### BICC36 — Central Chagos
**CRD page:** [Three Brothers & Central Chagos Coral Surveys (Mar 17-20, 2015)](https://coralreefdiagnostics.com/three-brotherscentral-chagos-20150317-to-0320)

**20 Mar 2015**
**GPS:** 6.23205°S, 71.58827°E
**Setting:** intermediate-exposure submerged lagoonal bank reef, forereef aspect outside the lagoon, 34.4 m
**Coral cover:** 5.8% (13 formal transects across 2 depth strata, D3–D4; **algal cover 60.4%**) · **published image bleaching:** 6.4%
**Live hard coral cover across methods:** formal transects 5.8% · CoralNet points 13.1% · Coralscapes masks 7.7% (15 frames). These products have different sampling footprints and denominators and should not be averaged.
**Temperature:** 29.5 °C · **Salinity:** 34.4
**Photo review:** all 15 of 15 frames examined (single dive, 20 Mar 2015, 14:37–15:04 UTC by file `DateTimeOriginal`; folder `BiTb36-finished` still carries the old Three Brothers code)
**Representative photos:** P3200455.jpg, P3200454.jpg, P3200430.jpg, P3200432.jpg, P3200439.jpg

BICC36 is a deep submerged bank reef in Central Chagos — 34.4 m, forereef aspect but outside the lagoon, intermediate exposure — and it is **one of the lowest-cover sites in the whole archipelago: 5.8% coral against 60.4% algae, ranking 106th of 109 Chagos sites for coral cover and 6th of 109 for algal cover.** The substrate breakdown explains what "algae" means here and it is not a weedy reef: crustose coralline algae (PCCATS) alone accounts for **25.9% of total benthos, 8th-highest of 109 sites**, and turf-plus-sediment (PTSTS) for a further 24.6%, while fleshy turf on its own (PTTS) is just 4.85% — **96th of 109, i.e. among the lowest turf values recorded.** Macroalgae (PMTS) is 3.5% and erect algae (PETS) 1.6%. Non-coral invertebrates (PITS) take another 25.8% and bare substrate (PB) 7.9%. The four site-level components — coral, algae, invertebrates and bare rock — sum to exactly 100.0%, so nothing is unaccounted for at the site level. This is a hard, well-grazed, coralline-encrusted deep pavement carrying a very thin veneer of living coral, not a degraded algal reef.

Genus richness is correspondingly low: **11 nonzero genera, 104th of 109 Chagos sites** — and with so little coral to divide, the assemblage is inevitably concentrated. The 11 genus percentages sum to 5.8000% of the benthos, an exact match to the published cover figure. All values below are percent of total benthos, not percent of coral. *Acropora* leads at 1.78% of the benthos (31% of the coral present), then *Porites* 1.52% (26%), *Pocillopora* 1.21% (21%) and *Montipora* 0.49% (8%) — those four hold 86% of all coral recorded. The tail is *Leptastrea* 0.19%, *Favites* 0.18%, *Turbinaria* 0.18%, *Galaxea* 0.10%, *Hydnophora* 0.06%, *Coscinaraea* 0.05% and *Goniastrea* 0.03%. Note the absolute scale: the site's *leading* genus occupies under two points of benthos, so ratios between genera here rest on very few points and small differences should not be over-read.

**The two depth strata are the real finding at this site, and they run in opposite directions for coral and for coralline algae.** D3 (11 transects, the shallower stratum sampled) carries 9.8% coral against 63.5% algae, and the coral there is a recognisable four-genus assemblage: *Acropora* 3.36% of the benthos (34% of that transect's coral), *Porites* 3.04% (31%), *Pocillopora* 1.53%, *Montipora* 0.78%, *Turbinaria* 0.36% and *Galaxea* 0.20%. Its algal budget is turf-and-sediment dominated — PTSTS 37.8%, PTTS 8.0%, PMTS 4.5%, PETS 3.0% — with CCA only 10.2%. **D4, the deeper stratum (2 transects only), collapses to 1.8% coral — less than a fifth of the shallower value — while crustose coralline algae quadruples to 41.6% of benthos and turf-plus-sediment falls to 11.3%.** The genus order also inverts: *Pocillopora* becomes the leading genus at 0.90% (half of all coral on that stratum), with *Acropora* down to 0.20%, level with *Favites*, *Leptastrea* and *Montipora* at 0.20% each and *Coscinaraea* at 0.10%; those six values account for the full 1.80%. **The site-level figure of 1.78% *Acropora* therefore describes neither stratum**: it averages a shallower station where *Acropora* is the clear leader at 3.36% against a deeper one where it is a trace taxon and the ground has become a coralline-algal pavement. The D4 result rests on just two transects and should be treated as indicative rather than well replicated.

The photographs — all 15 frames — corroborate the transects closely and show two distinct habitats. The wide shots are dominated by pale, low-relief limestone pavement and coarse sand with only scattered small colonies: P3200429 and P3200455 are almost coral-free, showing a green-tinged turf-and-sediment film over broken pavement with isolated centimetre- to decimetre-scale colonies and a few dark wire corals rising into blue water; P3200431 and P3200444 show the same open ground with small encrusting and submassive colonies and a scatter of small tabular/plate forms. P3200430 documents the drop-off itself — a steeply inclined, largely bare wall crusted pink-purple with coralline algae, carrying gorgonian sea fans, sea whips and antipatharian bushes rather than scleractinians, which is the visual counterpart of the CCA-heavy, coral-poor substrate signal. The one genuinely coral-abundant frame in the set is **P3200454**, a shallower patch where branching, digitate and corymbose colonies consistent with *Acropora*, plus knobby verrucose clumps consistent with *Pocillopora*, form a discontinuous field over pavement — a fair picture of the D3 stratum and a reminder that cover at this site is patchy rather than uniformly sparse. P3200432 is a close-up mixing an olive-brown verrucose colony consistent with *Pocillopora* against a large pale grey-lavender encrusting/foliose sheet, with pink coralline crust visible between them. Water clarity is good and the ambient light is dark blue throughout, consistent with the 34.4 m survey depth.

**Condition reads as unremarkable from both sources.** The published image-based bleaching figure is **6.4%, ranking 58th of 98 Chagos sites carrying a value — mid-pack** — and the 15-frame review found no colony with unambiguous, colony-scale tissue pallor. The large pale grey-lavender sheet in P3200432 is the only structure in the set light enough to raise the question, and it is flagged rather than read as bleaching: pale-lavender pigmentation is normal in several encrusting genera and a single strobe-lit still cannot separate that from pallor or from an exposed skeletal surface. **No crown-of-thorns starfish, no *Drupella* aggregations, no disease lesions or tissue-loss margins, and no predation scarring were seen in any of the 15 frames** — an explicit absence-of-evidence finding rather than an omission.

Macrofauna, by contrast, is the strength of this photo set and is dominated by cryptic and mobile invertebrates rather than by fish — appropriate for a site where non-coral invertebrates hold 25.8% of the benthos. Two frames (P3200423, P3200427) document a spiny lobster tucked under a ledge, its banded antennae and legs clearly visible against red and orange encrusting sponge; the ledge itself is a good illustration of the sponge-and-crust invertebrate cover in the substrate data. Three frames (P3200438, P3200439, P3200440) record a large octopus wedged into the same kind of undercut, dark red-brown and fully alert, framed against a cream barrel-type sponge and red encrusting sponge. P3200442 and P3200443 are two close-ups of a single large cowrie with the mantle fully extended over the shell — cream and tan reticulation edged with black papillae — grazing across the corallite-textured surface of a live massive coral. P3200425 is a shoal of glassfish/cardinalfish-type small silvery fishes swarming over a black-coral bush and sponge-covered overhang. Reef fish in the open-ground frames are limited to scattered blue-and-yellow damsels and anthias (P3200444, P3200454) and a distant diver silhouette in P3200444.

<b>Assemblage composition.</b> Formal transects give 5.8% coral against 60.4% algae across only 11 nonzero genera, led by *Acropora* (1.78% of benthos), *Porites* (1.52%), *Pocillopora* (1.21%) and *Montipora* (0.49%), which together hold 86% of the coral present. Two depth strata split the site sharply: D3 at 9.8% coral with a four-genus *Acropora*/*Porites* assemblage over turf-and-sediment ground, and D4 at 1.8% coral where *Pocillopora* leads a trace assemblage over a 41.6% crustose-coralline pavement. The photographs match — mostly bare, coralline-crusted pavement, sand and a gorgonian-and-black-coral drop-off, punctuated by one genuinely coral-abundant *Acropora*/*Pocillopora* patch (P3200454) — with no bleaching, disease, COTS or predation evident, and a notably strong cryptic-invertebrate fauna (lobster, octopus, cowrie, glassfish shoal).

**Flags:** (1) **SITE-CODE ALIAS — this site is filed under two codes.** The formal dataset carries it as **BICC36**, island and region "Central Chagos"; the photo folder (`BiTb36-finished`) and the multi-method table both key it as **BITB36**, a Three Brothers code. The same alias affects BICC35/50/51 and BIGC80 (= BINI80). All numbers in this shard are read from the BICC36 rows of the published tables and the multi-method row is read from BITB36. (2) **A pre-existing thin shard for this site already exists as `shards/desc_BITB36.md` (2.8 KB) and points at the WRONG CRD page** — the Mar 25–28 Three Brothers page, for a dive dated 20 Mar 2015. This shard uses the Mar 17–20 page, consistent with the site's date and with `chagos_site_to_page.json`'s BITB36 entry. The stale duplicate should be reconciled or retired before publishing, so that only one description reaches `crd_dive_sites.json`. (3) **Multi-method deviation runs in the UNUSUAL direction here.** CoralNet points (13.1%) and Coralscapes masks (7.7%) both read **HIGHER** than the formal transect figure (5.8%; deviation +1.9 to +7.3 pp), where the large majority of Chagos sites have the CV products reading low. This is consistent with the known behaviour of CoralSCOP-type mask models over-reading low-cover reefs. Do not average the three figures; the transect value is treated as ground truth for cover. (4) **The multi-method row's `note` field reads "formal alias BICC36; separate denominators"** — the denominators for the CV and transect products are not the same and the deviation should not be read as a measurement error. (5) **The CoralNet points figure is a CV product, not verified ground truth**; point-annotation data corpus-wide is roughly 76% unconfirmed machine output. (6) **`chagos_pct_bleached_recomputed.csv` carries a recomputed value for this site (7.81% from 5 bleached of 64 coral points across 450 points in 15 images) with `status = no_value_in_master`** — because the master markdown keys the site as BITB36, not BICC36. The recomputed figure is close to the published 6.4% but is **not** used here: the published column is authoritative and bleaching is never recomputed from point counts. The blank-lookup status is a keying artefact of the alias in flag 1, not a missing value. (7) **D4 rests on 2 transects only** (against 11 at D3), so the 1.8% deep-stratum cover and the *Pocillopora*-led deep genus order are weakly replicated. (8) **Depth codes D3–D4 carry no metre boundaries** in `Chagos benthic data by site-depth (n=336 rows).csv`; they are ordered shallow-to-deep strata, and the 34.4 m in the header is the site-level survey depth, not the depth of any one transect. **No D1 or D2 stratum exists for this site**, so the shallowest reef zone is unsampled. (9) **The pale grey-lavender encrusting/foliose sheet in P3200432** is the only pallor candidate in the set and is ambiguous from a single still — recorded as a caveat, not as bleaching. (10) **No COTS, disease, *Drupella* or predation scarring was observed** across the full 15-frame review. (11) **Cryptic macrofauna records worth keeping:** spiny lobster (P3200423, P3200427), a large octopus (P3200438–440), a large cowrie with mantle extended over live coral (P3200442–443), glassfish/cardinalfish shoal over black coral (P3200425), plus gorgonian sea fans, sea whips and antipatharian bushes on the drop-off (P3200430). (12) **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. (13) **Ranks off the `Mean(%bleaching)` column are approximate** — that column carries at least one known bad value (BITB58 listed at 22.22% against a corrected 0.77%), so the 58th-of-98 placement has not been independently audited. (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 **6.45%, rank 66 of 110** (1st training: 6.45%, rank 65 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 BICC36 benthic survey data (CSV)](../benthic_data/BICC36_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).