### BIDI13 — Danger Island
**Date:** 2015-03-13
**GPS:** 6.46106°S, 71.24386°E
**Frames reviewed:** 16 of 16 (100% of the site folder, `BiDi13-finished`)
**Coral cover:** 7.8% (formal transect surveys) · **% bleached:** 2.6% (CoralNet analysis of informal survey images; Mayfield & Dempsey 2026, Oceans)
**Live hard coral cover across methods** (n=16 photos): transect 7.8% · CoralNet points 13.8% · Coralscapes 12.3%. Deviation vs transect **+4.5 to +6.0 pp** — both photo methods read HIGH, and crucially **they agree closely with each other**. **Mean of 3: 11.3% ± 3.1.** (At low cover the photo methods are expected to read high — see the assemblage note; this site is the well-behaved version of the pattern that BIDI12 violates.)  
**Coral genera identified:** 6 scleractinian — *Acropora* (tabular and plating), *Porites* (massive domes), *Pocillopora*/*Stylophora* (branching, not separable), *Montipora*/*Turbinaria* (foliose), *Astreopora*, plus encrusting faviids. Growth-form calls from photographs, given as "compatible with". Coral is sparse and scattered throughout — the low genus count reflects a genuinely impoverished, eroded reef.
**Representative photos:** P3130358.jpg, P3130388.jpg, P3130355.jpg, P3130357.jpg, P3130391.jpg

BIDI13 is the second consecutive Danger Island site whose defining feature is **the near-absence of reef framework and the dominance of algae**, and taken with BIDI12 it turns a one-off observation into a pattern. The reef here is **low-relief, heavily eroded pavement** — P3130347 shows dark algal-crusted hardground blotched with bare white rock, carrying one small tabulate *Acropora* and scattered rubble across the whole frame, and P3130368 and P3130382 show the same substrate with widely spaced plating *Acropora*, brown domal *Porites* and a great deal of open white rubble. Coral is present but never forms a continuous framework, which is exactly what the 7.8% transect figure says.

**Two frames, P3130388 and P3130389, show a dense brown fleshy macroalgal bed** — olive and brown blades covering the seafloor over pale rubble with only sparse coralline patches showing through. This is a different alga from BIDI12's bright-green *Caulerpa*-type meadow (it reads as a brown alga, compatible with *Sargassum*, *Turbinaria* the alga, or *Lobophora*), but the ecological signal is the same: **on this stretch of the Great Chagos Bank western rim, two adjacent sites surveyed a day apart are both substantially macroalgae-dominated.** For a remote, unfished, unpolluted archipelago that is worth Anderson's attention as a regional observation rather than a site quirk.

Where coral persists, the macro frames are the interesting ones. **P3130357** packs a camouflaged grouper compatible with *Epinephelus*, a powder-blue surgeonfish (*Acanthurus leucosternon*), yellow anthias, ruffled yellow-green foliose colonies compatible with *Montipora* or *Turbinaria*, brown massive *Porites* and extensive red-pink coralline into one frame. **P3130367** is a clean close-up of a brown, densely verrucose branching colony — *Pocillopora* or *Stylophora*, not separable here — beside a grey-white massive *Porites*. **P3130381** is the condition frame: two large brown massive colonies with **a dark brown lesioned or dead patch between them**, surrounded by white CCA-crusted substrate, with a single small yellow *Acropora* recruit settled nearby — loss and recruitment in one field of view again.

Bleaching is minimal and the 2.6% figure holds; no unambiguously bleached colony appears in any of the 16 frames.

Faunally, BIDI13 is dominated by **grazers**, which fits an algal-dominated site. Dense schools of surgeonfishes and unicornfishes compatible with *Acanthurus*, *Ctenochaetus* and *Naso* fill P3130347, P3130354 and P3130356, mostly as silhouettes against the surface, and parrotfishes compatible with *Scarus* or *Chlorurus* work the pavement in P3130347 and P3130355. **A whitetip reef shark (*Triaenodon obesus*) cruises the rubble ridge in P3130355** — the second whitetip record after BIDI08. **P3130358 is the best fish photograph of the expedition so far**: an oriental sweetlips, *Plectorhinchus vittatus*, sheltering under a ledge — white with crisp black longitudinal stripes and yellow, black-spotted fins, an unambiguous species-level identification. Most unusually, **three frames record jellyfish**: a small translucent medusa near the substrate in P3130390, a large bell with oral arms drifting past a diver's yellow fins in P3130391, and a third at the surface in P3130393 — the only jellyfish in the Chagos frames reviewed to date.

<b>Assemblage composition.</b> The cover line carries the key numbers, and this site is the useful control for the BIDI12 flag. Here **both photo methods read above the transect (+4.5 and +6.0 pp) but agree with each other to within 1.5 pp** — which is precisely the archipelago-wide pattern documented in CHAGOS_MULTIMETHOD_README.md, where the photo-vs-transect gap is positive at low cover and strongly negative above 40%. BIDI13 is that pattern behaving normally. **BIDI12 is not**: there the two photo methods split by nearly 18 pp (19.5% vs 1.7%), which no cover-dependent bias explains. **So the BIDI12 flag should rest on the photo-method disagreement, not on CoralNet merely reading high** — reading high at low cover is expected, and BIDI13 shows what that looks like when nothing is wrong.

**Flags:** (1) **Second macroalgae-dominated site in a row** (P3130388, P3130389 — a brown alga here, distinct from BIDI12's green *Caulerpa*-type). Two adjacent sites a day apart; treat as a regional observation for the western Great Chagos Bank, not a site quirk. (2) **Refines the BIDI12 flag:** photo methods reading high at low cover is the EXPECTED archipelago pattern (BIDI13 shows it cleanly); what makes BIDI12 anomalous is the ~18 pp disagreement *between* the two photo methods. (3) **P3130358: oriental sweetlips (*Plectorhinchus vittatus*)** — a rare species-level ID from these frames. (4) **Whitetip reef shark** in P3130355, second record after BIDI08. (5) **Three jellyfish frames** (P3130390, P3130391, P3130393) — the only ones in the reviewed set; left unidentified. (6) P3130381 shows a dark lesioned/dead patch between two massive colonies with a coral recruit alongside — cause unresolved. (7) The white disc-shaped objects on the pavement in P3130381 are unresolved — possibly bleached juvenile colonies, molluscs or forams; do not score as coral. (8) No per-frame depth data.

**Benthic data:** [Download BIDI13 benthic survey data (CSV)](../benthic_data/BIDI13_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).