### BITB30 — Redemption Rock
**Date:** 2015-03-18 (dive window 13:55:55–14:31:52, `DateTimeOriginal` EXIF on all 33 files, in four bursts. The afternoon dive — BITB28's frames share the camera counter but run 08:21–09:18, which is why the two folders' filenames collide. Flag 1.)
**GPS:** 6.16396°S, 71.53207°E
**Frames reviewed:** 33 of 33 (100% of `BiTb30-finished`; none evicted). **CV basis: 33 of 33 — complete coverage. Every frame reviewed is a frame both models scored.**
**Coral cover:** 42.5% (formal transect; algal cover 40.68%) — **the highest-cover Three Brothers site written so far** · **% bleached:** 13.14% (Mayfield & Dempsey 2026, Oceans) — **the most bleached site of the Three Brothers series.**
**Reef type:** fringing, protected · **Temperature:** 29.6 °C
**Live hard coral cover across methods** (n=33 photos, complete): transect 42.5% · CoralNet points 18.2% · Coralscapes 21.4%. Deviation **−24.3 to −21.1 pp — the largest method deviation in the entire redo, roughly double BITB27/28/29 and BIEA26, and unlike BITB27 it is carried on COMPLETE coverage.** **Mean of 3: 27.4% ± 13.2.** **There is no unscored remainder to blame: this is a real, fully-sampled 21–24 pp shortfall. It is almost entirely a DENOMINATOR artefact — renormalise either photo method over labelled benthos and both land on the transect (39.9% and 41.9%). Flags 2–4.**
**Coral genera identified:** 9 scleractinian groups plus two octocoral forms — ***Acropora*** in four forms (**tabular plates to ~2 m**, corymbose/digitate, arborescent staghorn, and a **purple-mauve branching form** in P3180462); ***Porites*** (massive/domal, columnar-digitate, knobby-lobed); **fine compact verrucose pocilloporids compatible with the *P. damicornis*/*P. acuta* group** (P3180421, P3180463); **fine dichotomous branching compatible with *Seriatopora*/*Stylophora*** (P3180423; Coralscapes assigns 5.0% of live cover to a *Stylophora* class); meandroid brains compatible with *Platygyra*/*Leptoria* (P3180423, P3180440, P3180450, P3180462); lobed meandroid compatible with *Lobophyllia*/*Symphyllia* (P3180451, P3180461); cerioid honeycomb compatible with *Astreopora* or a faviid (P3180417); foliose plates compatible with *Echinopora*/*Turbinaria* (P3180394, P3180424); and salmon-orange encrusting sheets compatible with *Montipora* (P3180440, P3180450). Plus **large gorgonian sea fans** in five frames, a red branching octocoral compatible with *Melithaea* (P3180443), a golden branching colony compatible with *Millepora* or an alcyonacean (P3180424), and green *Didemnum*-type ascidians (P3180421–422). Growth-form calls are from photographs and given as "compatible with"; no species is asserted.
**Representative photos:** P3180452.jpg, P3180441.jpg, P3180434.jpg, P3180461.jpg, P3180465.jpg

**BITB30 is the best reef of the Three Brothers redo, and the frames say so more loudly than the 42.5% does.** The dive opens in blue water — the first six frames (P3180394–P3180402) are given almost entirely to **fusilier and sardine schools banked against the drop-off**, with **large olive-gold gorgonian sea fans** silhouetted along the slope edge in P3180398, P3180401 and P3180418. From 14:07 the camera drops onto the reef and does not leave it, and what it finds is a **continuous tabular-*Acropora* pavement**: P3180452 is a single olive plate filling the lower half of the frame with a second behind it; **P3180441** stacks three tan tables, the largest well over a metre, beneath a school of dark surgeonfish; **P3180434, P3180436, P3180451 and P3180462** repeat it at wider angle, plate on plate to the limit of visibility, with corymbose and staghorn *Acropora* filling every gap and massive *Porites* domes anchoring the low ground. By eye these frames carry 45–60% living tissue. **The transect's 42.5% is entirely believable; the photo methods' 18–21% is not, and explaining that is the work of this entry.**

**Between the tables the reef is a fine mosaic, and that matters for the CV.** **P3180422 and P3180423** are the clearest cases — pavement carrying dozens of colonies a few centimetres across: pocilloporid clumps, *Acropora* recruits, encrusting *Montipora*-type sheets, green *Didemnum*-type ascidians and pink crustose coralline algae, interleaved at a scale a 30-point grid cannot resolve. **P3180440 and P3180461** are the highest-cover, and **P3180437** adds a **clown triggerfish (*Balistoides conspicillum*)** sitting in the middle of it.

**Both target pocilloporid groups are present, which is unusual for Chagos.** The CRD site page records ***P. acuta*/*P. damicornis* (ChPd16-20) and *Seriatopora* spp. (ChSe09-10)** here, with ChPa16-18 photographed and genotyped. The imagery is consistent: **P3180421** is a flash macro of a fine verrucose pocilloporid ringed by green ascidians and red encrusting sponge, **P3180463** a second, smaller, cream-tan pocilloporid on CCA pavement — both compatible with the compact *damicornis*/*acuta* habit rather than the blunt *verrucosa* form seen at BITB27 — and **P3180423** carries a fine dichotomously-branched colony compatible with *Seriatopora*. **These are hedged photographic impressions that agree with the survey record; they do not independently confirm it.**

**Mortality is present but old, and there is no methodology imagery at all — zero frames of 33.** The dominant structural loss is **long-dead skeleton already resurfaced in pink and mauve CCA**: the pavement in P3180448, P3180449, P3180463 and P3180464 is almost entirely CCA-plated old framework, and Coralscapes puts dead coral at 3.35% of image area with rubble at 2.66%. Against the 13.14% bleaching figure only a handful of frames show pale tissue directly (flag 10). No tape, quadrat, diver, slate or collecting gear appears anywhere — **this is the cleanest survey-only folder of the redo** — though **eight close-up macro or "spot" frames** (P3180417, P3180421–424, P3180463–465) are composed on a colony rather than on the reef (flag 8).

**The fauna is good and the last three frames are the best of it.** **P3180465 is the single best macro of the redo so far — an octopus (*Octopus cyanea*-type) wedged under a grey-blue sponge overhang with both eyes open and a black *Diadema*-type urchin on the sponge above it.** **P3180419** holds **three bluefin trevally (*Caranx melampygus*)** in clean profile over white sand. Beyond those: **powder-blue surgeonfish (*Acanthurus leucosternon*)** in P3180438 and P3180447; **moorish idols (*Zanclus cornutus*)** in P3180451 and P3180461; a large grey angelfish with white face mask and yellow caudal in P3180450, **compatible with *Pomacanthus xanthometopon* but not callable**; parrotfish, red bigeyes compatible with *Priacanthus*, blue-and-yellow damselfish, and continuous fusilier, anthias and dark-surgeonfish traffic throughout.

<b>Assemblage composition.</b> **Both methods rest on all 33 frames — this is one of the few complete-coverage sites in the redo.** Across the 33 frames the Coralscapes masks put live hard coral at 21.4% of image area, of which <i>Acropora</i> (non-tabular) is the largest component at 23.3%; the rest breaks down as tabular <i>Acropora</i> 20.9%, massive and meandroid 20.2%, other hard coral 18.6% and <i>Stylophora</i> 5.0% of live cover — **the most evenly balanced growth-form spectrum of any site in the redo, and the only one carrying a *Stylophora* component.** The independent 30-point annotation of the same filenames (990 points) gives live hard coral as 18.2% of all points and apportions it <i>Acropora</i> 49.4%, <i>Porites</i> 36.1%, other hard coral 10.6%, <i>Pocillopora</i> 3.9% of live coral. Non-coral cover is dominated by turf 18.6% and crustose coralline algae 6.0% of all points. Structural loss registers as dead coral 3.35% of image area and rubble 2.66%. Bleaching stands at 2.12% of annotated points — **21 points**, concentrated in <i>Acropora</i> 1.82% and other hard coral 0.30% — which is **10.4% of all coral points, close to the 13.14% transect figure**; Coralscapes by contrast reads bleached at 0.05% of image area, effectively zero. Frame-to-frame live cover runs 0–43.3% by point count and 0–47.0% by mask, and **the two barely agree: r = 0.278 here against 0.716 corpus-wide.** Conspicuous fauna and sessile invertebrates appear as sponges in 22 frames, urchins in 2 frames, giant clams in 1 frame, sea cucumbers in 1 frame, anemones in 1 frame, debris in 2 frames.

**Flags:** (1) **THE MANIFEST'S `coralnet_annotated: 12` IS WRONG. The correct basis is 33 of 33 — complete coverage.** All 33 filenames carry rows in both comparison CSVs, every `image_path` reads `BiTb30-finished`, and every Coralscapes overlay is folder-qualified (`chagos__bitb30_finished__p3180394_pred.png`). **Only 9 of the 33 filenames are unique in the tree — 21 collide with `BiTb28-finished` and 3 with `2015-3-18 topside` — and BITB30 won every collision** (BiTb28 was left with exactly its 22 unique names). 33 − 21 = 12, which is how the manifest's number arose, but it matches neither the true 33 nor the 9 unique names. **Log this as a manifest data-integrity error — the second caught in this redo after the malformed `gps` at BIEA24/25/26 — and see flag 16 for the corpus-wide scale.** The collisions are between genuinely different images: **all 24 collided pairs are byte-different, and BITB28's P3180394 is timestamped 08:22:54 against BITB30's 13:55:55.** (2) **CAVEAT ON THE POINT COLUMN, NOT ON THE HEADLINE — 4 collided frames' point rows are visually inconsistent with BITB30's pixels.** CoralNet only ever held one image per filename, and the annotation for a collided name could in principle be the other folder's. Four are checkable by eye and fail: **P3180394 is ~95% dark blue water with a fusilier school, scored 0.0% water and 33.3% live; P3180421, P3180422 and P3180423 are full-frame benthic macros with no water column at all, scored 40.0%, 43.3% and 26.7% water.** This does not change the headline — Coralscapes' 21.4% is unambiguously BITB30's pixels, the two photo methods agree with each other on the shortfall, and the gap survives every subset test — but **the CoralNet points column for collided filenames should be treated as provisional corpus-wide, and this is worth a targeted audit against BITB28.** (3) **THERE IS NO SUBSET ESCAPE, UNLIKE BITB27 — AND THE GAP IS A DENOMINATOR ARTEFACT. THIS IS THE HEADLINE FINDING.** Restricting to the 12 non-BITB28-colliding frames moves the point set only **18.2% → 17.2%** and drives Coralscapes down to 16.0%, i.e. the deviation gets **worse** (−25.3 / −26.5 pp); on the 19 wide benthic frames it is 19.6% / 26.5%. Every subset reads low. What closes it is the denominator: **54.4% of the point budget is assigned to no benthic class — water 23.1%, fish 1.8%, Unknown 29.5% — and Coralscapes calls 41.5% of image area `background_dark`.** A transect measures cover **of the substrate**; both photo methods report cover **of the image**. Renormalised, the gap all but vanishes: **live over labelled benthic points = 39.9% (44.6% including bleached) against a 42.5% transect, and Coralscapes over (100 − background − fish) = 41.9%**; on the 19 wide benthic frames, 39.9% and 50.6%. **Same lever as BITB27 flag 7 and BIEA26, but on complete coverage it accounts for the whole gap rather than part of it. Recommend renormalising photo-derived cover over labelled benthos corpus-wide before any transect comparison — on this evidence the −15 pp corpus-wide photo/transect offset in `CHAGOS_MULTIMETHOD_README.md` may be largely the same artefact.** (4) **BLUE-WATER DILUTION IS MINOR — CANDIDATE 1 REJECTED.** 8 frames are ≥40% water (P3180401 70.0, P3180402 56.7, P3180419 56.7, P3180462 50.0, P3180434 46.7, P3180436 46.7, P3180422 43.3, P3180421 40.0; mean 23.1%). Dropping them lifts CoralNet 18.2→20.5 and Coralscapes 21.4→23.2 — **still 22.0 and 19.3 pp low, under a tenth of the gap closed. The BIEA20 sign-flip does not occur; the gap narrows slightly and never reverses.** (5) **POINTS AND MASKS FAIL IN OPPOSITE DIRECTIONS ON THE FINE MOSAIC AND FRAME-LEVEL AGREEMENT COLLAPSES — r = 0.278 here against 0.716 corpus-wide.** Disagreement runs **−32.6 pp (P3180465: 40.0% by points on a frame of sponge, octopus and rubble) to +40.4 pp (P3180452: 6.7% by points, 47.0% by mask, on a frame dominated by one enormous tabular *Acropora*)**; also P3180440 +26.1, P3180449 +24.2, P3180451 +19.0, P3180443 +18.7. **On wide table frames the mask is far closer to the eye; on close-up macros the point set over-reads. Both are wrong in ways that partly cancel in the site mean, which is why the two methods agree with each other far better than either agrees with the reef.** (6) **THE BLEACHING SIGNAL IS CONCENTRATED, NOT DISTRIBUTED — THE BITB28 PATTERN REPEATS.** **8 of 33 frames carry all 21 bleached points; 25 frames carry none, and two frames hold 48% of the signal.** The worst is **P3180419 — three bluefin trevally over bare white sand, 56.7% water by the point set's own reckoning and 0.0% live coral by mask — carrying 6 bleached-*Acropora* points, 29% of the site's entire bleaching signal.** Remove that one frame and the point-based estimate falls from **10.4% to 7.8% of coral points**, well below the 13.14% transect figure; remove P3180397 (63.3% Unknown, pure blue water, 1 point) and P3180398 as well and it falls to 5.7%. **Do not treat the 13.14% as evenly distributed across this reef on photographic evidence.** With ~76% of Chagos point annotations being robot predictions, report P3180419 as annotation provenance, not observation. (7) **BLEACHING IS ALSO HANDLED INCOMPATIBLY BY THE TWO METHODS AT THE MOST BLEACHED SITE OF THE SERIES.** The point set separates bleached from live (2.12% of points, 10.4% of coral points); **Coralscapes reads bleached at 0.05% of image area, max 0.38% on any frame — folding pale tissue into live coral, exactly as at BIEA24.** Its 21.4% therefore already contains bleached tissue the 18.2% excludes; adding bleached points back gives 20.3% and removes two-thirds of the difference between the photo methods. **The two photo columns are not measuring the same quantity here, and at 13.14% bleached this is the site where that matters most.** (8) **NO METHODOLOGY IMAGERY AT ALL — 0 of 33 frames; candidate 4 rejected outright**, and a useful control against BITB27 where 6 of 16 scored frames were measurement shots. The **8 close-up macro/"spot" frames** (P3180417, P3180421–424, P3180463–465) are a milder version of the same bias: renormalised they read 34.1%/22.1% against 42.1%/48.5% for the 25 wide frames. (9) **A PATCHY REEF IS ALSO REJECTED** — from 14:07 to 14:31 every wide frame carries dense tabular and corymbose *Acropora*. If anything the imagery suggests the transect is **conservative**. (10) **PALING AND PARTIAL MORTALITY, SEVEN FRAMES, NONE RESOLVABLE PHOTOGRAPHICALLY** — P3180446 (white sector on a tabular plate), P3180450 (a fully white tabular colony plus a pale branching colony), P3180451 (white band on a table margin), P3180449 (white tissue at a corymbose colony's base), P3180448 (chalky white branch fragments on CCA pavement), P3180421 and P3180463 (pale-tan pocilloporid macros). Bleaching versus recent partial mortality versus pale morph cannot be separated from these images. **Note that none of these seven is among the two frames carrying half the point-set bleaching signal.** (11) **BOTH TARGET POCILLOPORID GROUPS PRESENT — *P. acuta*/*P. damicornis* AND *Seriatopora* — the opposite of BITB27's record.** P3180421 and P3180463 are the candidate ChPa16-18 subjects; **whether either is actually one of the genotyped colonies needs Anderson's judgement.** (12) **P3180465 (octopus + *Diadema*-type urchin under a sponge overhang) is the best macro of the redo so far** — flagged for illustrative use with **P3180452**, **P3180441**, **P3180434** and **P3180419**. **P3180464's cryptic animal is NOT callable** (scorpionfish only compatible), and a dark mass in **P3180449** may be a second octopus or a dark encrusting sponge; the BIEG04 lesson applies to both. (13) **No COTS, no disease lesions and no predation scars in any of the 33 frames.** (14) **Gorgonian sea fans in ≥5 frames (P3180394, P3180398, P3180401, P3180402, P3180418) and a red branching octocoral in P3180443** — structurally conspicuous, invisible to a hard-coral point scheme, as at BIEA18 and BIEA20. (15) No per-frame depth data. (16) **`coralnet_annotated` IS UNRELIABLE CORPUS-WIDE, NOT JUST HERE: it disagrees with the CSV row count at 39 of 105 sites, including six recorded as 0 that in fact carry 11–47 rows** (BITB35, BITB36, BITB50, BITB51, BITB52, BITB57). Recommend rebuilding the field from the comparison CSVs before any other site's write-up cites it.

**Benthic data:** [Download BITB30 benthic survey data (CSV)](../benthic_data/BITB30_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).