Chagos Archipelago - Victory Bank (BIVB99)

Setting

BIVB99 is a lagoonal patch reef on Victory Bank, leeward on Anderson's field sheet, surveyed 27 April 2015 on a dive to 31.2 m (see Dive record below).

General benthic features

BIVB99 is a blue-coral reef. Heliopora holds 49% of the benthos — 82% of all coral recorded. Every other genus is marginal: Stylophora 4.7%, Acropora 2.4%, Pocillopora 1.5%, Montipora 0.4%, Gardineroseris and Favia 0.3% each, and Pavona, Hydnophora, and Echinopora 0.2% each, with a further tail below that across 17 non-zero codes. Coralline algae hold 14% of the benthos, turf 9.8%, erect algae 8.0%, bare substrate 3.7%, macroalgae 3.3%, and non-coral invertebrates only 0.9%. Both depth strata are Heliopora stands and the shallower one is nearly a monoculture. Algal cover is high and steady in both bands (34% and 38%). Here the thicket is the subject. Two frames show extensive vertical relief where upright folded sheets occupy most of the available hard surface (e.g., Image 2); another shows repeated ridges and folded blades standing at multiple heights (e.g., Image 3); a short later run of frames retains that dense structure, interrupted by compact branching colonies and bright red-purple encrusting material.

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The sequence begins at a sandier margin, with open sand and scattered low colonies beside a table in the foreground, then moves quickly into the thicket (e.g., Image 1).

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The upright, folded, blade-like growth form in these frames is exactly what a blue coral (Heliopora) stand looks like, as in Image 3 and Image 8.

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The habitat is structurally intricate, its complexity generated by upright sheets rather than broad tables, with narrow channels and cavities between closely packed vertical blades. Red, pink, and purple encrusting surfaces are abundant beneath the sheets; some are compatible with crustose coralline algae — which the transects put at 14% — while others could be sponges or other encrusting organisms. The photographs document the blue-coral thicket itself — upright folded blades at multiple heights — plus heavy red-purple encrustation, a sandier outer margin, and localized bleaching restricted to compact branching colonies rather than the dominant sheets.

Coral cover

• Reference: Mayfield & Dempsey (2026)
• CoralNet submission package
• 60% against 35% algae.
• Transect mean 60 ± 4.0% SE (n = 16 transects; Living Oceans Foundation per-transect benthic data); the headline above is this all-transect mean, and the published station value, which weights each depth equally, is 56% (Mayfield & Dempsey 2026, Oceans, suppl. S1).
• D3 carries 64% coral with Heliopora at 54% — 84% of the coral in that stratum — across 12 genera.
• D4 falls to 47% with Heliopora at 34% (73% of coral) across 14 genera, and Acropora rising to second place at 4.7%.
• Formal surveys give 60% coral, 35% algae, and 0.9% non-coral invertebrates, the coral 82% Heliopora.

Stressors

Published bleaching is 6.8% (CoralNet analysis of the survey images; Mayfield & Dempsey 2026, suppl. S1), and Anderson's notes record numerous bleached pocilloporids and dying Seriatopora at 25-30 m, and all four Seriatopora colonies sampled were fully or likely fully bleached (see Dive record below). One frame shows several compact branching colonies nearly white across intact branches, immediately adjacent to purple-pigmented compact colonies and dark upright sheets (e.g., Image 10); smaller white colonies recur in wider frames (e.g., Image 4).

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The Living Oceans Foundation transects here recorded 0.1% cyanobacteria and no recently dead coral.

Fish and other fauna

Small reef fishes use the narrow spaces between folds and branches throughout.

Comparisons with other data

Apart from the Living Oceans Foundation survey shown here, we found no other surveys at this site as of 2026. If you would like to share yours, please email us.

Dive record

Anderson's early-morning dive on 27 April 2015, the one dive that day, entered the water at 07:27 and lasted 55 minutes, to a maximum depth of 31.2 m (the field sheet); the notes describe a site dominated by blue coral from 23 to 13 m (the shallowest depth, 13.5 m), a reef that looked spectacular below 30 m, mostly Halimeda and dying Seriatopora at 25-30 m, numerous bleached pocilloporids, and calm water with surge and a current that picked up as the dive progressed; there was no P. acuta. Anderson sampled one Pocillopora damicornis (confirmed by RFLP), CHPd141 (collected 07:56 at 24.0 m; healthy, the only one seen, at the base of the blue-coral reef, “likely to be the real P. damicornis”), and four Seriatopora hystrix: ChSe146 (07:34, 30.2 m; 100% bleached, common below 30 m, live tissue not evident), ChSe147 (07:38, 26.1 m; neon-green fluorescent, likely 100% bleached), ChSe148 (07:44, 25.1 m; 100% bleached, alone in the sand, possibly dead), and ChSe149 (07:54, 26.5 m; large, 100% bleached, being overgrown by algae, possibly dead). For a collaborator Anderson also sampled four Pocillopora sp. colonies: ChJo252 (07:49, 20.6 m; pale, likely healthy), ChJo253 (08:05, 15.9 m; 80% bleached), ChJo254 (08:10, 14.6 m; more than 90% bleached, with several 100% bleached colonies nearby), and ChJo255 (08:16, 14.4 m; more than 90% bleached, live tissue still evident). Pocillopora acuta colonies from this archipelago were analyzed in Thomas et al. (2026, Global Ecology and Biogeography 35:e70305, https://doi.org/10.1111/geb.70305).

Other notes

Aspect follows Anderson's field sheet (LOF fellowship 2016 workbook: 'lagoonal patch reef', 'leeward'); the expedition's 933-site table classes the station as 'protected' (patch reef). Esri World Imagery (no LOF habitat map covers Victory Bank; the LOF Chagos habitat map covers only Peros Banhos) shows only open water over the submerged bank at the pin, so the aspect cannot be checked from imagery.