Chagos Archipelago - Great Chagos Bank West (BIEA22)

Setting

Submerged bank reef, leeward (from its position on the west side of the Eagle Islands reef platform relative to the prevailing southeast trade winds; Anderson's field sheet: windward), Eagle Island, 6.27659°S, 71.27627°E · station CTD 29.1°C · salinity 34.5.

General benthic features

Assemblage composition from transect data — this is a Porites reef with a long thin tail. Twenty-three scleractinian genera are non-zero, and one of them carries almost half the coral: Porites 13% of the benthos, far ahead of Pocillopora 3.9%, Montipora 2.8%, and Stylophora 2.4%. Acropora is only 0.9% — a caution against reading the tabular and laminar plates visible in the wide frames as an Acropora-led assemblage. The remainder runs Favia and Astreopora 0.7% each, Pachyseris, Goniastrea, and Pavona 0.5% each, Galaxea and Favites 0.3% each, Fungia, Cantharellus, Acanthastrea, Goniopora, Hydnophora, Symphyllia, and Leptastrea 0.1% each, and Cyphastrea, Montastrea, Caulastraea, and Seriatopora <0.1% each. On the substrate side the transects give non-coral invertebrates 19% — consistent with the sponge, ascidian, and hydroid cover the frames show — bare substrate 16%, crustose coralline algae 10%, erect algae 10%, sediment-laden turf 7.9%, turf 7.2%, and macroalgae 2.5%. A modest, open, low-relief bank reef held up by Porites, with conspicuous non-coral invertebrate cover between the colonies. A large day octopus occupies 16 of the 23 frames; they are Image 2, Image 3, Image 4, Image 5, Image 8, Image 12, Image 13, Image 14, Image 15, Image 16, Image 18, Image 19, Image 20, Image 23, Image 25, and Image 27, and the camera stays on it for eleven straight minutes, moving from arm's length to near-macro.

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The animal cycles through its repertoire: flat near-black over pavement in Image 12 and Image 19; emerging from a shaded crevice under a red-sponge-encrusted overhang, with a pale pink leathery soft coral and a feather star beside it, in Image 2, Image 5, and Image 8; deep maroon with the pale funnel showing in Image 2, Image 3, Image 13, and Image 14; rust-red with a raised ocular papilla in Image 5, Image 18, and Image 20; crossing cream and tan digitate and nodular colonies through the middle of the sequence; wedged into a fissure in a huge salmon-pink Porites head in Image 23; and fully papillate, settled on sponge-crusted rock, in Image 4 and Image 15. Image 3 is the best portrait; Image 15 the best camouflage frame, the octopus almost invisible against a Porites head that fills the whole picture.

The reef itself is best read from the four wide frames, and it is modest. Image 9 and Image 22 look down over a low-relief pavement-and-rubble plain with coral as scattered small colonies — laminar and tabular plates compatible with Acropora, small corymbose colonies, pale massive Porites domes, mauve encrusting sheets, pink crustose coralline algae, turf, and fine green algae between them. Image 10 is the same plain at distance in poor light, coral visibly thin. Image 1 is the highest-relief and highest-cover frame in the set and the only one with real structure: cream Porites plates stepping down a slope, a large tan massive Porites to the right, brown ridged plating coral in the foreground and white sand channels cut through between them.

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Four frames are not a site, and this part of the account should be read as four windows onto a modest, low-relief reef rather than as a survey of it. The photographic genus inventory runs to seven scleractinian groups — Porites in four growth forms (columnar-nodular heads, massive/domal boulders, encrusting-plating sheets, and pale cream digitate colonies), tabular and laminar plates compatible with Acropora, corymbose Acropora-type colonies, bushy verrucose colonies compatible with Pocillopora, a vermiform meandroid brain, smooth brown encrusting plates compatible with Montipora, and ridged foliose plates — all given as “compatible with”. Non-scleractinian cover is conspicuous and matches the transect's 19% invertebrate figure: red, orange and pale-pink encrusting sponges, colonial ascidians, feather-like hydroids, and calcified green algae. Non-coral cover is dominated by turf (12% of all points), with crustose coralline algae 4.3% and macroalgae 4.2%.

Coral cover

• Reference: Mayfield & Dempsey (2026)
• CoralNet submission package
• 27%, 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 38% of the benthos.
• Transect mean 27 ± 3.3% SE (n = 12 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 22% (Mayfield & Dempsey 2026, Oceans, suppl. S1). One of these transects is labeled BIEA20 in the per-transect file, but its station code, position, and survey date, and Anderson's published supplementary data (Mayfield & Dempsey 2026), all place it at this station.
• Nothing photographic or model-derived below qualifies, softens, or disputes this value.
• Computer-vision method comparison — a large method divergence, and the frames explain it. Three of the four image methods in the method table read well above the transect here (CoralNet, DINOv3, and CoralSCOP, about 1.7 to 1.8 times); Claude reads close to it (about 1.1 times). This is a property of the photo set, not of the reef. Only 4 of the 23 frames are reef-scale wide views; the other 19 are close-focus frames built around a single octopus, eleven of them filled edge to edge by one coral head. Blue-water dilution cannot apply here: not one BIEA22 frame reaches 40% water — the maximum is 33% and the site mean 5.7%, so there are no open-blue frames pulling the photo mean down. n is effectively 4, not 23, and any site-level cover statistic from this photo set should be treated as non-random. That is the reason to trust the transect over the photo mean.
• Within the image products, the community apportionment is informative in its own right and is worth setting beside the transect. The 30-point annotation of the same frames (690 points) apportions live coral Porites 46%, other hard coral 41%, Acropora 11%, Pocillopora 1.4%.
• The photo annotation's Porites-dominated apportionment matches the transect's community structure closely even where its absolute cover does not — the two methods disagree about how much coral there is, not about what it is. Two known limitations of the annotation scheme show up here: colonial ascidians appear in six frames and are scored variously as CCA, “other” or Unknown, because the scheme has no tunicate class, and Unknown runs at 24% of all points site-wide, peaking at 57% on Image 8.
• Anderson's published per-site figure — 6.9% of coral bleached (CoralNet analysis of informal survey images; Mayfield & Dempsey 2026, Oceans) — is the authoritative statement of condition at BIEA22.
• Nothing here is converted into a percentage from frames, and the photographic account neither stands in for that figure nor argues with it.
• Frame-to-frame live cover runs 6.7% to 90% by point count.
• These products have different sampling footprints and denominators and should not be averaged.

Stressors

Qualitatively, no frame in this set shows a chalky, tissue-free colony, and no crown-of-thorns, disease lesion, or predation scar resolves anywhere; Anderson's sampling sheet does record one of the four Pocillopora colonies sampled here as 10% bleached (the other three healthy). What the close frames do show is pale-but-pigmented coral: the cream-tan digitate and nodular Porites that dominates the octopus sequence is naturally light-colored, and the two extreme macros (e.g., Image 21, Image 26) resolve the pale look as white growth tips and fully expanded white polyps over healthy tan coenosarc, not bleached skeleton.

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That distinction matters and is easy to get wrong at lower magnification.

As a matter of annotation quality rather than of reef condition, the image methods are all working on the same unrepresentative frame set. All 24 of the points the photo annotation scored as bleached sit on eight close-focus frames; not one falls on the four wide reef frames. Image 21 (4 points) and Image 26 (5) score expanded white polyps and white growth tips on a healthy tan branching colony; Image 19 (6), Image 25 (3), Image 27 (2), and Image 16 (2) score naturally pale cream Porites. The Living Oceans Foundation transects here recorded no cyanobacteria and under 0.1% recently dead coral.

Fish and other fauna

The macrofauna is thin but holds the site's other headline. Image 10 shows at least two, probably three, requiem sharks over the plain in mid-water; outline and caudal shape fit gray reef shark, but the frame is dim and distant, so the species cannot be confirmed from this image. The same frame carries a giant clam at lower right, mantle open. Image 7 is the best fish frame: a small bright red serranid, densely white-spotted with pale-yellow blotches and an orange-ringed eye — compatible with a coral hind — among bushy Pocillopora-type colonies.

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Red soldierfish- or squirrelfish-type fish sit in the shadows of Image 15, and yellow butterflyfish are just resolvable in the background of Image 1. Conspicuous fauna and sessile invertebrates appear as sponges in 15 frames, anemones in 9 frames, giant clams in 2 frames.

Comparisons with other data

Apart from the Living Oceans Foundation survey shown here, we found no other surveys at this site as of 2026. If you would like to share yours, please email us.

Dive record

Anderson sampled four pocilloporid colonies on this dive for a collaborator (species are field calls; the sampling sheet): Pocillopora sp. ChJo61 (collected 08:32 at 24.7 m; possibly P. elegans, very common at the site, healthy with a green hue), ChJo62 (08:39, 20.4 m; a large, healthy colony, likely P. elegans), ChJo63 (08:46, 15.7 m; 10% bleached, bleached part sampled), and ChJo64 (08:55, 14.5 m; healthy, with some fish bites). The dive notes record a surface current, a soft coral-dominated reef near Cow Island, a very gradual slope, and no P. damicornis, P. acuta, or Seriatopora seen on the dive (the transects recorded a trace of Seriatopora, under 0.1%). 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

Image-by-image analysis — BIEA22 is the octopus site, and that fact governs everything the image products say about it. There are 23 frames and no sampled-colony frames. Image 1 also carries a thin dark line running vertically through the frame from the top edge onto the reef, with a fainter second at upper left: a shot (descent) line from the surface, not debris. No quadrat, scale bar, or depth stamp appears anywhere in the set. Anderson's field sheet records the aspect (LOF fellowship 2016 workbook: 'submerged bank reef', 'windward'); the expedition's 933-site table classes the station as 'intermediate' (bank reef). The aspect shown, 'leeward', comes from the station's GPS position relative to the prevailing wind: the pin lies on the west side of the Eagle Islands reef platform, about 152° off the prevailing southeast trades. Esri World Imagery (no LOF habitat map covers the Great Chagos Bank) puts the pin on the open-ocean (west-northwest) side of the Eagle Islands platform, south-southwest of Sea Cow Island, facing west-northwest (about 300°): windward for the northwest monsoon winds of the March survey (December to March), though it would be leeward for the southeast trades of April to November.