Chagos Archipelago - Peros Banhos (BIPB60)

Setting

BIPB60 is a middling-cover, high-diversity fringing forereef on the outer rim of Peros Banhos: 42% coral cover ranks 39th of the 98 Chagos sites with a cover figure, but the assemblage carries 18 non-zero coral genera, against seven at a site like BITB58.

General benthic features

It is emphatically a Porites reef. Porites alone holds 22.83% of the benthos — 54% of all coral recorded — with Acropora a distant second at 8.39%, then Montipora 3.14% and Pocillopora 3.12%, and a fourteen-genus tail below 0.7%: Millepora, Heliopora, Stylophora, Pavona, Leptastrea, Goniastrea, Montastrea, Astreopora, Goniopora, Favia, Favites, Galaxea, Cyphastrea, and Gardineroseris. Crustose coralline algae hold 21.21% of the benthos, turf 11.61%, bare substrate 9.23%, non-coral invertebrates 9.02%, erect algae 3.62%, and macroalgae 1.32% (e.g., Image 26, Image 44).

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The depth structure is a clean genus handover. The compositional shift across that peak is the real story: above 8 m, Acropora leads at 19.0% of the benthos with Porites second at 12.8%. From 8 m down the order inverts and never recovers — Porites takes 33.0% at 8–12 m against Acropora's 6.5%, then 25.6% versus 5.2% at 12–18 m, and 19.9% versus 4.8% at 18–25 m. The site-level figure of 22.8% Porites is therefore not a uniform value but the average of a reef that is Acropora-fronted in the shallows and Porites-built below them. Algal cover runs inversely, highest in the shallows (47.5%) and lowest at depth (28.7%). Those six photographs are accounted for as follows: two are wide silhouette shots taken into the surface light, showing a school of large unicornfish or surgeonfish over a reef whose bottom is entirely in shadow — legible only as profile, and revealing gorgonian sea fans and thin whip or wire corals standing off the substrate, neither of which appears in the transect genus list (e.g., Image 1, Image 25).

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Assemblage composition. Formal surveys give 42.2% coral, 39.5% algae, and 9.0% non-coral invertebrates, the coral dominated by Porites below 8 m and Acropora above it. The photographs contribute three facts and no cover information: gorgonians and whip corals are present, a substantial surgeonfish aggregation was working the site, and at least one phyllidiid nudibranch was on the pavement (e.g., Image 43, Image 45).

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Coral cover

Coral cover peaks in the 8–12 m band at 49.3%, then declines steadily — 39.1% at 12–18 m, 36.3% at 18–25 m — while the shallowest band holds 44.3%.

Stressors

Condition is the weakest element. The published image-based bleaching figure is 11.8%, which ranks 31st of 109 Chagos sites — an order of magnitude above sites like BITB58 (0.8%) and well above the archipelago's low-bleaching majority (e.g., Image 27, Image 36, Image 40).

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Combined with 39.5% algal cover and only 42.2% coral, this reads as a reef under more pressure than its Peros Banhos neighbors, though nothing in those six photographs bears on that either way. No crown-of-thorns starfish (COTS) or other corallivores evident.

Fish and other fauna

One elongate fish in one of the silhouette shots may be a small shark or a barracuda; the silhouette does not permit a call. The remaining four of them are a macro sequence of a single phyllidiid nudibranch — black ground, pale blue-white ridges, yellow-orange tipped tubercles, photographed from four angles on coralline-crusted pavement (e.g., Image 2, Image 3, Image 4, Image 8).

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Genus is Phyllidia-type; species is not resolvable from these images alone.

Comparisons with other data

No independent survey was made at this exact site; the nearest ones are listed in the table below.

Other notes

The six photographs this account was written from cannot describe this reef.