### BITB37 — South Brother
**CRD page:** [Three Brothers & Central Chagos Coral Surveys (Mar 17-20, 2015)](https://coralreefdiagnostics.com/three-brotherscentral-chagos-20150317-to-0320)
**Date:** 2015-03-20
**GPS:** 6.17699°S, 71.54402°E
**Frames reviewed:** 78 of 78 (100% of `BiTb37-great site-finished`; all present, non-zero and readable). The published `simplified image data` table contains 57 BITB37 rows, while the paired point/mask audit has 56 usable matches; 21 folder frames were never scored and P3200393 is lost from the paired audit to a same-name topside collision. Those absences are upstream of the analysis and cannot be repaired by re-keying the present table.
**Coral cover:** 38.77% (formal transect surveys; **algal cover 35.46%**) · **published % bleached lookup:** 5.82% (`% bleaching (2nd training)`, Mayfield & Dempsey 2026 supplement). This March value is **not corroborated as a site-scale event by the diver record or the complete 78-frame review**; see the bleaching paragraph and flags.
**Reef type:** exposed, emergent **fringing fore reef**, outside the lagoon · **Temperature:** 29.6 °C · **Salinity:** 34.3 · **Survey effort:** 17 transects across four depth strata: D1 n=1, D2 n=7, D3 n=6, D4 n=3.
**Live hard coral cover across methods** (n=56 paired photos): transect **38.8%** · CoralNet points 28.9% · Coralscapes 27.7%, deviations −9.9 and −11.1 percentage points. Mean of three methods 31.8% ± 6.1. The supplement's differently normalised image estimate is **37.27%**, only 1.49 points below the transect. These numbers should not be used to claim that imagery inherently under-read Three Brothers: the methods use different and partly undocumented denominators, 30.1% of paired points are `Unknown`, 22 of 78 available frames are absent from the paired audit, and the formal transects and edited photographs do not share a documented sampling footprint.
**Formal benthic composition:** *Acropora* 13.97%, *Porites* 7.95%, *Pocillopora* 3.81%, *Montipora* 3.15%, then *Leptastrea* 1.05%, *Favites* 1.02%, *Pavona* and *Astreopora* 0.84% each, *Goniastrea* 0.71%, *Favia* 0.70%, *Millepora* 0.66%, *Echinopora* 0.64% and 15 additional genera at <0.6% each. Algae is mostly CCA 19.93%, turf 8.29% and sediment-laden turf 4.40%, with erect algae 1.61% and macroalgae 1.24%. Barren substrate is only 5.08%; non-coral invertebrate cover (`PITS`) is 20.69%.
**Representative photos:** P3200403.jpg, P3200466.jpg, P3200469.jpg, P3200471.jpg, P3200393.jpg, P3200425.jpg

**BITB37 is a high-diversity coral terrace wrapped around a steeper fore-reef face, not a uniformly carpeted reef.** The opening and wall-facing frames show a broken, sloping framework with plates, massive domes, encrusting sheets and small branching colonies separated by shadowed ledges and open blue water. **P3200403** is the cleanest wall view: overlapping tan and green plates, small digitate and corymbose colonies, massive and meandroid heads and vivid pink CCA descending along an exposed face. Later frames open onto a broad upper-slope terrace where large tabular colonies compatible with *Acropora* and *Montipora*, corymbose and arborescent *Acropora*-type colonies, massive *Porites*-type domes and smaller pocilloporid clumps form a dense three-dimensional mosaic. **P3200466, P3200469 and P3200471** are the defining wide views. They make the formal 38.77% cover credible while also showing why it is not 60–80%: pale pavement channels, old consolidated framework and occasional sandy openings remain visible between colonies.

**The formal transects resolve a real depth turnover hidden by the site mean.** D2 is the coral maximum at **47.77%**, with *Acropora* alone at **27.46%**; one 100-point D2 transect reaches 79% coral and 64% *Acropora*. D3 falls to 35.97% coral, and D4 to **31.33%**, while *Acropora* declines to just **0.67%** in D4. In the opposite direction, *Porites* rises from 5.4% in D1 and 6.31% in D2 to **12.33% in D4**. The deepest stratum is also more open: barren substrate rises to **14.87%** and sediment-laden turf to **10.6%**, versus 1–2.4% barren in D1–D3. Thus “38.8% coral” averages an Acropora-dominated shallow/mid-depth terrace and a more Porites-led, sediment-retaining deeper surface. D1 and D4 rest on one and three transects respectively, so the direction is strong but the endpoints should not be over-generalised.

**The photographs and the transects agree on the major coral groups.** Across the 56 paired frames, the point set assigns 66.7% of live coral points to *Acropora*, 15.0% to *Porites*, 2.3% to *Pocillopora* and 16.0% to other hard coral. Coralscapes divides live area among non-tabular *Acropora* (32.4%), tabular *Acropora* (19.9%), massive/meandroid forms (21.4%), other hard coral (14.2%) and *Pocillopora* (7.5%). Those image classes are coarser than the 28 non-zero formal genera and should not be treated as genus identifications, but they independently recover the same Acropora-led architecture. Photographic genus calls remain morphological compatibilities: plates compatible with *Acropora*/*Montipora*/*Pavona*, massive and lobate colonies compatible with *Porites*, verrucose bushes compatible with *Pocillopora*, and meandroid or honeycomb colonies compatible with several faviid lineages.

**The site also carries unusually memorable mobile fauna.** **P3200391–P3200393** show a close school of large silver trevallies, several with dark flank blotches and yellow-green spotting, filling the water column; species-level identification is not secure from these angles. The same large-bodied fish presence returns in P3200491–P3200492 over the coral terrace. **P3200425** is a close, well-lit octopus resting in a crevice among encrusting sponges, ascidians and small corals. **P3200427** shows a collector urchin heavily covered with algae and debris—not a crown-of-thorns starfish. Reef fishes include surgeonfishes, butterflyfishes, wrasses, anthias and small damselfishes, but the photographs are illustrative rather than a fish census.

**No crown-of-thorns starfish, disease lesion or clear corallivore predation scar is visible in any of the 78 frames.** The sole COTS line inherited from the automated summary is P3200403 at a rounded **0.00% of frame area**. Full-resolution review shows an ordinary coral-covered wall and no starfish; this is a threshold/rounding artefact and should be removed as a biological observation. Coralscapes' paired-frame means of 2.25% dead coral and 1.11% rubble are compatible with scattered old exposed framework, not an acute disturbance pulse.

**BLEACHING — the published 5.82% lookup is not visually corroborated at site scale.** Anderson's field record is that no bleaching was observed on March or April dives, with bleaching beginning in May, and the full 78-frame review supports that distinction. Most colonies are normally pigmented across brown, olive, green, cream, lavender and blue-grey forms. Pale branch tips occur, but intact, regularly distributed tips on otherwise pigmented colonies are compatible with growth, flash and natural colour and are not sufficient evidence of bleaching. A few close frames, especially P3200462, contain small mottled or pale patches that could represent local paling or recent tissue loss, but nothing approaches 5.82% of coral across the photographed reef.

The point audit's apparent bleaching is highly concentrated and substantially non-benthic. It assigns **21 of 1,680 points (1.25% of all points)** to bleached coral across nine frames. **Ten of those 21 points sit on P3200483**, where a diver is holding a small pale coral fragment close to the camera. That is a specimen/methodology image, not a random view of benthic cover; Coralscapes likewise assigns **11.91% of the frame** to bleached coral, accounting for **83.5% of the site's total mask-bleaching signal**. Removing only P3200483 lowers mean point bleaching from 1.25% to **0.67% of points** and mean mask bleaching from 0.255% to **0.043% of image area**. Other point calls are visibly misplaced: P3200437 is an octopus macro with bright carbonate and flash highlights; P3200466 and P3200471 are normally pigmented wide reef views; P3200491 is dominated by fish and open water. P3200441 and P3200445 show pale-tipped small colonies, but the photographs do not resolve bleaching from growth tips or natural pigmentation. The honest conclusion is **no site-wide bleaching signal in March**, with a few ambiguous pale colonies and one collected fragment that must not be allowed to represent the reef.

**Coverage and provenance limit every photo-derived percentage here.** All 78 available photographs were visually reviewed, but only 57 filenames occur in `simplified image data`, and only 56 are usable in the paired point/mask table. The 21 never-scored files include important ecological frames—trevallies, the octopus, the collector urchin and several wall/terrace views—so the quantitative photo subset is not equivalent to the full folder. P3200393 is additionally misjoined to an unrelated `2015-3-20 topside` sunset because both folders contain that filename; the BITB37 copy is actually a close trevally school. This is a genuine filename-collision loss, not something that a later relabelling of the joined row can recover. Finally, roughly **506 of 1,680 paired points (30.1%) are `Unknown`**, and the Chagos point set is predominantly unconfirmed CoralNet robot output. Treat the point table as an imperfect image-analysis product, never as a complete human re-annotation of the dive.

<b>Assemblage composition.</b> Across 56 paired frames, Coralscapes estimates live hard coral at 27.73% of image area, dead coral at 2.25%, rubble at 1.11% and bleached coral at 0.255%; its live-area morphology is led by non-tabular and tabular <i>Acropora</i>-type forms. The 30-point set gives 28.93% live hard coral, 1.25% bleached hard coral, 12.7% turf and 6.8% CCA, with 30.1% Unknown. The formal transects—17 replicates across four depth strata—give the preferred benthic-cover estimate: 38.77% coral, 35.46% algae, 5.08% barren substrate and 20.69% non-coral invertebrates. The formal and photographic records agree qualitatively on an Acropora-led, structurally complex reef, but only the formal dataset supports the site-level cover value.

**Flags:** (1) The old sentence calling bleaching “moderate overall, consistent with 5.8%” was incorrect and has been removed. The 5.82% value is a published lookup, not a field observation; the diver record and 78-frame review do not corroborate a March site-scale event. (2) P3200483 is a close specimen image of a pale coral fragment in a diver's hand and drives 10/21 bleached points plus 83.5% of mask bleaching; it is not valid cover imagery. (3) P3200393 is lost from the paired audit to a same-name topside collision; the BITB37 file is a trevally school, while the joined model path is a sunset. (4) Twenty-one of 78 available frames were never scored, and the missing data cannot be recovered by re-keying. (5) P3200403's COTS value is 0.00% after rounding and is a false automated trace; no COTS is present. (6) D2 and D4 describe different assemblages—Acropora-dominated versus Porites-led/open—and the four-stratum structure should be retained whenever the single 38.77% mean is interpreted. (7) No per-frame depth data are available.

**Benthic data:** [Download BITB37 benthic survey data (CSV)](../benthic_data/BITB37_benthic.csv) — scleractinian cover by genus with full names, plus substrate and algal categories (bare substrate, CCA, turf, macroalgae, invertebrates), from the LOF BIOT-1 survey (Mayfield & Dempsey 2026, *Oceans*, supplemental file S1).