### BIPB75 — Peros Banhos
**CRD page:** [Peros Banhos Coral Surveys (18–19 Apr 2015)](https://coralreefdiagnostics.com/peros-banhos-20150418-to-0419)

**19 Apr 2015**
**GPS:** 5.30500°S, 71.97903°E
**Setting:** exposed (windward) emergent fringing forereef, outside the lagoon, 31.0 m
**Coral cover:** 43.9% (3 depth transects, D2–D4; **algal cover 32.8%**) · **published image bleaching:** 17.4%
**Live hard coral cover across methods:** formal transects 43.9% · CoralNet points 29.6% · Coralscapes masks 31.4% (9 frames). These products have different sampling footprints and denominators and should not be averaged.
**Temperature:** 30.0 °C · **Salinity:** 34.4
**Photo review:** all 9 of 9 frames examined
**Representative photos:** P4190855.jpg, P4190854.jpg, P4190868.jpg, P4190837.jpg, P4190866.jpg

BIPB75 is an emergent fringing forereef on the windward, exposed outside of Peros Banhos, surveyed at 31.0 m — the deepest of this batch and the warmest, at 30.0 °C. **Coral cover is above average at 43.9%, 38th of 109 Chagos sites, against relatively low algal cover of 32.8%, which ranks 81st of 109 — i.e. in the bottom third of the archipelago for algae.** The composition of the non-coral benthos is where this site departs from its neighbours: **non-coral invertebrates (PITS) account for 17.4% of the total benthos, an unusually large share**, with fleshy turf next at 13.2% (28th of 109, high), crustose coralline algae at only 6.8% (97th of 109, near the bottom of the archipelago), sediment-laden turf 5.4%, macroalgae 4.9%, erect algae 2.5% and bare substrate 5.9%. The five algal categories sum exactly to the published 32.75%. A windward forereef with this little coralline pavement and this much sessile invertebrate cover is unusual, and the photographs explain it directly (below).

**The assemblage is dominated by *Porites* to a degree rarely seen on an exposed Chagos forereef.** Twenty nonzero genera are recorded — 77th of 109 for genus richness, in the lower third — and the twenty genus percentages sum to 43.9167% of the benthos, an exact match to the published cover figure. All values below are percent of total benthos, not percent of coral. *Porites* leads at 22.42% of the benthos, **51% of all the coral at the site**, followed at a great distance by *Montipora* 3.75% (9% of coral), *Pocillopora* 3.25% (7%) and — remarkably for a windward forereef — ***Acropora* at only 3.00% (7%)**. Then *Favia* 2.00%, *Stylophora* 1.50%, *Pavona* 1.33%, *Leptastrea* 1.17%, *Psammocora* 1.17%, *Millepora* 1.08% and *Platygyra* 0.92%, with a tail of *Goniastrea* and *Montastrea* (0.50% each), *Physogyra* 0.33%, *Astreopora* and *Heliopora* (0.25% each), *Favites* and *Symphyllia* (0.17% each), and *Isopora* and *Turbinaria* (0.08% each). At most Chagos forereefs *Acropora* is the leading genus; here it holds barely an eighth of what *Porites* does, and the site's structure is carried by massive, mound-forming colonies rather than tables and thickets.

**The depth profile is a straightforward, steep decline: cover almost halves from the shallowest station to the deepest, and it is *Porites* that falls away.** D2 carries 63.25% coral against only 24.25% algae — a high-cover, low-algae shallow station — and ***Porites* alone reaches 40.75% of the benthos there, 64% of all the coral on that transect**, with *Acropora* 6.00%, *Pocillopora* 5.00%, *Favia* 2.75%, *Pavona* 2.50% and *Montipora* 2.25% following. At D3 cover drops to 35.5% and algae rises to 32.5%; *Porites* falls to 14.00% (still 39% of that transect's coral) ahead of *Stylophora* 4.50%, *Psammocora* 3.50%, *Millepora* 3.25%, *Favia* 2.25% and *Acropora* 1.50%. At D4 cover falls again to 33.0% while algae climbs to its profile maximum of 41.5%; *Porites* 12.50% (38% of coral) still leads, but ***Montipora* rises to 8.00% — 24% of that transect's coral and by far its peak in the profile** — with *Pocillopora* 3.50%, *Leptastrea* 2.50%, *Acropora* 1.50% and *Favia* 1.00% behind. **The site-level figure of 22.42% *Porites* describes no single transect**: it averages a shallow station at nearly double that value against two deeper stations at roughly half of it. Note also that coral and algae together leave 12.5 pp of the benthos unaccounted for at D2 but 32.0 pp at D3 and 25.5 pp at D4 — consistent with the high site-level non-coral-invertebrate and bare-substrate readings, which are not itemised by depth in this dataset.

The photographs — all nine frames in the folder — cover two distinct habitats and are dominated by condition rather than composition. **P4190837 is the odd frame out and the one that explains the high non-coral-invertebrate reading:** it looks down a pale sand slope at the edge of a drop-off into blue water, with large fan gorgonians, dark bushy colonies consistent with antipatharian (black coral) growth, wire/whip corals and scattered small sponges rooted along the rock/sand interface — essentially no scleractinian cover in frame. The remaining eight frames document the reef proper: high-cover, structurally complex pavement carrying large brain-patterned colonies consistent with *Platygyra* (P4190852, and a full-frame close-up in P4190854), mustard and olive massive and lobate mounds consistent with *Porites* (P4190867, P4190869), abundant knobby and digitate branching colonies consistent with *Pocillopora* and small *Acropora* (P4190853, P4190866, P4190868), encrusting and plating colonies, purple-pink coralline crusts, and open sand-and-rubble pockets between coral stands. Two large flat plate colonies that appear dead and sediment-veneered are visible in P4190853 and P4190867. Fish are present but not abundant in frame — damselfish, a lone sailfin/rabbitfish and small planktivores over the reef in P4190855 and P4190867 — and the set is not standardised for fish abundance.

**Condition is the headline finding, and the imagery is unusually direct about it.** The published image-based bleaching figure is **17.4%, ranking 10th of 98 Chagos sites carrying a value — inside the top decile of bleaching severity for the archipelago** — and the photographs corroborate that at colony scale rather than marginally. **P4190855 shows two large, fully white plating/tabular colonies side by side, stark against neighbouring normally pigmented corals in the same frame** — a within-frame contrast that exposure or white balance cannot explain — with a third white plate at the left edge. P4190868 is a macro of a branching colony (growth form consistent with *Pocillopora* or *Stylophora*) whose branch tips are pale lavender-white against brown tissue on the lower branches: **classic tip-paling, an early-stage bleaching signature rather than full whitening**. P4190854, the full-frame brain colony, carries an overall pale mauve-lavender cast with only the valleys still pigmented — again consistent with paling or a pink/purple chromoprotein stress response rather than healthy pigmentation. Additional white or conspicuously pale colonies appear in P4190853 (a white lobate cluster left of centre), P4190867 (a white patch on rock centre-left) and P4190869 (two large cream-pale massive mounds). Taken together these are frame-level bleaching and paling records that support the high published figure; **no new site percentage is calculated from them.** No disease lesion margins or tissue-loss bands were identified, so the white colonies are logged as bleaching rather than disease. No *Drupella* aggregations, no parrotfish scarring, and no crown-of-thorns starfish or COTS feeding scars were seen in any of the nine frames.

<b>Assemblage composition.</b> Formal transects give 43.9% coral against 32.8% algae across 20 nonzero genera, with *Porites* alone holding 51% of all coral (22.4% of the benthos) and *Acropora* an unusually minor 3.0% for an exposed forereef. The depth profile falls steeply — 63.25% cover at D2, where *Porites* is 64% of the coral, to 35.5% at D3 and 33.0% at D4, where *Montipora* rises to a quarter of the coral present. The nine photographs show a *Porites*- and brain-coral-dominated pavement with abundant small branching colonies, plus one deep sand-slope frame carrying gorgonians, black corals and sponges that matches the site's high non-coral-invertebrate reading, and they document multiple bleached and paling colonies consistent with this site's top-decile published bleaching figure.

**Flags:** (1) **BLEACHING OBSERVATION — this site should be carried into the running COTS/bleaching/disease log.** Frame-level evidence: two fully white plating colonies plus a third white plate in **P4190855**; lavender-white branch-tip paling in **P4190868**; whole-colony pale mauve cast on the brain colony in **P4190854**; further pale/white colonies in P4190853, P4190867 and P4190869. Published image bleaching 17.4%, 10th of 98 sites. (2) **The published bleaching figure is a COMPUTER-VISION product, not an in-situ observation.** It comes from CoralNet annotation of the informal survey images (Mayfield & Dempsey 2026, *Oceans*). Per-image human-vs-model agreement is substantially weaker for bleaching than for cover: r = 0.40 for bleached against r = 0.72 for live hard coral (`CHAGOS_MULTIMETHOD_README.md`). Never read it as a diver's field estimate. (3) **Any rank taken off the `Mean(%bleaching)` column is approximate.** That column carries at least one known bad value (BITB58 listed at 22.22% against a corrected 0.77%), so the 10th-of-98 placement has not been independently audited — though here the photographs independently support a high value. (4) **Multi-method deviation is moderate and one-directional:** CoralNet points 29.6% and Coralscapes masks 31.4% sit 14.3 and 12.5 pp below the formal transect figure of 43.9%. Do not average the three; the transect is treated as ground truth for cover. (5) **The CV/photo figures rest on a 9-frame scored subset**, which happens to equal the folder count here — a coincidence that does not make the denominators comparable, and **one of those nine frames (P4190837) is a sand-slope/blue-water shot with almost no benthos in it**, which will depress any image-derived cover estimate for this site. (6) **`coralnet_points_pct` is documented as a human 30-point read, not a model prediction** (`CHAGOS_MULTIMETHOD_README.md`), but only roughly a quarter of Chagos image annotations corpus-wide are Anderson-confirmed; do not present 29.6% as verified ground truth. (7) **Non-coral invertebrates are 17.4% of the benthos here** — a large share, matched in the imagery by gorgonians, antipatharians and sponges on the deep slope. Note the near-coincidence of that value (17.4166) with the published bleaching figure (17.4136); they are different quantities and must not be confused when transcribing. (8) **CCA is scarce (6.8%, 97th of 109) on a windward, exposed forereef**, which runs against the usual Chagos pattern and is worth carrying into any cross-site algal comparison. (9) **Two large plate colonies appear dead and sediment-veneered (P4190853, P4190867)** — recorded as an observation; standing dead framework is not separated from bare substrate in this dataset. (10) **Formal replication is thin: `N Rows` = 3**, and **there is no D1 transect** — the shallowest reef zone is unsampled and the profile starts at D2. (11) **The depth codes D2–D4 carry no metre boundaries** in `Chagos benthic data by site-depth (n=336 rows).csv`; they are ordered shallow-to-deep stations, and the 31.0 m in the header is the site-level survey depth, not the depth of any one transect. (12) **At D3 and D4, 32.0 and 25.5 pp of the benthos are unaccounted for** by the coral and algae columns (vs 12.5 pp at D2) — most likely non-coral invertebrates and bare substrate, which are not itemised by depth. (13) **No COTS, disease or predation scarring was observed** across the full 9-frame review — an explicit absence-of-evidence finding, though nine frames cannot support a strong site-level absence claim. (14) **NON-CANONICAL BLEACHING RANK — added 2026-08-26.** Any "of 98" bleaching rank above was computed off the site table's `Mean(%bleaching)` column, which equals the **1st** CoralNet training run. Anderson's decision of 2026-08-26 (HOW_TO rule 4e) makes **`% bleaching (2nd training)` canonical**, The file carries TWO pools for it, and they are not interchangeable: a **110-row pool** ranking every dive (revisit dives included, so BIPB70B/72B/69C compete) and a **99-site pool** ranking one value per site. The rank quoted below is on the 110-row dive pool. For this site the canonical value is **17.41%, rank 18 of 110** (1st training: 17.41%, rank 13 of 110). The two runs agree to <0.05 pp on 100 of 110 rows and diverge only on the bleaching-onset dives, so for most sites the distinction changes the rank but not the number.

**Benthic data:** [Download BIPB75 benthic survey data (CSV)](../benthic_data/BIPB75_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).