Chagos Archipelago - Great Chagos Bank West (BITB56)

Setting

BITB56 is a protected, submerged bank reef inside the Three Brothers lagoon, surveyed 27 March 2015.

General benthic features

Porites is by far the dominant coral genus, holding nearly half of all recorded coral, with Acropora, Montipora, Pocillopora, Favites, and Favia trailing well behind (e.g., Image 1, Image 3, Image 9, Image 11).

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Coral cover and diversity peak at mid-depth and collapse at the deepest transect to a near two-genus assemblage of Porites and Acropora.

Coral cover

Formal transects recorded 21% live coral cover against a much higher 48.9% algal cover, making this an algae-dominated hardground of open pavement and rubble rather than a continuous coral wall (e.g., Image 4, Image 6, Image 12, Image 13, Image 16).

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Image-based methods read this reef 6–7 points lower than the transects (CoralNet 15.6%, Coralscapes 14.5%); the three are not interchangeable and should not be averaged (see the full report).

Stressors

Despite a notably high published bleaching figure of 16.0%, the surveyed photographs show no visibly bleached coral, and the reef instead stood out for a sheltering spiny lobster found beneath a ledge of large plating corals (e.g., Image 2, Image 5, Image 8, Image 15).

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The bleaching percentage is a computer vision figure — CoralNet annotation of the informal survey images, not a diver's field estimate — and human-vs-model agreement is weaker for bleaching (r = 0.40) than for coral cover (r = 0.72). No crown-of-thorns starfish (COTS) or other corallivores evident.

Comparisons with other data

No prior survey data from this site. If you would like to share yours, please email us.