### BIVB81 — Victory Bank
**CRD page:** [Blenheim, Nelson Island & Victory Bank Coral Surveys (Apr 2015)](https://coralreefdiagnostics.com/new-page-4)

**21 Apr 2015**
**GPS:** 5.53439°S, 72.21585°E
**Setting:** protected (leeward) submerged rim reef, inside the lagoon, 29.4 m
**Coral cover:** 51.7% (3 depth transects, D2–D4; **algal cover 44.3%**) · **published image bleaching:** 5.1%
**Live hard coral cover across methods:** formal transects 51.7% · CoralNet points 32.7% · Coralscapes masks 36.6% (5 frames). These products have different sampling footprints and denominators and should not be averaged.
**Temperature:** 29.7 °C · **Salinity:** 34.4
**Photo review:** all 5 of 5 frames examined
**Representative photos:** P4210851.jpg, P4210850.jpg, P4210848.jpg, P4210849.jpg, P4210852.jpg

BIVB81 sits on the submerged rim of Victory Bank, inside the lagoon and on the protected side, at 29.4 m — a deep, sheltered station rather than a wave-worked forereef. It carries **well above-average coral cover for the archipelago: 51.7% coral against 44.3% algae, ranking 22nd of 109 Chagos sites for coral cover, while its algal cover ranks 36th of 109 — high in absolute terms.** What makes the algal budget unusual is its composition. Of the 44.3 percentage points of algae, **22.8 pp is sediment-laden turf (PTSTS) and 11.8 pp is macroalgae (PMTS), while crustose coralline algae accounts for only 4.8% of the benthos (103rd of 109 Chagos sites) and clean fleshy turf for just 3.9% (101st of 109).** Bare substrate is low at 3.0% and non-coral invertebrates at 1.1%. This is the inverse of the well-grazed, coralline-pavement signature seen on the exposed forereefs: at BIVB81 the non-coral surface is soft, sediment-trapping turf and fleshy macroalgae on a low-energy lagoonal bank, with almost no coralline pavement. The five substrate/algal categories sum exactly to the published 44.25% algal cover.

Coral cover is concentrated in remarkably few taxa. **Only 17 nonzero genera are recorded — 88th of 109 Chagos sites for genus richness, in the bottom quintile** — and the 17 genus percentages sum to 51.6667% 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 overwhelmingly at 30.17% of the benthos — **58% of all the coral at the site** — followed by *Porites* 8.75% (17%), the hydrocoral *Millepora* 4.92% (10%) and *Pocillopora* 2.83% (5%). **Those four genera hold 90% of all coral recorded**, and the remaining thirteen together contribute only 5.0 pp of benthos: *Echinopora*, *Favia*, *Platygyra* and *Psammocora* at 0.67% each, *Favites* 0.50%, *Montipora* and *Stylophora* 0.42% each, *Leptastrea* 0.33%, *Galaxea* 0.25%, *Astreopora* 0.17%, and *Cyphastrea*, *Fungia* and *Pavona* at 0.08% each. Note that *Millepora* is a hydrocoral rather than a scleractinian but is carried in the LOF genus columns and therefore in the cover total.

**The three depth transects are the real finding here: cover dips at mid-depth and then more than doubles at the deepest station, which is almost a monoculture of *Acropora*.** D2, the shallowest station sampled, carries 43.0% coral against 48.5% algae, and it is the one transect where *Acropora* does **not** dominate — *Acropora* 13.50% and *Porites* 13.00% are effectively tied (31% and 30% of that transect's coral respectively), with ***Millepora* at 11.75%, 27% of the coral on that transect and by far its peak anywhere in the profile** (the site-level 4.92% is the mean of 11.75, 3.00 and 0.00). *Psammocora* 1.75%, *Favia* 1.50% and *Stylophora* 0.75% follow. At D3 cover falls to its profile minimum of 34.0% while algae rises to 64.25%, the highest algal value in the profile; *Acropora* 12.00% (35% of that transect's coral) leads, then *Pocillopora* 8.25% — 24% of the coral there and again its peak in the profile — with *Porites* 5.25%, *Millepora* 3.00%, *Montipora* 1.25% and *Leptastrea* 1.00%. Then D4, the deepest station, **jumps to 78.0% coral against only 20.0% algae, and *Acropora* alone reaches 65.00% of the benthos — 83% of all the coral on that transect** — with *Porites* 8.00%, *Echinopora* 2.00%, *Platygyra* 2.00% and *Favites* 1.00% making up nearly all the rest, and *Millepora* falling to zero. **The site-level figure of 30.17% *Acropora* describes no single transect**: it averages two shallower stations at 12–14% against a deep station at 65%. Anyone reading the site row alone would badly understate how completely *Acropora* takes over the deep rim.

The photographs — all five frames in the folder — split cleanly along that same axis and corroborate it. P4210848 and P4210849 show the low-relief, mixed, sediment-dusted ground: encrusting and small massive colonies, brain-patterned colonies consistent with *Platygyra*, a conspicuous barrel/vase sponge with a purple oscular rim, open sand-and-rubble pockets, purple-pink coralline crusts on dead framework, and fleshy and filamentous algal turf draped over much of the surface — a visual match for the high PTSTS/PMTS reading and for the diverse but individually minor genus tail. Tabular and yellow-tinged branching colonies consistent with *Acropora* occupy the mid-ground of P4210848. P4210850 then opens onto an extensive field of overlapping tabular plates — dozens of tables in one frame, consistent with the *Acropora* dominance of the deeper transects. **P4210851 and P4210852 document something different again: a near-continuous, apparently monogeneric stand of tiered, whorled foliose coral running to the limit of visibility**, one of the more striking single-species stands in the Chagos photo set, with a sand channel breaking it on the right of P4210852. Water clarity in these two frames is poor (strong blue-grey veiling, minimal strobe return), consistent with a deep, sheltered, sediment-loaded lagoonal setting. Fish are present but sparse — a butterflyfish in P4210849, a surgeonfish silhouette and scattered damselfish in P4210850 — and the set is not standardised for fish abundance.

**Condition reads as broadly good, with a caveat about what the pale colonies in two frames actually are.** The published image-based bleaching figure is **5.1%, ranking 69th of 98 Chagos sites with a value — in the lower half of bleaching severity for the archipelago**. Consistent with that, most colonies across the five frames carry normal olive, tan, yellow-green or brown pigmentation. However, P4210848 and P4210849 each contain several conspicuously white elements: fine, bushy, white branching colonies and, in P4210849, a smooth pale-white massive/lobate colony and a whitish foliose plate at upper left. These are recorded as ambiguous rather than as confirmed bleaching — from a single still they cannot be separated from naturally white hydrozoan or hydrocoral growth, from exposed dead skeleton, or from strobe over-exposure on a pale colony — and no site bleaching figure is recalculated from them. No disease lesions or tissue-loss margins, no *Drupella* aggregations, no parrotfish bite scarring, and no crown-of-thorns starfish or COTS feeding scars were seen in any of the five frames.

<b>Assemblage composition.</b> Formal transects give 51.7% coral against 44.3% algae — the algae being overwhelmingly sediment-laden turf and macroalgae rather than coralline pavement — across just 17 nonzero genera, of which *Acropora* (30.2%), *Porites* (8.8%), *Millepora* (4.9%) and *Pocillopora* (2.8%) hold 90% of all coral. The depth profile runs 43.0% cover at D2 (where *Porites* and *Millepora* are co-important), down to 34.0% at D3, then up to 78.0% at D4 where *Acropora* alone accounts for 83% of the coral present. The five photographs match that range directly: sediment-dusted mixed low-relief ground with a barrel sponge and open sand in the first two frames, an extensive tabular *Acropora* field in the third, and a near-continuous whorled foliose stand in the last two.

**Flags:** (1) **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 5.1% as the weaker of the two published numbers and never read it as a diver's field estimate. (2) **Any rank taken off the `Mean(%bleaching)` column is approximate.** That column carries at least one known bad value (BITB58 listed at 22.22% against a corrected 0.77%), so the 69th-of-98 placement has not been independently audited. (3) **Multi-method deviation is moderate but one-directional:** CoralNet points 32.7% and Coralscapes masks 36.6% sit 19.0 and 15.1 pp below the formal transect figure of 51.7%. Do not average the three; the transect is treated as ground truth for cover. (4) **The CV/photo figures rest on a 5-frame scored subset**, which happens to equal the folder count here — a coincidence that does not make the denominators comparable. Five frames is an extremely thin photographic basis for a site-level image estimate. (5) **`coralnet_points_pct` is documented as a human 30-point read, not a model prediction** (`CHAGOS_MULTIMETHOD_README.md`), but only roughly a quarter of Chagos image annotations corpus-wide are Anderson-confirmed; the remainder is unconfirmed machine output. Do not present 32.7% as verified ground truth. (6) **The large whorled foliose stand in P4210851–852 is not represented in the transect genus list.** The plausible candidates — foliose *Echinopora* (0.67% of benthos site-wide) and *Montipora* (0.42%) — are both minor in the formal data, so either the transects missed this stand or the stand is a genus not resolved from the imagery. Image-based genus calls are tentative and no genus is asserted here; worth a targeted re-look if these frames are ever point-annotated. (7) **Several bright-white branching colonies and one pale massive colony (P4210848, P4210849) could not be resolved** as bleaching versus naturally white hydrozoan growth, exposed skeleton or strobe over-exposure. Logged as ambiguous, not counted as bleaching. (8) **No COTS, disease or predation scarring was observed** across the full 5-frame review — an explicit absence-of-evidence finding, not an omission, but note that five frames is far too small a sample to support a site-level absence claim. (9) **Formal replication is thin: `N Rows` = 3**, and **there is no D1 transect** — the shallowest reef zone at this site is unsampled and the profile starts at D2. (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 29.4 m in the header is the site-level survey depth, not the depth of any one transect. (11) **Genus richness is low (17 genera, 88th of 109) while cover is high (22nd of 109)** — a high-cover, low-diversity site, and the two rankings should not be read as one "reef quality" signal. (12) **CCA is nearly absent (4.8%, 103rd of 109) and clean turf almost as scarce (3.9%, 101st of 109)** while sediment-laden turf runs to 22.8% — an algal budget unlike the exposed Chagos forereefs and worth carrying into any cross-site algal comparison. (13) **HOUSEKEEPING — the CRD page URL is the `new-page-4` placeholder** shared with the other Blenheim / Nelson Island / Victory Bank sites, and needs repointing once that page is renamed. (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 **5.08%, rank 76 of 110** (1st training: 5.08%, rank 76 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 BIVB81 benthic survey data (CSV)](../benthic_data/BIVB81_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).