### BIPB73 — Peros Banhos
**CRD page:** [Peros Banhos Coral Surveys (18–19 Apr 2015)](https://coralreefdiagnostics.com/peros-banhos-20150418-to-0419)

**18 Apr 2015**
**GPS:** 5.26732°S, 71.88862°E
**Setting:** exposed emergent fringing reef, 30 m
**Coral cover:** 50.08% (3 formal transects, D2–D4; **algal cover 23.08%**) · **published image bleaching:** 6.37%
**Live hard coral cover across methods:** formal transects 50.1% · CoralNet points 36.1% · Coralscapes masks 25.5% (45 frames). These products have different sampling footprints and denominators and should not be averaged.
**Temperature:** 30.2 °C · **Salinity:** 34.2
**Photo review:** all 45 frames examined
**Representative photos:** P4180813.jpg, P4180829.jpg, P4180836.jpg, P4180762.jpg, P4180835.jpg

BIPB73 is a coral-abundant forereef, **50.08% cover ranking 25th of 109 Chagos sites** and above both the Peros Banhos regional mean (46.55% across 20 sites) and the dataset mean (37.92%). Against that, algal cover is a modest 23.08% and non-coral invertebrates hold an unusually large 16.03% of the benthos (`PITS`) — by far the largest non-coral, non-algal component of the substrate here, well above what several other Chagos sites carry, and visibly explained in the photographs by frequent giant clams, sponges and a resident anemone with its fish. Bare substrate is 10.8%, turf 9.05%, crustose coralline algae 5.33%, macroalgae 3.5%, erect algae 3.37%.

Seventeen scleractinian genera were recorded, and the assemblage is carried by two of them: ***Acropora* at 20.98% of the benthos (42% of live coral) and *Porites* at 15.72% (31%)** — together nearly three-quarters of all coral on the reef. *Montipora* is a distant third at 3.12%, then *Pocillopora* 2.57%, *Echinopora* 1.50%, *Astreopora* 1.47%, *Pavona* 0.98%, ***Ctenella* 0.67%** (the Chagos-endemic monotypic genus, not to be confused with *Cynarina*), *Fungia* 0.67%, *Psammocora* 0.65%, *Goniastrea* 0.50%, *Leptastrea* 0.48%, *Stylophora* 0.27%, *Favia* 0.22%, *Favites* 0.17%, *Lobophyllia* 0.07% and *Montastrea* 0.07%.

**The depth structure here is unusually clean and worth stating in full, because the site-level averages describe no single transect.** D2, the shallowest stratum, carries by far the highest cover of the three at **68.25% coral against only 22.75% algae**, and it is essentially a two-genus reef: *Acropora* 34.75% (51% of the coral present) and *Porites* 18.25% (27%), with *Pocillopora*, *Astreopora*, *Montipora* and *Ctenella* filling out the rest at 2–4% each. D3 drops sharply to **37% coral, the site's low point**, and the genus dominance inverts — ***Porites* leads at 16.40% (44% of the coral) over *Acropora* at 12.20% (33%)** — with *Montipora*, *Astreopora*, *Pocillopora* and *Psammocora* each in the low single digits. D4 partially recovers to **45% coral** and *Acropora* regains the lead at 16.00% (36%), but the composition is not simply D2 repeated: *Porites* holds 12.50% (28%), while ***Echinopora* and *Montipora* each reach 4.50% (10% of coral apiece) and *Fungia* appears at 2.00%** — none of which feature meaningfully in the shallower strata. **The published site mean of 50.08% coral sits between D3 and D4 and matches neither transect**, and the 20.98%/15.72% *Acropora*/*Porites* split is an average of a shallow reef that is essentially two genera and a deeper reef where a third and fourth genus (*Echinopora*, *Fungia*) become relevant.

The photographs bear this structure out extensively. Massive, tiered plate-like colonies compatible with *Porites* dominate several frames (P4180760, P4180761), often with anthiine fish swarming above them. Dense, high-cover reef sections combine tabular and corymbose *Acropora* with large massive/faviid domes and pink CCA-encrusted framework (P4180813, P4180818, P4180823, P4180830, P4180832, P4180834), consistent with the mixed *Acropora*-*Porites*-plus-others assemblage the transects describe. A large table coral shading a massive dome below it (P4180835) is a textbook forereef terracing structure. Free-living and encrusting forms — small fungiids, foliose plates compatible with *Montipora*/*Pavona* — recur throughout (P4180777, P4180809). One frame (P4180812) shows a sparse hard-pavement patch with only scattered small coral recruits, the photographic counterpart of the site's 10.8% bare substrate.

**Condition at BIPB73 illustrates why a single site-level bleaching figure can understate what is visible in the water.** The published image-based bleaching rate is 6.37% (43rd of 109 Chagos sites with a value, i.e. mid-to-low range), and a separate 30-point human-annotated tally for this site gives an even lower 2.44% bleached (by taxon: *Acropora* 1.85%, other hard coral 0.44%, *Pocillopora* 0.15%) — both figures describe a reef that is mostly unaffected. Yet the photo set documents several colony-scale bleaching events. P4180829 and P4180831 (consecutive frames of the same colony) show a large table coral that is almost entirely stark white, with only a narrow band of pigmented brown tissue at one edge — an unambiguous, tissue-intact bleaching event. P4180836 is a close frame of a second table coral split roughly in half between white and tan-pigmented tissue on the same colony. Smaller partial-bleaching sectors appear on a massive colony in P4180792 (a bright white patch bridging two otherwise pigmented plates) and P4180825 (a stark white wedge cutting across an otherwise olive-yellow massive coral). None of this contradicts the low published rate — a handful of severely affected individual colonies within a 45-frame set is entirely compatible with a reef-wide incidence near 2–6% — but it does mean the published number should not be read as "no visible bleaching here."

Reef fauna is well represented: a resident sea anemone hosts both anemonefish and a cluster of three-spot dascyllus (P4180762, P4180763); a honeycomb-patterned grouper rests repeatedly under coral overhangs across several frames (P4180769, P4180770, P4180805, P4180806); a reef shark is visible cruising in the background of at least three wide shots (P4180779, P4180819, P4180821); a turtle appears cruising past table *Acropora* in two consecutive frames (P4180810, P4180811); giant clams (*Tridacna*) are embedded in the reef framework in multiple images (P4180813, P4180830, P4180832); and a large gastropod shell, consistent with a conch, appears in close-up (P4180799). Powder blue surgeonfish and schooling fusiliers/chromis recur throughout. No COTS, disease lesions or predation scars were observed in any frame.

<b>Assemblage composition.</b> Formal transects give 50.08% coral against 23.08% algae and an unusually high 16.03% non-coral invertebrates, with *Acropora* and *Porites* together holding nearly three-quarters of all coral across 17 genera. That two-genus dominance is a shallow-water pattern (D2, 68.25% coral); by D3 the ranking inverts to *Porites*-led, and by D4 *Echinopora* and *Fungia* join the assemblage for the first time. The photographs document massive *Porites*-type plate stacks, dense mixed *Acropora*/faviid reef sections, and — set against a low published bleaching rate — several individually striking colony-scale bleaching events concentrated in tabular *Acropora*.

**Flags:** (1) **Photographic bleaching is locally severe despite a low published site rate.** Published `Mean(%bleaching)` = 6.37% (recomputed from point data as 6.35%, a confirmed match) and a separate 30-point human-annotated tally gives 2.44%; both describe the reef as mostly unaffected. However P4180829/P4180831 show a table coral that is almost entirely bleached white, and P4180836 shows a second table coral roughly half white/half pigmented — colony-scale, tissue-intact pallor, not algae-covered mortality. Smaller partial-white sectors appear in P4180792 and P4180825. Treat the published rate as correct for the reef as a whole; the photographs show that a small number of individual colonies carried much more severe bleaching than the site average implies. (2) **Exposure field conflict.** The site record carries `exposure-original` = "leeward" and `exposure-revised` = "exposed" for the same site; this shard follows the revised value per convention, but the discrepancy is unresolved in the source data. (3) **Non-coral invertebrate cover (16.03% of the benthos) is unusually high** for a Chagos forereef and is the largest non-algal, non-coral substrate category recorded here; the photographs corroborate this with giant clams, sponges and an anemone-anemonefish association, but no corpus-wide rank is available to say how extreme this value is relative to other Chagos sites. (4) **Wide multi-method spread.** Formal transects (50.1%), CoralNet points (36.1%) and Coralscapes masks (25.5%) span a −24.6 to −14.0 percentage-point deviation range on the same 45 frames used for both photo methods (`n_photos` matches the folder count exactly, so there is no frame-count mismatch here) — these are different sampling footprints and denominators and must not be averaged or treated as validating one another. (5) **`CNEL` = *Ctenella*, the Chagos-endemic monotypic genus**, present in this dataset at 0.67% of the benthos and 2.00% of D2 coral cover — do not read it as *Cynarina* per the known stale-lookup-table error. (6) No COTS, disease lesions or predation scars observed in any of the 45 frames reviewed.

**Benthic data:** [Download BIPB73 benthic survey data (CSV)](../benthic_data/BIPB73_benthic.csv) — scleractinian cover by genus with full names, plus substrate and algal categories, from the LOF BIOT-1 survey (Mayfield & Dempsey 2026, *Oceans*, supplemental file S1).