Chagos Archipelago - Peros Banhos (BIPB68)

Setting

BIPB68 is a leeward back-reef coral framework on the lagoon side of the Peros Banhos north rim (LOF habitat map), dived to 21.5 m on 16 April 2015, at a station CTD-cast midpoint of 30.9°C and 34.5 salinity; Anderson's notes call it a stunning site, among the best of the BIOT dives, with live coral covering 100% of the reef from 2 to 12 m in the field notes (the page's cover headline comes from the expedition transects).

General benthic features

The reef has two faces. The first is a continuous, gently descending slope of branching and tabulate Acropora: Image 8 through Image 16 and Image 19 through Image 23 look along an unbroken thicket of thin-branched staghorn and corymbose colonies running out of sight into blue, with flat tabulate plates a meter and more across stacked in overlapping tiers through it (Image 17 is the clearest — three large tables shingled one above the next).

8910111213141516192021222317

Between the tables the framework is not bare but crusted pink and mauve in coralline algae. The second face is a mounded, far more mixed assemblage on shallower, better-lit ground: Image 4, Image 26, Image 30, Image 32, and Image 43 show salmon and pink corymbose Acropora, chartreuse-green and mustard corymbose and digitate colonies, lilac and blue-tipped branching colonies, tan and yellow tabulate plates, cream and ochre massive domes with the fine, even porosity of Porites, brown honeycombed and meandering-valley massives, ruffled foliose and bowl-forming plates, and orange-tan encrusting sheets, all packed shoulder to shoulder with almost no open substrate between them.

426303243

Image 2 is given over to one large ruffled foliose colony, its concentric fronds edged in pink coralline crust; Image 7 shows tan plating and vase-shaped colonies with an orange encrusting growth beside them; Image 33 and Image 42 are anchored by big pale cream boulder-massives.

273342

Twenty-seven coral genera were recorded, yet Acropora still carries roughly half of all coral present. Composition swings hard with depth: the shallowest stratum is close to a two-genus reef of Acropora and Porites at 58% cover, while the deepest drops to 35% cover with algae peaking at 47% and hands the reef over to Echinopora and Goniastrea.

Coral cover

• Reference: Mayfield & Dempsey (2026)
• CoralNet submission package
• 59% (the mean of all 14 transects, across four depth strata) against 32% algae, with bare substrate at 3.8%: between coral, algae, and crustose coralline algae, almost the entire reef surface is biologically occupied.
• Transect mean 59 ± 4.5% SE (n = 14 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 55% (Mayfield & Dempsey 2026, Oceans, suppl. S1).

Stressors

The wide slope frames show scattered pale to bone-white patches within the branching stands (e.g., Image 11, Image 13, Image 16), which may be exposed skeleton and rubble or bleached tissue; no crown-of-thorns or obvious disease lesion was seen. The published image-based bleaching figure is 0.87%, but Anderson's sampling sheet records three of the 11 colonies sampled here as 80% or more bleached, and three others as healthy but with pale branch tips or sections (see Dive record below). The Living Oceans Foundation transects here recorded 0.2% cyanobacteria and 0.3% recently dead coral.

Fish and other fauna

Fish are sparse and incidental: small planktivores over the slope, a single blue-and-yellow fish beside the mushroom coral in Image 6.

6

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's dive on 16 April 2015 entered the water at 13:49 and lasted 65 minutes, to a maximum depth of 21.5 m (the field sheet), in calm water. Anderson sampled 10 colonies identified in the field as Pocillopora acuta, found at all depths (CHPd81–CHPd90, collected 13:55–14:50 at 6.5–19.9 m): CHPd81–CHPd87 healthy (three with pale branch tips or sections), CHPd88 80% bleached and likely unhealthy, CHPd89 95% bleached with live tissue and polyps still extended, and CHPd90 (collected by Sam) apparently healthy. Anderson also sampled the only Seriatopora hystrix colony seen at the site, ChSe71 (14:04, 19.8 m; more than 90% bleached, likely unhealthy, in an area of very high coral cover). 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 6 itself is a near-macro portrait of one large solitary free-living fungiid, radial septa in fine detail. Reef type and aspect follow the LOF habitat map: the pin lies 35 m from 'back reef coral framework' on the lagoon side of the north rim, sheltered from the open ocean. Anderson's field sheet (LOF fellowship 2016 workbook) records 'fringing reef', 'leeward'; the expedition's 933-site table classes the station as 'protected' (fringing reef).