Chagos Archipelago - Great Chagos Bank West (BITB39)

Setting

BITB39 sits on the emergent fringing forereef off South Brother in the Three Brothers group, windward on Anderson's field sheet, surveyed 21 March 2015 on a dive to 30.2 m.

General benthic features

Genus richness is the other headline: 34 genera, including the Chagos-endemic brain coral Ctenella at trace level. Acropora leads at 22% of the benthos, 43% of all coral, ahead of Porites 6.3% (e.g., Image 2, Image 11).

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Coral cover

• Reference: Mayfield & Dempsey (2026)
• CoralNet submission package
• Formal transects recorded 52% coral against 36% algae, with coralline crust at 17% of the benthos and a substantial fleshy turf component at 14% (e.g., Image 9, Image 23).
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• The depth finding is a genus handover at the top of the reef rather than the bottom: cover runs 42%, 54%, 55%, 50% down the profile, but the shallowest stratum is a massive-Porites zone on hard coralline pavement (Porites 18% against Acropora 11%), while every deeper stratum is Acropora-led, peaking at 30% of the benthos.
• 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 36% of the benthos.
• Transect mean 52 ± 2.4% SE (n = 19 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, supplemental S1).
• 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 4.2% (CoralNet analysis of the survey images; Mayfield & Dempsey 2026, suppl. S1), and the frames show partly bleached branching colonies: Image 10 holds a brown branching pocilloporid with several stark-white branch clusters whose polyps are still visible, and a second frame on file shows the same pattern.

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The other colonies mostly hold normal pigmentation, and the conspicuous white areas elsewhere are exposed framework and rubble rather than pale tissue, though some pallor observations are left unresolved (e.g., Image 6, Image 24).

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No crown-of-thorns, disease front, or predation scarring appears in the photographs from the 21 March dive. The expedition's BIOT-1 crown-of-thorns sheet lists 11 starfish (13 to 31 cm) under BITB39, with no date recorded. Anderson's sampling sheet records one of the six colonies sampled on 21 March, a Pocillopora acuta at 24 m, as 40% bleached, "possibly due to disease", and another as healthy though relatively pale. 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). The Living Oceans Foundation transects here recorded no cyanobacteria and no recently dead coral.

Fish and other fauna

Fish are the standout, with fusilier and baitfish schools, two bluefin trevally, a giant clam, and a moray (e.g., Image 5, Image 19, Image 22, Image 31, Image 38).

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Comparisons with other data

Social Ecological Research Frontiers (SERF) and Thriving Oceans surveyed a nearby station (GCB South Brother Out E2) in 2010, 2019, and 2023, 0.22 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, 2.48 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' (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 western tip of South Brother, facing west (about 265°): windward for the northwest monsoon winds of the March survey and leeward for the southeast trades of April to November.