### BIDI08 — GCB-West (Danger Island)
**Date:** 2015-03-12
**GPS:** 6.39417°S, 71.23412°E
**Frames reviewed:** 36 of 36 (100% of the site folder, `BiDi08-meter site-finished`)
**Coral cover:** 59.0% · **% bleached:** 1.7% — **the highest coral cover of any site reviewed**
**Live hard coral cover across methods** (n=16 photos): transect 59.0% · CoralNet points 35.0% · Coralscapes 33.6%. Deviation vs transect -25.4 to -24.0 pp. **Mean of 3: 42.5% ± 14.3.** (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 overwhelmingly dominant, plus corymbose and digitate), *Porites* (massive domes and lobed/columnar colonies), *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:** P3120277.jpg, P3120282.jpg, P3120254.jpg, P3120298.jpg, P3120285.jpg

BIDI08 carries the **highest transect cover in the entire archipelago set at 59.0%**, and it is the one site where the wide frames make that number look conservative rather than generous. The reef is a **tabular *Acropora* plain of exceptional development**: P3120277, P3120278, P3120279, P3120286 and P3120312 show dozens of large olive and gold plates overlapping edge to edge and stepping away to the limit of visibility, with massive *Porites* bommies wedged between them. P3120255 and P3120266 add big yellow-green tables in the shallows. Between the plates the substrate is a pink and mauve crustose-coralline pavement carrying corymbose *Acropora*, knobby and lobed *Porites*, *Pocillopora*, faviids and encrusting *Montipora*. Two macro frames are outstanding: **P3120285**, a large green meandroid brain compatible with *Platygyra* or *Leptoria* with tight sinuous valleys, flanked by pale-green knobby *Porites*; and **P3120257**, a plocoid colony with raised polygonal corallites among golden *Porites* and white-tipped *Acropora*. P3120311 records a distinct habitat — discrete corymbose *Acropora* colonies scattered like garden plantings across clean white sand.

Bleaching is minimal and the 1.7% figure holds up: no bleached colony appears in any of the 36 frames. Two signals are worth logging without over-reading them. **P3120278** shows a large table whose centre has gone pale pink-white across a broad area while its margins remain pigmented, and **P3120288** shows a pale grey-white knobby colony in the foreground. Both read as **localised mortality or partial paling rather than active thermal bleaching**, and on a 12 March dive at a site with 59% cover that is the more plausible interpretation. The extensive bare pink CCA pavement between the tables (P3120294, P3120297, P3120312) is likewise better read as normal reef architecture at this site than as loss — the tables are healthy and the pavement is well consolidated.

**The site's methodological content is the strongest in the Chagos set, and it explains the folder's "meter site" tag.** Three frames show the survey in progress — **P3120280** has a diver holding the white PVC quadrat frame squarely on the reef with a second diver at right; **P3120279** and **P3120290** put divers and the frame over the tabular plain. But **P3120298 is the important one**: a grey cylindrical instrument housing, cable-tied to a mount with two upright black rods, sits deployed on the pavement beside a yellow mesh collecting bag and a blue-and-white sampling tube on a *Porites* bommie. That is **deployed instrumentation, not a hand-held tool** — most consistent with a logger or meter left in place, which would account for this being designated the "meter site". Anyone reconstructing where in-situ instrument records came from should start with this frame.

Faunally the dive is dominated by two things. First, **fusiliers**: P3120259 through P3120269 are an almost unbroken run of blue-and-yellow *Caesio*- or *Pterocaesio*-type schools streaming over the reef, some frames holding well over a hundred fish. Second, **whitetip reef sharks** — **P3120281 and P3120282 are unambiguous *Triaenodon obesus***, slender and blunt-headed with white-tipped dorsal and caudal lobes, one photographed at close range resting on sand beneath a tabular *Acropora*. That is a different animal from the grey reef and silvertip sharks at Egmont, and gives Danger Island a third shark species for the trip. **P3120254 is the single best invertebrate image reviewed so far**: a giant clam compatible with *Tridacna*, mantle fully extended, deep purple-black shot through with iridescent blue-green. Reef fish include raccoon butterflyfish compatible with *Chaetodon lunula* (P3120289, P3120294), powder-blue surgeonfish compatible with *Acanthurus leucosternon* (P3120277, P3120296), a large jobfish or snapper compatible with *Aprion* or *Lutjanus* (P3120284, P3120286), parrotfish, and dense *Chromis* clouds over the plates.

<b>Assemblage composition.</b> The Coralscapes and CoralNet figures are given in the cover line above, and this site produces **the largest photo-vs-transect gap in the archipelago — around -25 pp**. That is worth stating plainly: the two photo methods agree closely with each other (35.0% and 33.6%) and both fall roughly 25 points below the transect. The frames explain why. This reef's cover is carried by **large horizontal tabular plates**, which a downward or oblique photograph foreshortens badly and which mask-based segmentation tends to under-call where plate edges shadow the substrate beneath. It is the clearest single illustration in the Chagos set of the archipelago-wide finding that the photo-vs-transect gap grows with cover, and it should be cited as such rather than treated as a model failure.

**Flags:** (1) **P3120298 documents deployed instrumentation** — a cable-tied cylindrical housing with upright rods, plus collecting bag and sampling tube. Almost certainly the origin of the "meter site" folder name; worth cross-checking against any in-situ logger records for Danger Island. (2) Three quadrat/diver methodology frames (P3120279, P3120280, P3120290). (3) **P3120278 and P3120288 are logged as partial paling / localised mortality, NOT active bleaching** — the site is 1.7% bleached and no frame shows a bleached colony. (4) **Whitetip reef shark (*Triaenodon obesus*) confirmed** in P3120281/P3120282 — a third shark species for the expedition, distinct from the Egmont grey reef and silvertip records. (5) The grey ruffled structure in P3120291 and P3120294 is read as a sponge, not a coral, and should not be counted toward benthic coral cover. (6) The brown lobed colonies in P3120297 could be *Porites rus* or a fleshy mussid; not separable here. (7) This site's -25 pp method gap is the archipelago's largest and is attributed to tabular growth form, not model error. (8) No per-frame depth data.

**Benthic data:** [Download BIDI08 benthic survey data (CSV)](../benthic_data/BIDI08_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).