Chagos Archipelago - Great Chagos Bank West (BITB53)

Setting

BITB53 is a fringing reef off North Brother in the Three Brothers group, windward on Anderson's field sheet, surveyed 26 March 2015 on a dive to 18.2 m.

General benthic features

Porites is the leading genus, carrying over a third of all coral recorded, followed by Acropora, though the balance shifts with depth: the shallow zone is algae-dominated with the two genera nearly tied, while the deepest stratum, a single transect, is more coral-abundant and clearly Porites-led. Underwater photographs show large gorgonian sea fans and massive coral bommies sheltering dense fish schools, alongside open rubble flats with only scattered colonies (e.g., Image 1, Image 2, Image 3, Image 5, Image 6, Image 10, Image 14, Image 16, Image 18, Image 21, Image 24, Image 25).

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Coral cover

• Reference: Mayfield & Dempsey (2026)
• CoralNet submission package
• Formal transects recorded 33% live coral cover against 43% algae.
• Transect mean 33 ± 3.6% SE (n = 13 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 37% (Mayfield & Dempsey 2026, Oceans, suppl. S1).

Stressors

Published bleaching is 3.2%. 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). Image 20 shows a small branching colony with its right half stark white, polyp texture still visible, and its left half brown: partial bleaching.

20

Two of the colonies Anderson sampled here were logged as 10% diseased or bleached (see Dive record below), and no crown-of-thorns is visible in the photographs. Small white bare patches between the branches of a pink-tan colony with extended polyps are recent mortality, cause not stated (e.g., Image 4).

4

The Living Oceans Foundation transects here recorded no 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, 3.76 km away, read 21% in 2015.

Dive record

Anderson's morning dive on 26 March 2015 entered the water at 08:25 and lasted 63 minutes, to a maximum depth of 18.2 m (the field sheet); the notes describe a calm dive on a very gradual slope from 13 to 18 m. The sampling sheet lists one Seriatopora hystrix colony, ChSe45 (about 09:00, logged at 28 m; collected by Andy and photographed only at the surface; appears healthy), and three pocilloporids sampled for a collaborator (Pocillopora sp.): ChJo142 (collected 08:36 at 15.1 m; a large, healthy colony between the candelabrum and P. eydouxi forms, common at the site), ChJo143 (08:41, 15.9 m; 10% diseased or bleached; the sickly part was sampled), and ChJo144 (09:10, 17.7 m; about 500 m from the other two, 10% diseased; the diseased part was sampled). The notes record no Pocillopora damicornis, P. acuta, or Seriatopora hystrix at the depths Anderson dived. 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) shows open water at the pin about 1.1 km north of North Brother, where sun glint covers the water; no bottom or reef edge is visible, so the imagery cannot test the aspect.