Chagos Archipelago - Salomon / Blenheim / Speakers Bank

Salomon and Blenheim Reef are two of the five true atolls that Spalding et al. (2001) recognize in the Chagos Archipelago, and Speakers Bank is one of its submerged banks. Salomon, like the other northern atolls, has only small islands scattered along its rim, and the atlas pictures a small coral cay sitting on its reef flat. It is also the atoll the authors single out for human impact. As of 2001 the outer islands had no residents, but visiting yachts were thought to be causing local anchor damage and sewage pollution in Salomon's enclosed lagoon. A 1996 photograph of shallow Salomon lagoon reefs is captioned as showing reefs later devastated by the 1998 bleaching. Across the archipelago, coral had typically covered 50–80% of seaward and lagoon slopes down to about 40 m before 1998, and seaward coral loss in that event averaged an estimated 80–85%. The Chagos reefs hold some 220 scleractinian species, among the richest coral faunas in the Indian Ocean, including Ctenella chagius, a coral that may occur only here apart from a single reported sighting at Mauritius. Spalding et al. (2001) estimate about 3,770 km² of reef across the whole archipelago, and Souter et al. (2021), using a newer map, give 2,731 km² for the Chagos ecoregion.

Reefs in the Chagos showed encouraging recovery in the years after 1998, according to Wilkinson's (2006) summary of the 2004 status report. Souter et al. (2021) trace coral cover in the Chagos subregion rising until about 2015 and falling again after the 2016 bleaching, an event that killed more than 42% of living hard coral across South Asia. Region-wide, smaller and more localized bleaching between 2017 and 2019 prolonged the decline, and algal cover crept up from about 10% before 2008 to 14% by 2018. Because the Chagos atolls combine restricted access with the region's largest marine protected area (MPA), Souter et al. (2021) rank them among the South Asian reefs under least human pressure and name bleaching as the chief threat. Fishing was not absent, though: as of 2001, Spalding et al. reported illegal fishing for sharks and sea cucumbers, countered by a British Indian Ocean Territory (BIOT) fisheries protection vessel.

This unit pools 25 stations in three neighboring northern survey areas dived between 22 April and 3 May 2015: the Blenheim forereefs, the lagoon and outer forereefs of Salomon atoll, and the lagoon and bank reefs of Speakers Bank. Logged maximum depths ran from 15.5 m at BISA97 to 31.1 m on the Blenheim forereef at BIBL82 (BISA98 and BISA106 have station CTD casts only). Speakers Bank reefs are dominated by Acropora tables. Acropora makes up 80% of the coral at BISP101, and at BISP96 overlapping tables form a multi-layered canopy. Salomon's lagoon reefs are often not Acropora reefs at all. Porites, Lobophyllia, and Goniopora lead at BISA105, and Acropora is almost absent at BISA94. At BISA91 the lead passes with depth from Acropora through Goniopora and Porites to a deep transect of 77% Montipora.

Cover ranged from 26% on the BISA92 wall, where sponges, sea squirts, meter-wide sea fans, and black wire corals outweigh the coral, to 82% at BISP96, with BISA107 (25%) at the low end beside BISA92 and BISA98 at 71% from a single transect. Coral diversity is high: BIBL87 carries 37 genera, and BISA93 carries 34. The Chagos-endemic Ctenella was recorded at BISA93, BISA104, BISP90, BISP95, BISP96, and BISP115. At Blenheim, fleshy turf covers 22% of the benthos at BIBL86, and fire coral (Millepora) is its leading coral taxon. BISP96 also has 12% algal cover.

The Salomon forereefs carried published bleaching of 34% at BISA84, 31% at BISA104, 24% at BISA103, and 19% at BISA102 (CoralNet analysis of the survey images; Mayfield & Dempsey 2026, suppl. S1), with stark-white colonies beside pigmented neighbors. Published values at Speakers Bank ran from 2.1% at BISP90 to 9.2% at BISP89, but Anderson's dive notes and sampling sheets record heavy bleaching there as well: all four Seriatopora hystrix sampled at BISP90 were 95-100% bleached, 90%-bleached Pocillopora verrucosa were very common at 20 m at BISP95, and most pocilloporids at BISP100 were bleached, diseased, or partly dead. In the Salomon lagoon, Anderson's notes describe most corals at 10-20 m as bleached or bleaching at BISA105 (published 5.6%) and many bleached corals in the shallows at BISA94 (2.8%); at Blenheim, published values near 10% sit beside the notes that most corals were bleaching at BIBL86, particularly fire coral, and that at least half the colonies had bleached at BIBL87. BISP115 has no published figure; its photographs show several bleached colonies, and three of the five pocilloporids Anderson sampled were 90-100% bleached, though the dive note judged the bleaching not severe in terms of percentage of colonies. At BISA107 an abandoned fishing net was photographed draped across live coral inside the no-take marine protected area. No crown-of-thorns was reported at the sites that recorded its absence, but Anderson's sampling sheet records diseased or possibly diseased Pocillopora colonies among those sampled at BIBL83, BISP89, BISP96, BISP100, and BISP101. Wildlife included a hawksbill turtle resting at BIBL83, a silvertip-type shark and two giant morays at BISA102, and three sharks and a spiny lobster at BISA107.