### BITB53 — Three Brothers
**CRD page:** [Three Brothers/Central Chagos Coral Surveys March 25-28, 2015](https://coralreefdiagnostics.com/three-brotherscentral-chagos-20150325-to-0328)

**26 Mar 2015**
**GPS:** 6.12684°S, 71.50318°E
**Setting:** intermediate-exposure, submerged fringing reef inside the lagoon, 18.3 m
**Coral cover:** 36.7% (3 formal transects, D2–D4; **algal cover 34.8%**) · **published image bleaching:** 3.2%
**Live hard coral cover across methods:** formal transects 36.7% · CoralNet points 27.4% · Coralscapes masks 25.7% (22 frames). These products have different sampling footprints and denominators and should not be averaged.
**Temperature:** 29.5 °C · **Salinity:** 34.7
**Photo review:** all 22 frames examined
**Representative photos:** P3260405.jpg, P3260408.jpg, P3260411.jpg, P3260417.jpg, P3260430.jpg

BITB53 sits mid-pack for the Chagos set: 36.7% coral cover ranks 61st of 109 sites and its 34.8% algal cover ranks 72nd (i.e., lower than most), with a low published bleaching mean of 3.2%, ranking 79th of 98 sites with a value — in the least-bleached fifth of the archipelago. Twenty-three genera were recorded, none dominant enough to define the site on its own: *Porites* leads at 14.07% of the benthos (38% of all coral recorded), followed by *Acropora* at 7.67% (21%), *Montipora* at 3.79% (10%), *Pavona* at 2.20% (6%), *Pocillopora* at 1.88% (5%), *Symphyllia* at 1.56% (4%), *Stylophora* at 1.15% (3%) and *Pachyseris* at 1.00% (3%). Those eight genera carry 91% of the coral; the remaining fifteen — *Leptoseris*, *Favites*, *Leptastrea*, *Psammocora*, *Goniastrea*, *Favia*, *Cyphastrea*, *Turbinaria*, *Astreopora*, *Platygyra*, *Isopora*, *Acanthastrea*, *Coscinaraea*, *Montastrea* and *Plesiastrea* — sum to only 1.20% and each sits below 0.55%. The genus values total 36.69%, matching the published coral-cover figure exactly (23 nonzero codes, no rounding gap). Non-coral invertebrates hold 12.62% of the substrate cover — a comparatively large share reflecting the large gorgonian sea fans and encrusting fauna documented in the photographs — alongside 12.93% crustose coralline algae, 15.87% bare substrate, and the algal categories (7.30% turf, 11.40% turf+sediment, 2.57% macroalgae, 0.62% erect algae) that sum to the reported 34.82% algal cover.

**The three depth transects describe two different reefs, not one.** D2, the shallowest transect run, carries only 24% coral against a dominant 50.7% algae — the highest algal share recorded anywhere at this site — with *Acropora* (7.33%) essentially tied with *Porites* (6.67%) as co-dominants, *Montipora* third at 5.00%, and trace *Pocillopora*, *Psammocora* and *Turbinaria*. D3 lifts coral to 33.1% while algae falls to 43.8%; *Porites* pulls decisively ahead here at 12.53%, nearly double *Acropora*'s 9.67%, with *Montipora*, *Pocillopora*, *Favites* and *Goniastrea* filling out the low single digits. D4, the deepest transect, is a different reef entirely: coral cover reaches 53% — more than double D2's figure — while algae collapses to just 10%, its lowest value at the site. *Porites* alone carries 23.00% of the benthos here, 43% of all the coral on that transect, with *Acropora* down to 6.00% (tied with *Pavona*), and *Montipora*, *Symphyllia* and *Pachyseris* each in the 3–4% range. The site-level average of 14.07% *Porites* and 7.67% *Acropora* is therefore a blend of a shallow, algae-choked flat where the two genera run nearly co-dominant and a deep, coral-abundant *Porites*-dominated zone where *Acropora*'s relative share is roughly halved. This progression matches the imagery: the barer, rubble-and-sand frames (P3260386, P3260388, P3260414) read as the algae-heavy shallow end, while the massive-coral, gorgonian-thicketed bommies (P3260405–P3260413) are the coral-abundant, low-algae deep end.

The photographs split cleanly into these two visual regimes. Several frames (P3260386, P3260388, P3260414, and to a lesser extent P3260419) show open, largely bare rubble-and-sand hardground with only scattered colonies — a small staghorn *Acropora*-type clump, isolated massive lobate heads consistent with *Porites*, and, in P3260388, a blue linckia-type sea star crossing open pavement; a lone dark surgeonfish or triggerfish patrols this ground in P3260414. The other regime, concentrated in P3260405–P3260413, is a series of high-relief coral/gorgonian bommies: tall brown sea fans rise over massive lobate and meandroid colonies (including a large brain-coral dome in P3260408 and P3260416), with dense clouds of anthias, orange-and-yellow cardinalfish/glassfish and chromis sheltering in the shadow beneath the overhangs (P3260406, P3260409, P3260412, P3260413). P3260409 additionally shows a cluster of white-spined sea urchins and small tube-dwelling fanworms tucked into the crevices of one bommie — consistent with the elevated non-coral invertebrate share (12.6%) noted in the substrate data. P3260387 documents a dense, high-cover thicket combining a yellow-green digitate coral, brown massive colonies and encrusting purple framework, and P3260415 and P3260430 show healthy *Acropora*-type table/staghorn growth with fully extended polyps in close-up, salmon-tan and clearly pigmented. A striped emperor-type angelfish shelters under a coral ledge in P3260416, and macro frames P3260402 and P3260417 resolve individual small encrusting colonies against crustose coralline algae and bare pavement.

**Condition is consistent with, rather than contradicted by, the low published bleaching figure.** At 3.2%, BITB53 sits in the least-bleached fifth of the Chagos sites surveyed, and the great majority of colonies photographed — the massive *Porites*-type domes, the *Acropora* tables in P3260415 and P3260430, the brain coral in P3260408 — show normal tan-to-olive pigmentation with no visible paling. One clear exception is P3260417, a close-up macro frame in which a small encrusting coral colony is roughly half normally pigmented (brown, textured coenosarc) and half stark white with intact polyp structure — an unambiguous partial-colony bleaching signature rather than a growth-tip or lighting artefact. A single partial-colony case in a 22-frame set is exactly the kind of sporadic, colony-scale event a low reef-wide mean would be expected to contain; it does not imply the published 3.2% figure understates conditions here, but it is worth recording as ground-truth confirmation that some bleaching stress was present and photographically documented on 26 Mar 2015.

<b>Assemblage composition.</b> Formal transects give 36.7% coral, 34.8% algae and a comparatively high 12.6% non-coral invertebrate cover, with *Porites* (38% of the coral) and *Acropora* (21%) the leading genera across 23 recorded taxa — though the two switch relative dominance with depth, from near-parity in the shallow, algae-heavy D2 transect to a *Porites*-led, low-algae assemblage at D4. The photographs document both faces of that structure: open rubble-and-sand ground with scattered colonies at the algae-heavy end, and dense gorgonian-and-massive-coral bommies sheltering large reef-fish schools at the coral-abundant end, plus one documented case of partial-colony bleaching.

**Flags:** (1) **Severe filename overlap across three photo folders shot the same day.** Seventeen of the 22 BITB53 frames (P3260386–388, 403–404, 406–415, 418–419) share filenames with images in `BiTb55-finished-octoporn` and/or `2015-3-26 topside`, all dated 26 Mar 2015. File sizes differ in every checked case (e.g. P3260403: 2.94 MB in BiTb53-finished vs 1.65 MB in BiTb55-finished-octoporn), confirming these are genuinely different photographs from the same camera-roll numbering sequence, not duplicates. **Any tool or process that keys on bare frame filename alone will misassign these images between BITB53, BITB55 and the topside folder — always carry the full folder path.** (2) **Partial-colony bleaching documented in close-up.** P3260417 shows a small coral colony roughly half normally pigmented and half stark white with intact polyp structure. This is consistent with, not contradictory to, the low published site bleaching mean (3.2%, 79th of 98 sites) — a single sporadic colony-scale event against a low reef-wide background — but is recorded here as photographic ground-truth. (3) **Site-level genus means describe no single transect.** The published 14.07% *Porites* / 7.67% *Acropora* is the average of an algae-dominated shallow transect (D2: 50.7% algae, *Acropora* ≈ *Porites*) and a coral-abundant deep transect (D4: 10% algae, *Porites* more than triple *Acropora*). Do not quote the site mean as representative of any one depth. (4) **Multi-method divergence:** formal transects (36.7%) read 9.3–11.0 percentage points higher than CoralNet points (27.4%) and Coralscapes masks (25.7%) on the same 22 frames — consistent with the transect method sampling more of the coral-abundant bommie zone than the photo set, which includes several open-rubble frames. Do not average the three figures. (5) No COTS, disease lesions or predation scarring were observed in any of the 22 frames reviewed. (6) **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`). Treat the site bleaching percentage as the weaker of the two published numbers, and never read it as a diver's field estimate. (7) **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 **3.25%, rank 89 of 110** (1st training: 3.25%, rank 88 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 BITB53 benthic survey data (CSV)](../benthic_data/BITB53_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).