Chagos Archipelago - Great Chagos Bank West (BITB30)

Setting

Fringing reef, leeward on Anderson's field sheet, Redemption Rock on the western rim of the Great Chagos Bank, 6.16396°S, 71.53207°E · station CTD 29.6°C · salinity 34.3.

General benthic features

Assemblage composition from transect data — a broad, well-shared assemblage, not a monoculture. Twenty-seven scleractinian genera are non-zero, plus the fire coral Millepora. Acropora leads at 12% of the benthos, but only just: Porites 8.9% is close behind, then Pocillopora 3.6%, Favia 3.1%, the hydrocoral Millepora 2.8%, Montipora 2.3%, and Echinopora 1.6%. The tail is long and genuinely populated: Platygyra 1.0%, Pavona 0.9%, Favites 0.8%, Montastrea 0.8%, Leptastrea 0.7%, Gardineroseris 0.6%, Goniastrea 0.5%, Astreopora 0.5%, Cyphastrea, Stylophora, and Psammocora 0.4% each, Echinophyllia 0.3%, Pachyseris 0.2%, Symphyllia 0.3%, Leptoseris 0.2%, Hydnophora 0.2%, Galaxea 0.2%, Fungia 0.2%, Ctenella 0.1% (the Chagos-endemic monotypic genus), Turbinaria 0.1% and Coscinaraea <0.1%. No single genus holds even a third of the coral. On the substrate side the transects give crustose coralline algae 15%, turf 16%, bare substrate 10%, sediment-laden turf 6.3%, non-coral invertebrates 5.7%, macroalgae 2.1%, erect algae 1.8%, and cyanobacteria 0.1%. The dive opens in blue water: Image 1, Image 7, and Image 32 are given almost entirely to fusilier and sardine schools banked against the drop-off, and Image 9 and Image 13 look along a reef edge dropping into open white sand, with large olive-gold gorgonian sea fans standing off the lip in Image 6, Image 9, and Image 10 and stacked foliose plates below them.

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On the reef itself the camera finds a continuous tabular-Acropora pavement: Image 15 is a single olive plate filling the lower half of the frame with a second behind it; Image 33 stacks three tan tables, the largest well over a meter, beneath a school of dark surgeonfish; Image 18, Image 26, Image 28, and Image 35 repeat it at wider angle, plate on plate to the limit of visibility, with corymbose and staghorn Acropora filling every gap and massive Porites domes anchoring the low ground.

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Between the tables the reef is a fine-grained mosaic. Image 2 and Image 19 are the clearest cases — pavement carrying dozens of colonies a few centimeters across: pocilloporid clumps, Acropora recruits, encrusting Montipora-type sheets, green ascidians, and pink crustose coralline algae, interleaved at a fine scale. Image 3, Image 8, Image 16, Image 20, and Image 28 show the same fabric at wider angle: consolidated pink-and-mauve coralline pavement carrying a dense, close-packed assortment of small colonies — corymbose and digitate Acropora in gold, tan, and vivid yellow-green, brown staghorn, small tan tables, pocilloporid clumps, cerioid and meandroid massives, low brown domes, and encrusting sheets — with very little of any one growth form dominating a frame. Image 8 and Image 37 are dense, packing salmon-pink encrusting plates, gold corymbose Acropora, brown branching thickets, and massive domes into one view, and Image 25 adds a clown triggerfish sitting in the middle of it.

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The photographic genus inventory runs to nine scleractinian groups plus two octocoral forms: Acropora in four forms (tabular plates to ~2 m, corymbose/digitate, arborescent staghorn, and a purple-mauve branching form in Image 28); Porites (massive/domal, columnar-digitate, knobby-lobed); fine compact verrucose pocilloporids; fine dichotomous branching compatible with Seriatopora/Stylophora; meandroid brains (e.g., Image 2, Image 16, Image 28, Image 37); lobed meandroid (e.g., Image 8, Image 35); cerioid honeycomb compatible with Astreopora or a faviid (e.g., Image 21); foliose plates compatible with Echinopora/Turbinaria (e.g., Image 1, Image 12); and salmon-orange encrusting sheets compatible with Montipora (e.g., Image 16, Image 37).

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Plus large gorgonian sea fans in five frames, a red branching octocoral in Image 27, a golden branching colony compatible with Millepora or an alcyonacean in Image 12, and green ascidians in Image 22.

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Both target pocilloporid groups are present. Anderson's sampling sheet lists Pocillopora acuta (CHPd16–20) and Seriatopora hystrix (ChSe9–10) here; CHPd16–18 were confirmed as P. acuta by RFLP (including CHPd18, field-called P. damicornis) and CHPd19–20 were not determined. 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 imagery is consistent: Image 22 is a flash macro of a fine verrucose pocilloporid ringed by green ascidians and red encrusting sponge; Image 31 is a second, smaller, cream-tan pocilloporid on CCA pavement — both compatible with the compact damicornis/acuta habit — and Image 2 carries a fine dichotomously-branched colony compatible with Seriatopora.

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Coral cover

• Reference: Mayfield & Dempsey (2026)
• CoralNet submission package
• 43%, from the Khaled bin Sultan Living Oceans Foundation's BIOT-1 in situ transect survey (the mean of all of this station's transects); the photographs illustrate the site. Algal cover is 41% of the benthos.
• Transect mean 43 ± 3.2% SE (n = 16 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 42% (Mayfield & Dempsey 2026, Oceans, suppl. 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.
• No tape, quadrat, diver, slate, or collecting gear appears anywhere in the 33 frames.

Stressors

The photographic impressions below are hedged: they agree with the survey record but do not independently confirm it.

Structural loss is present but old, and there is no methodology imagery at all — zero frames of 33. The dominant structural loss is long-dead skeleton already resurfaced in pink and mauve CCA: the pavement in Image 11, Image 17, Image 31, and Image 36 is almost entirely CCA-plated old framework.

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Anderson's published per-site figure — 13% of coral bleached (Mayfield & Dempsey 2026, Oceans) — is the authoritative statement of condition at BITB30. Nothing below is converted into a percentage from frames, and the photographic and annotation material is not offered as a check on that figure. Qualitatively, seven frames carry paling or partial mortality and none of it is resolvable photographically: Image 3 (a white sector on a tabular plate), Image 16 (a fully white tabular colony plus a pale branching colony), Image 35 (a white band on a table margin), Image 17 (white tissue at a corymbose colony's base), Image 36 (chalky white branch fragments on CCA pavement), and Image 22, Image 31 (pale-tan pocilloporid macros). Bleaching versus recent partial mortality versus a naturally pale morph cannot be separated from these images, and none is separated here. No crown-of-thorns, no unambiguous disease lesion, and no fresh predation scar resolves in any of the 33 frames.

Separately, the bleached points in this page's photo-comparison set are concentrated rather than distributed: eight of the 33 frames carry all 21 of them, 25 frames carry none, and two frames hold 48% of them. The extreme case is Image 14, three trevally over bare white sand with no live coral in frame, which carries 6 bleached-Acropora points (29% of that set's bleached points); none of the seven frames that actually show pale tissue is among the two frames carrying half of them.

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These are the point labels listed for comparison in the photo-comparison table; the published 13% stands as this site's bleaching figure. The Living Oceans Foundation transects here recorded 0.1% cyanobacteria and no recently dead coral.

Fish and other fauna

The fauna is good and the last three frames are the best of it. Image 4 is a macro of an octopus wedged under a gray-blue sponge overhang with both eyes open and a black urchin on the sponge above it. Image 14 holds three bluefin trevally in clean profile over white sand.

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Beyond those: powder-blue surgeonfish in Image 5 and Image 23; Moorish idols in Image 8 and Image 35; a large gray angelfish with white face mask and yellow caudal in Image 16, not callable to species; parrotfish, red bigeyes, blue-and-yellow damselfish, anthias, and chromis, and continuous fusilier and dark-surgeonfish traffic throughout.

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Two animals are deliberately left uncalled: the cryptic animal in Image 11 is NOT callable (scorpionfish only compatible), and a dark mass in Image 17 may be a second octopus or a dark encrusting sponge. Conspicuous fauna and sessile invertebrates appear as sponges in 22 frames, urchins in 2 frames, and giant clams, sea cucumbers, and anemones in 1 frame each.

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, 1.59 km away, read 21% in 2015.

Dive record

Anderson's afternoon dive on 18 March 2015 entered the water at 13:48 and lasted 62 minutes, to a maximum depth of 26.5 m (the field sheet).

Other notes

Image-by-image analysis — the frames show this reef's condition more strongly than the 43% does. All 33 frames are habitat frames; close-ups of the colonies sampled are on file but are not shown here. Aspect follows Anderson's field sheet (LOF fellowship 2016 workbook: 'fringing reef', 'leeward'); the expedition's 933-site table classes the station as 'protected' (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 east side of a small islet between Middle Brother and South Brother, facing east-northeast (about 70°): side-on (close to the leeward cutoff) for the northwest monsoon winds of the March survey and side-on for the southeast trades of April to November, so the imagery neither confirms nor contradicts the sheet.