Archipelago - Great Chagos Bank West (BITB54)

Setting

BITB54 lies on a fringing forereef off South Brother in the Three Brothers group, surveyed 26 March 2015 on a dive to 30.2 m; it is windward on Anderson's field sheet.

General benthic features

Acropora is the leading genus overall, but the reef's depth transects tell a layered story: coral cover builds to nearly 64% at mid-depth under heavy Acropora dominance, then drops at the deepest transect (a single transect), where Pachyseris takes over instead. Thirty-four coral genera were recorded on the transects, and the photo record shows a structurally complex reef with abundant fish life, including a whitetip reef shark (e.g., Image 51) and a nurse shark that Anderson's field notes call “aggressive” Image 15 and Image 17, and the photographs show no sign of disease, predation, or crown-of-thorns activity (e.g., Image 2, Image 8, Image 9, Image 20, Image 21, Image 26, Image 34, Image 44).

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Anderson's sampling sheet records three colonies sampled here: Pocillopora acuta (confirmed by RFLP) CHPd47 (21.8 m, large, healthy, 5% bleached or dead); Seriatopora hystrix ChSe46 (30.1 m, healthy) and ChSe47 (28.6 m, large, healthy). It also records two other pocilloporids sampled for a collaborator (species by field call): Pocillopora sp. ChJo145 (10.5 m, healthy, toppled) and ChJo146 (8.8 m, sickly, 20% diseased). 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 transects recorded 56% coral cover against 33% algal cover, dominated by crustose coralline algae rather than turf, and a published image-based bleaching estimate of 4.3%.
• Transect mean 56 ± 3.5% SE (n = 17 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 50% (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 (Mayfield & Dempsey 2026, suppl. S1).

Stressors

Anderson's sampling sheet records one of the colonies sampled here, Pocillopora sp. ChJo146 (8.8 m), as sickly and 20% diseased; no disease lesion is visible in the photographs (see General benthic features). 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, 1.75 km away, read 21% in 2015.

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' (channel reef). Esri World Imagery (no LOF habitat map covers this station; the LOF Chagos habitat map covers only Peros Banhos) puts the pin on the west side of the small unnamed islet between Middle Brother and South Brother, at the edge of the channel, facing west (about 270°): windward for the northwest monsoon winds of the March survey and leeward for the southeast trades of April to November; the sheet's 'windward' matches the monsoon reading.