Chagos Archipelago - Great Chagos Bank West (BITB29)

General benthic features

Porites was the most common coral genus on this station's transects (12% of the benthos), followed by Acropora (4.6%), Pocillopora (3.8%), Montipora (3.4%), Favia (0.9%), and Pavona (0.7%); bare substrate covered 11%, behind crustose coralline algae (20%), the largest single living non-coral group (LOF transects; all algae together 53%). Among the algal classes, crustose coralline algae led (20% of the benthos), followed by turf (19%) and erect algae (4.8%). The reef photographs as three distinct grounds, and the dive crosses all of them.

First, a plate-and-dome ground carried by large pale Porites. Image 8, Image 11, Image 22, Image 24, Image 25, Image 33, Image 35, and Image 53 are all built around broad cream-to-fawn laminar and lobate Porites – large overlapping shingled plates stacked down the slope, with small massives, mustard encrusting sheets, and occasional pink corymbose Acropora tucked between them. Image 8 shows the polyp texture clearly on one tan colony; Image 23 frames the underside of one such plate at close range.

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Second, a tabular-Acropora ground, and the Acropora here is present, large, and healthy where it occurs. Image 5 is the signature frame – one large olive-gold tabular plate standing alone over rubble pavement, margin unbroken, a small healed pale scar at the center.

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Image 49 is the best mixed frame: gold tables over tan lobed Porites with three large dark barred groupers behind. Image 18, Image 34, Image 38, Image 41, and Image 51 each carry one or more clean tables; in Image 18 the table's center is bare and algal-crusted while its margin is alive.

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Third – and it takes up a large share of the swim – low-relief consolidated pavement, rubble, and old dead tabular plates already resurfaced in maroon and pink crustose coralline algae. Image 4 states it most plainly: a broad sheet of pink-maroon coralline-crusted rock carrying nothing but a few tiny yellow Acropora recruits. Image 39 is a downward view of dead tables crusted maroon carrying only scattered tan Acropora recruits; Image 32, Image 34, Image 40, Image 43, Image 45, and Image 54 are the same ground at wide angle, and Image 44 puts a whitetip reef shark over a rubble plain with almost no coral in frame.

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This low-cover habitat is real and is not an artifact of the photography – roughly a third of the swim crosses it, and it is part of what this reef is. Set against it are the mosaic frames (e.g., Image 19, Image 30, Image 31): dense fields of small colonies packed shoulder to shoulder – mauve and pink pocilloporid clumps, corymbose Acropora, tan Porites knobs, faviids, and yellow-green foliose and Montipora-type encrusting sheets, all fringed with pink coralline algae and turf.

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Image 9 is the close settlement frame – cream-tan branching recruits established directly on a coralline-crusted ledge beside dark encrusting coral, composed for settlement or size rather than for cover.

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Coral cover

• Reference: Mayfield & Dempsey (2026)
• CoralNet submission package
• 30%, 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 53% of the benthos.
• Transect mean 30 ± 2.6% 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 30% (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.

Stressors

Structural loss here is old rather than acute. The dead plates are already coralline-resurfaced and carrying recruits, and there are no crown-of-thorns, no disease lesions, and no fresh predation scars in any of the 49 frames. Survey apparatus appears once: a thin yellow survey line runs across the lower part of Image 7 – the only apparatus visible in the whole set.

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The one condition question the imagery cannot settle here is bleaching. The cream color of the big Porites plates in Image 11, Image 25, Image 35, and Image 53 is the dive's dominant visual impression, and it is not photographically separable from paling – Chagos Porites is often this color at depth. Image 51 holds one distinctly cream-white massive dome among normally pigmented neighbors. Beyond that the reef reads as pigmented. No bleaching percentage is estimated from these frames and none is tested against them; Anderson's published per-site percentage bleached is the authoritative figure for this reef. The 20 points the photo annotation scored as bleached reflect which frames were taken rather than the reef – 9 of them fall in the eight under-ledge lobster macros, where there is essentially no coral in frame at all. Anderson's field sampling record shows that Seriatopora spp. were present here and three colonies (ChSe6–8) were sampled, plus other pocilloporids for a collaborator. Their sheet rows read Seriatopora hystrix ChSe6 (29.4 m, healthy) and ChSe7 (20.7 m, 5% bleached, likely healthy), with no condition recorded for ChSe8. The collaborator samples were Pocillopora sp. ChJo85 (29.5 m, small, healthy), ChJo86 (10.0 m, 5–10% partial mortality, overall healthy), and ChJo87 (9.9 m, large, 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). Nothing in the 49 frames unambiguously shows it: the fine-branched pocilloporid clumps in Image 3, Image 19, Image 30, and Image 31 are compatible in habit, but at this resolution Seriatopora is not separable from small Pocillopora or Stylophora.

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The field record stands; the photographs neither confirm nor contradict it. The Living Oceans Foundation transects here recorded 0.1% cyanobacteria and 0.1% recently dead coral.

Fish and other fauna

Image 23 is the standout – a giant moray looking out from beneath a broad pale Porites plate, in the highest-cover frame at the site. Image 48 holds a large silver jack in open blue water, compatible with giant or bluefin trevally, not callable at that range; Image 36 a heavy dark grouper with pale saddle bars, compatible with a dark-phase blacksaddled coralgrouper.

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Beyond those: bluestripe snapper in Image 12; powder-blue surgeonfish in Image 7, Image 17, and Image 24; yellowback fusiliers in Image 27, Image 36, and Image 55, with further blue-and-yellow fusiliers in Image 7, Image 11, Image 24, and Image 33; a rudderfish or dark snapper school in Image 43; dense orange anthias in Image 14, Image 41, and Image 55; and a large gorgonian sea fan beside a table in Image 12.

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Sponges register in 29 of the 49 frames, giant clams in 2, urchins in 2, and sea cucumbers in 2.

Eight frames are not a reef survey at all. Image 6, Image 10, Image 20, Image 26, Image 29, Image 42, Image 46, and Image 47 are macros of a spiny lobster den under a plate overhang – at least three and probably four lobsters with boldly black-and-white banded legs and striped carapaces, not callable to species from these frames – under a red-sponge-crusted ledge over coarse rubble.

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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, Sheppard et al. (2008) at Great Chagos Bank, 3.12 km away, read a mean of 41% in 2006.

Other notes

All forty-nine habitat frames were reviewed. Apart from the eight lobster-den macros and the close settlement frame, the 49 frames are wide or medium reef views – there are no sampled-colony close-ups – and a handful (e.g., Image 21, Image 50, Image 54) are hazy or backscattered.

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No quadrat or scale bar appears anywhere, so no colony dimension quoted here is measured. 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 west side of North Brother, facing west (about 280°): windward for the northwest monsoon winds of the March survey and leeward for the southeast trades of April to November.