Chagos Archipelago - Great Chagos Bank West (BITB28)

Setting

Reef type: channel reef, leeward on Anderson's field sheet.

General benthic features

The transect genus list numbers 29 genera and is Porites-led (LOF BIOT-1 survey, the mean of all of this station's transects): Porites 13%, Acropora 5.4%, Pocillopora 2.2%, Stylophora 1.6%, Montipora 1.3%, Favia 1.1%, Pachyseris and Gardineroseris 0.8% each, Platygyra and Favites 0.7% each, Leptastrea and Echinopora 0.6% each, Montastrea 0.4%, Pavona 0.3%, Symphyllia and Mycedium 0.2% each, Echinophyllia, Hydnophora, Leptoseris, Alveopora, Ctenella, Galaxea, and Psammocora 0.1% each, and Fungia, Cyphastrea, Seriatopora, Acanthastrea, Goniastrea, and Heliopora <0.1% each. The blue coral is Anderson's own determination: the field data are Heliopora, determined by A. Mayfield (expedition coral-physiology lead). The substrate and algal categories run crustose coralline algae 17%, turf 14%, non-coral invertebrates 12%, bare substrate 11%, sediment-laden turf 11%, macroalgae 2.7%, cyanobacteria 0.5%, and erect algae 0.4%. This channel reef is a patchwork: tiered gold tabular Acropora, white sand aprons, bare CCA-crusted pavement, dense mauve-and-gold coral gardens, and more sand. Many of the wide frames — Image 16, Image 17, Image 18, Image 19, Image 21, Image 28, Image 33, and Image 34 — show pale carbonate rock and rubble with turf film, sand pockets, and colonies distributed across the ground rather than covering it.

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Image 22 is the best image of the good end — tiered stacks of golden tabular Acropora running into the blue under fusiliers and a school of yellow snapper, with Image 1 and Image 44 repeating it with a diver silhouetted over the plates.

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Image 18 is the other end: a bare pale carbonate plain carrying perhaps five small tables in the whole field. Some of the ground carries high cover. Image 5 is a clear, high-cover benthic frame (43% by points) — gray-lilac digitate Acropora, brown Pocillopora-type knobs, sheets compatible with Montipora, a meandroid brain and a bivalve compatible with a giant clam, all under a broad tabular plate, with the transect tape running down the right edge. Image 26 is its equal and the site maximum by points (47%): pale plate-form Porites left, ridged foliose colonies center, branching Acropora and Porites domes right.

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Image 13 and Image 39, both unscored, are as good, and Image 3, Image 41, and Image 46 belong with them. Image 24 is the most atmospheric image — a gold plate and a pale Porites dome framing an overhang packed with orange anthias, a giant clam wedged beneath.

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Image 27 holds a single very large table standing clear of the pavement over a sand-floored undercut, with branching colonies and damselfish sheltering beneath it.

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Massive Porites is the structural backbone, and both the transect and the photo annotation show it: domes anchor (e.g., Image 25, Image 35, Image 36, Image 39, Image 46), and pale encrusting plates fill the foreground of Image 7.

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Porites is just as dominant in the unscored imagery, and the transect puts it at 13% against Acropora's 5.4%. A taxonomic note carries over from Anderson's field sampling record, which records that neither Seriatopora spp. nor P. acuta/P. damicornis were present at this channel reef, and that other pocilloporids were collected here for a collaborator. The sheet rows for those collaborator samples read Pocillopora sp. ChJo82 (21.1 m, healthy, some old mortality), ChJo83 (16.0 m, pale and sickly, 40% bleached or dead), and ChJo84 (10.5 m, healthy). 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: pocilloporid material in Image 5, Image 13, and Image 19 is blunt and knobby, compatible with a P. verrucosa-type habit, nothing reads as Seriatopora, and the photo annotation puts Pocillopora at only 2.3% of live coral. A hedged photographic impression, not a survey result. Image 47 is a cave mouth filled with golden sweepers or cardinalfish, and orange anthias swarm the overhang in Image 24 as well as Image 33 and Image 46.

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Fusiliers work the water column in ten frames (e.g., Image 1, Image 2, Image 7, Image 8, Image 12, Image 15, Image 16, Image 22, Image 29, Image 44). Image 14 holds olive-yellow gorgonian sea fans over sand — octocorals are structurally present here and invisible to a hard coral point scheme.

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Coral cover

• Reference: Mayfield & Dempsey (2026)
• CoralNet submission package
• 31%, 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 46% of the benthos.
• Transect mean 31 ± 5.0% 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 28% (Mayfield & Dempsey 2026, Oceans, suppl. S1).
• Anderson's published percentage bleached for this site is 11% (Mayfield & Dempsey 2026, Oceans).
• 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

Mortality is present, old, and concentrated in the tables. Image 8 is the clearest frame — two broad Acropora tables reduced to bare pink-white CCA-plated slabs with no live tissue, living tables and Porites domes alongside; 23% of its points are CCA. Image 16 shows the same thing: a broad, flat slab filmed with turf and coralline algae, with the outline of the plate intact and no live tissue remaining. Anderson's published per-site percentage bleached, 11% (Mayfield & Dempsey 2026), is this site's authoritative bleaching figure, and no bleached fraction is derived from the photographs. What the frames show is limited and is reported as such: the stark-white lumpy mass in Image 13 is a white sponge (a smooth body with large dark openings), not whitened coral; Image 7 is a wide expanse of cream-gray plate-form colonies with intact tissue, paler than the tan plates of the same form beside them, so they are read as paling (partial bleaching), and Image 23 carries a similar broad pale-cream tabular plate above the ledge.

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The image products' bleaching outputs are listed for comparison only: 11 of the 17 bleached points in this page's photo-comparison set fall on Image 31, essentially bare white sand with no coral in the foreground, and Image 12 and Image 47 carry a few more on similar ground.

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No disease lesion or predation scar resolves in any of the 43 frames, and no crown-of-thorns is identifiable. The Living Oceans Foundation transects here recorded 0.5% cyanobacteria and no recently dead coral.

Fish and other fauna

Sponges appear in 17 frames, sea cucumbers in 4, giant clams in 1, urchins in 1, and anemones in 1. Image 43 carries the site's highest dead reading, 11%, over pale turf- and CCA-covered pavement with a pair of yellow-bodied fish above it (possibly rabbitfish or a surgeonfish).

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The fauna is notable. Image 23 and Image 42 hold a nurse shark resting on sand beneath a ledge — blunt-headed and brown-gray, with two near-equal dorsal fins set far back and no white fin tips, close and sharply lit in Image 42; Anderson's sampling sheet records a nurse shark on this dive.

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Image 4 is a standout fauna frame: an adult emperor angelfish, two powder-blue surgeonfish, a palette surgeonfish, an anemonefish, and a parrotfish in one field over well-covered ground.

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Image 10 and Image 20 are close portraits of an octopus in its den — a pale mantle and webbed arm curtain filling the ledge mouth, one eye with its horizontal slit pupil clearly resolved in Image 10, a dark maroon central mass behind, framed by red encrusting sponge and pink coralline crusts.

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Enlarged, Image 10 shows no eel head, jaw, or tubular nostril, so these are not moray portraits. Image 31 is a sand plain carrying a single mottled bottom-resting fish that the frame does not resolve to family.

Comparisons with other data

Sannassy Pilly et al. (2024) surveyed a nearby station (North Brother), 0.24 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, Sheppard et al. (2008) at Great Chagos Bank, 2.40 km away, read a mean of 41% in 2006.

Dive record

Survey conditions on 18 Mar 2015 at 6.1384°S, 71.5091°E: station CTD temperature 29.4°C, salinity 34.4.

Other notes

The transects were run in four depth strata (D1 8.0%, D2 28%, D3 37%, and D4 41% coral). Image-by-image: a mosaic site. All 43 habitat frames were reviewed (the morning dive of 18 March). There are no sampled-colony frames and no scale object anywhere in the set. Aspect follows Anderson's field sheet (LOF fellowship 2016 workbook: 'channel reef', 'leeward'); 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 off the east side of North Brother, facing east (about 95°): leeward for the northwest monsoon winds of the March survey and windward for the southeast trades of April to November.