### BIDI09 — GCB-West (Danger Island)
**Date:** 2015-03-12
**GPS:** 6.37934°S, 71.23359°E
**Frames reviewed:** 31 of 31 (100% of the site folder, `BiDi09-finished`)
**Coral cover:** 37.7% · **% bleached:** 2.4%
**Live hard coral cover across methods** (n=31 photos): transect 37.7% · CoralNet points 24.6% · Coralscapes 23.2%. Deviation vs transect -14.5 to -13.1 pp. **Mean of 3: 28.5% ± 8.0.** (SD here reflects systematic method bias, not precision — photo methods read low at high cover; see CHAGOS_MULTIMETHOD_README.md.)  
**Coral genera identified:** 7 scleractinian + 1 hydrocoral — *Acropora* (tabular dominant, plus corymbose and digitate), *Porites* (massive domes and broad plates), *Platygyra*/*Leptoria*, *Favites*/*Goniastrea*, *Pocillopora*, *Montipora*, *Astreopora*; hydrocoral *Millepora*. Also giant clam compatible with *Tridacna*. Growth-form calls from photographs, given as "compatible with".
**Representative photos:** P3120357.jpg, P3120320.jpg, P3120352.jpg, P3120328.jpg, P3120339.jpg

BIDI09 sits immediately north of BIDI08 on the same Great Chagos Bank western rim and shares its architecture — a **tabular *Acropora* plain over consolidated pavement** — but at 37.7% rather than 59.0% cover, and the full 31-frame pass shows why. The dive's first two-thirds cross genuinely excellent reef: P3120314, P3120315, P3120319, P3120320 and P3120352 show large olive and gold tables overlapping across the whole frame, with massive *Porites* wedged between them and, in P3120352, a fine green meandroid brain compatible with *Platygyra* or *Leptoria* in the foreground. P3120316 and P3120322 are dominated by broad, smooth, pale-green **massive *Porites* plates** carrying purple and crimson encrusting sponges — a growth form more prominent here than at any other Danger Island site. But the last three frames are a different reef entirely: **P3120365, P3120382 and P3120383 show pale rubble and sand plains carrying only small, widely spaced corymbose and digitate colonies**, with bare substrate dominating. That end-of-dive zone, not model error, is a large part of what pulls the site mean down from its neighbour's.

The single most diverse benthic frame is **P3120328**, a mid-slope pavement packed with meandroid brains, *Pocillopora*, purple digitate *Acropora*, faviid domes, pink crustose coralline algae and encrusting sponges — with a **peacock grouper compatible with *Cephalopholis argus***, blue-spotted and unmistakable, resting at the left of frame. Bleaching is minimal, consistent with the 2.4% figure: **P3120317 is the only clear case**, a bone-white branching colony on the reef below a rudderfish school. The bare white sand patches recurring through P3120329, P3120356 and P3120359 are sediment, not bleached coral, and should not be scored as such.

**This site is the best-documented in the archipelago set for survey-methodology imagery — eight frames.** P3120357 and P3120359 show a diver in yellow fins carrying and then placing a large white PVC quadrat frame over the tabular plain, with dense fusilier schools filling the water. P3120343 catches a diver deploying a transect reel with a slate. P3120361 shows a diver with camera alongside a white transect bar laid across the reef. P3120337 and P3120326 show divers running line at depth, and **P3120339 has the transect bar visible in-frame beside a large giant clam compatible with *Tridacna***, its fluted shell wedged among *Porites* plates. Together with BIDI08's deployed instrument and BIEG05's quadrat series, Danger Island now has a near-complete photographic record of how these numbers were produced.

Faunally BIDI09 is defined by **sheer planktivore biomass**. Fusiliers compatible with *Caesio* and *Pterocaesio* — brilliant blue with dark dorsal stripes — fill at least fourteen frames (P3120329 through P3120361), in schools of many hundreds that in P3120340 and P3120338 obscure the reef behind them. Layered above and among them are large dark-bodied rudderfish or snappers compatible with *Kyphosus*, *Macolor* or *Lutjanus* (P3120317, P3120319, P3120323, P3120324), and **blue-striped snapper compatible with *Lutjanus kasmira*** — bright yellow with electric blue lines — appear in P3120329, P3120337 and P3120343. Larger predators include a grey jobfish compatible with *Aprion virescens* (P3120353), a red-orange hogfish or wrasse compatible with *Bodianus* (P3120355), red bigeyes compatible with *Priacanthus*, Moorish idols and a powder-blue surgeonfish. The overall impression — thousands of planktivores, multiple large predators, no fishing pressure — is exactly what an unfished remote atoll should look like.

<b>Assemblage composition.</b> The Coralscapes and CoralNet figures are given in the cover line above; at 24.6% and 23.2% they agree closely with each other and sit ~14 pp below the transect, the same systematic offset as the Egmont sites and about half the -25 pp seen at BIDI08. That difference between the two neighbouring Danger Island sites is itself informative: BIDI08's cover is carried almost entirely by large horizontal tables (badly foreshortened in photographs), whereas BIDI09 mixes tables with massive *Porites* and low-relief pavement that photographs capture more faithfully.

**Flags:** (1) **Eight methodology frames** — P3120326, P3120337, P3120339, P3120343, P3120357, P3120359, P3120361 (quadrat, transect reel, transect bar, divers working). The most complete methods set in the archipelago. (2) **P3120317 is the only clear bleached colony**; the site is 2.4% bleached and the imagery agrees. (3) **The white sand patches in P3120329/P3120356/P3120359 are sediment, not bleaching** — flagged explicitly because they are the kind of feature a mask model may misclassify. (4) The last three frames (P3120365, P3120382, P3120383) document a genuinely low-cover rubble zone; the site is not uniform and its mean averages two states. (5) Peacock grouper (*Cephalopholis argus*-type) in P3120328 and giant clam (*Tridacna*) in P3120339. (6) The dark-bodied schooling fish are left at family level — *Kyphosus* vs *Macolor* vs *Lutjanus* cannot be separated in these silhouettes. (7) No per-frame depth data; zonation inferred from framing and light.

**Benthic data:** [Download BIDI09 benthic survey data (CSV)](../benthic_data/BIDI09_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).