### BICC51 — Three Brothers / Central Chagos
**CRD page:** [Three Brothers/Central Chagos Coral Surveys March 25-28, 2015](https://coralreefdiagnostics.com/three-brotherscentral-chagos-20150325-to-0328)

**25 Mar 2015**
**GPS:** 6.30018°S, 71.56963°E
**Setting:** intermediate-exposure submerged bank reef inside the lagoon, 29 m
**Coral cover:** 15.4% (3 depth transects, D2–D4; **algal cover 70.0%**) · **published image bleaching:** 4.5%
**Live hard coral cover across methods:** formal transects 15.4% · **no CoralNet or Coralscapes product exists for this site** (BICC51 has no row in `chagos_multimethod_cover_by_site.csv`). These products have different sampling footprints and denominators and should not be averaged — including any CV figure generated for this site later.
**Temperature:** 29.7 °C · **Salinity:** 34.4
**Photo review:** all 19 of 19 stills examined (folder `BiTb51-finished`; single dive, 10:57:57–11:49:17 local from `DateTimeOriginal` EXIF, Olympus TG-2)
**Representative photos:** P3250461.jpg, P3250452.jpg, P3250481.jpg, P3250478.jpg, P3250450.jpg

BICC51 is a submerged bank reef inside the Central Chagos lagoon at 29 m, and its defining number is not its coral but its algae. **At 70.0% algal cover it is the single most algae-dominated of the 109 Chagos sites — 1st of 109 — against 15.4% coral cover, which ranks 98th of 109.** The algal budget is dominated by a category that is easy to misread: *Percent Turf+Sediment* (PTSTS) alone accounts for **31.6% of total benthos**, followed by crustose coralline algae (PCCATS) at 20.8% — 19th-highest of 109 — then erect algae 8.2%, macroalgae 6.3% and clean fleshy turf (PTTS) at just 3.0%. **That last figure ranks 104th of 109 and must not be read as a low-algae signal**: BICC51's turf is almost all bound up in the sediment-laden turf category rather than the clean-turf one, and taking PTTS alone would invert the site's actual standing. Bare substrate is 9.0%, non-coral invertebrates 5.6%, cyanobacteria 0.06%. Coral, algae and the three residual categories sum to 100.05% of benthos.

Coral cover is spread across **18 nonzero genera, ranking 83rd of 109 for genus richness**, and the 18 genus percentages sum to 15.3778% of the benthos — an exact match to the published cover figure. All values below are percent of total benthos, not percent of coral. **What is unusual here is how evenly that small coral budget is divided**: no genus reaches a quarter of the coral present. *Pocillopora* leads at 3.78% of the benthos (25% of the coral), then *Acropora* 3.19% (21%), *Porites* 3.02% (20%) and *Stylophora* 1.84% (12%) — those four hold 77% of all coral recorded, and the leader's margin over fourth place is under two percentage points of benthos. The remaining 14 genera each sit below 0.53%: *Favites* 0.52%, *Pavona* 0.46%, *Montipora* 0.40%, *Platygyra* 0.28%, *Millepora* 0.28%, *Astreopora* 0.24%, *Leptastrea* 0.23%, *Hydnophora* 0.22%, *Favia* 0.20%, *Psammocora* 0.18%, *Ctenella* 0.17%, *Turbinaria* 0.17%, *Goniastrea* 0.14% and *Goniopora* 0.06%.

**The three depth transects show a mid-depth cover peak with the lead genus changing hands at every station, and — the more important finding — algal cover never falls below 67.8% anywhere on the profile.** D2, the shallowest station sampled, carries the lowest coral cover of the three at 12.7% against the profile's highest algal cover, **73.0%**; *Acropora* and *Pocillopora* are exactly tied at 3.33% of the benthos apiece (26% of that transect's coral each), ahead of *Stylophora* 1.67%, *Porites* 1.33%, *Hydnophora* 0.67% and *Montipora* 0.67%. Cover then rises to its profile maximum at D3 — **20.2% coral against 69.3% algae — and the assemblage becomes *Pocillopora*-led**, that genus reaching 6.00% of the benthos (30% of the transect's coral), with *Porites* 3.83%, *Acropora* 2.83%, *Stylophora* 1.67%, *Favites* 0.83% and *Pavona* 0.83% behind it. At D4, the deepest station, cover falls back to 13.3% against 67.8% algae and **the lead changes again, to *Porites* at 3.90%** (29% of that transect's coral), ahead of *Acropora* 3.40%, *Stylophora* 2.20%, *Pocillopora* 2.00%, *Astreopora* 0.40% and *Favites* 0.40%. **The site-level ordering — *Pocillopora* first, *Acropora* second, *Porites* third — therefore describes no single transect**: it averages a shallow station where *Acropora* and *Pocillopora* tie, a mid station where *Pocillopora* runs away with it, and a deep station where *Porites* leads. What does hold at every depth is the algae: 73.0 / 69.3 / 67.8%, a band of nine percentage points and never below two-thirds of the benthos. This is an algae-dominated bank reef top to bottom, not a reef with an algal shallow zone.

The photographs — all 19 stills in the folder — corroborate that reading directly and are among the clearest visual counterparts to a low-cover, high-algae figure in the Chagos set. The habitat is a broad, low-relief carbonate pavement plain with almost no vertical structure: **P3250461**, **P3250462** and **P3250474** are wide views over pale, pink-and-white encrusted hardground on which individual colonies are small, discrete and widely spaced, nothing touching its neighbour, and much of the surface reads as sediment-veiled encrusting growth rather than clean rock — the visual signature of the 31.6% turf-plus-sediment figure. **P3250450** shows the same ground at closer range, small tan corymbose *Acropora*-type colonies and low domal colonies scattered on pavement with pink and magenta coralline crust between them. Close-range frames pick up the leading genera: **P3250452** is a near-vertical view of dark olive verrucose clumps consistent with *Pocillopora*, spreading over white pavement, and **P3250476** shows pale cream-and-brown digitate branchlets alongside a smooth olive-brown digitate colony, consistent with the *Acropora*/*Stylophora* pairing in the transects. **P3250453** looks down a low slope to a ruffled foliose colony consistent with *Pavona* or *Turbinaria*, with a black antipatharian bush beside it, over ground that is otherwise pale rubble. **P3250481** is the one genuinely three-dimensional structure in the set — a low staghorn/tabular *Acropora* thicket standing clear of the pavement, sheltering soldierfish (*Myripristis*) beneath —. Growth-form calls from photographs are given as "consistent with"; the genus percentages above are transect identifications.

**Condition reads as good, and the photographs give no reason to revise the published bleaching figure upward.** That figure is **4.48%, ranking 74th of 98 Chagos sites carrying a value — in the lower half of bleaching severity for the archipelago** — and the 19-frame review found no colony with unambiguous, colony-scale tissue pallor. The pale branchlets in **P3250476** and the white apical tips on the branching colony in **P3250451** are the only candidates, and both are better explained as normal pale growing tips over pigmented tissue than as bleaching; they are recorded as ambiguous rather than counted. **No crown-of-thorns starfish, no *Drupella* aggregation, no disease lesion or tissue-loss margin, and no predation scarring was seen in any of the 19 frames.** The pale exposed hardground here is old, well-encrusted pavement — pink and lavender coralline crust, not fresh skeleton — and should not be read as recent mortality.

Fauna is conspicuous despite the thin coral. A **whitetip reef shark** is documented resting under a yellow-green plating *Porites*-type colony across three consecutive frames (**P3250477**, **P3250478**, **P3250479**), and dense schooling planktivores dominate the water column in a third of the set: blue-and-yellow fusiliers streaming low over the pavement in **P3250461**, **P3250462** and **P3250480**, a heavy cloud of purple anthias in **P3250463**, and loose schools of larger fusiliers in the two closing frames **P3250484** and **P3250485**, which are essentially water-column shots with little benthic content. A black feather star (crinoid) sits on pavement in **P3250450**, an antipatharian bush in **P3250453**, and butterflyfish, damselfish and soldierfish appear through the set. The fish life at BICC51 is markedly out of proportion to its coral cover.

<b>Assemblage composition.</b> Formal transects give 15.4% coral against 70.0% algae — the highest algal cover of the 109 Chagos sites and the 98th-ranked coral cover — across 18 nonzero genera divided unusually evenly, with *Pocillopora* (3.8%), *Acropora* (3.2%), *Porites* (3.0%) and *Stylophora* (1.8%) each holding between 12 and 25% of the coral present. The depth profile peaks at mid-depth (12.7 → 20.2 → 13.3% coral from D2 to D4) with the lead genus changing at every station, while algal cover stays between 67.8 and 73.0% throughout. The photographs document exactly that: a flat, pale, pink-encrusted pavement plain carrying small, widely spaced *Pocillopora*-, *Acropora*- and *Porites*-type colonies, a single conspicuous staghorn thicket, a resting whitetip reef shark, and heavy schooling fish traffic — with no bleaching, disease, predation or COTS visible in any of the 19 stills reviewed.

**Flags:** (1) **NO MULTI-METHOD ROW EXISTS FOR THIS SITE.** BICC51 is absent from `chagos_multimethod_cover_by_site.csv` — no CoralNet points, no Coralscapes mask, no `n_photos` subset. The 15.4% transect figure is the only cover number for this site; do not pair it with the folder frame-count (19) as a CV sample size. (2) **THE LOW TURF RANK IS MISLEADING AND SHOULD NOT BE QUOTED ALONE.** `PTTS` (Percent Turf) is 3.03%, 104th of 109 — but the separate `PTSTS` (Percent Turf+Sediment) category is **31.6%, the largest single benthic component at the site**. Any analysis that treats PTTS as "turf cover" will rank BICC51 among the least turfy reefs in Chagos when it is in fact the most algae-dominated. Worth resolving corpus-wide: the two turf categories need to be reported together or the ranking on either alone is uninterpretable. (3) **Highest algal cover in the archipelago: 70.0%, 1st of 109.** Paired with 15.4% coral (98th of 109). Both ranks are from `CHAGOS_CENTRAL_RANKS.json`. (4) **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. (5) **The bleaching rank (74th of 98) is approximate.** The `Mean(%bleaching)` column carries at least one known bad value — BITB58 is listed at 22.22% against a corrected 0.77% — so the ordering of that column has not been independently audited. (6) **No COTS, no disease, no predation scarring, and no unambiguous bleaching was observed across the full 19-frame review** — an explicit absence-of-evidence finding, not an omission. Two frames (P3250451, P3250476) show pale branch tips read as normal growth rather than pallor; recorded as ambiguous, not as bleaching. (7) **SITE-CODE / FILE-KEY MISMATCH.** The site is `BICC51` in the benthic tables, the photo folder is `BiTb51-finished`, and `chagos_site_to_page.json` keys the CRD page under **`BITB51`** — there is no `BICC51` key in that file. CC and TB are both Three Brothers / Central Chagos and resolve by number, but a lookup on the literal site code returns null and will silently drop the page link. Same trap applies to BINI80/BiGc80. (8) **Exposure was revised between fields.** `exposure-original` records this site as **leeward**; `exposure-revised` gives **intermediate**, and the header above uses the revised value. Both are recorded; the revision is documented in the source table rather than being a conflict between sources, but downstream users pulling the original column will get a different class. (9) **No D1 transect exists** — the profile starts at D2, so the shallowest reef zone at this site is unsampled, and the site-level figures represent D2–D4 only (`N Rows` = 3). (10) **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 29 m in the header is the site-level survey depth, not the depth of any one transect. (11) **Two of the 19 frames (P3250484, P3250485) are water-column fish shots** with negligible benthic content — relevant if this folder is ever fed to a CV cover pass, since frames dominated by open blue water are where human/model agreement is weakest corpus-wide. (20) **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 **4.48%, rank 82 of 110** (1st training: 4.48%, rank 82 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 BICC51 benthic survey data (CSV)](../benthic_data/BICC51_benthic.csv) — scleractinian cover by genus with full names, plus substrate and algal categories (bare substrate, CCA, turf, turf+sediment, macroalgae, erect algae, invertebrates, cyanobacteria), from the LOF BIOT-1 survey (Mayfield & Dempsey 2026, *Oceans*, supplemental file S1).