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, the northernmost Chagos survey area. Logged maximum depths ran from 15.5 m at BISA97 to 31.1 m on the Blenheim forereef at BIBL82 (BISA98 and BISA106 have no logged depth). Speakers Bank reefs are dominated by Acropora tables. Acropora makes up 83% 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 24.5% on the BISA92 wall, where sponges, sea squirts, meter-wide sea fans, and black wire corals outweigh the coral, to 56.9% at BISP95, with BISA107 (24.6%) at the low end beside BISA92. BISP96 recorded 86% and BISA98 71%, the latter from a single transect, and both are best read as best-patch values rather than site means. Coral diversity is high: BIBL87 carries 36 genera, the highest count stated on any Chagos station, and BISA93 carries 34. The Chagos-endemic Ctenella was recorded at BISA93 and BISP90. At Blenheim, BIBL86 is heavily turf-dominated, with fire coral (Millepora) rather than Acropora as its leading taxon. BISP96 also has the lowest algal cover of any Chagos station, 10.3%.

Condition split by area. The exposed Salomon forereefs carried some of the highest bleaching in Chagos: 34.0% at BISA84 and 31.5% at BISA104, second and third only to BIPB112 at Peros Banhos among all 112 Chagos stations, then 24.1% at BISA103 and 19.2% at BISA102, with stark-white colonies beside pigmented neighbors. Speakers Bank values were lower, from 2.1% at BISP90 to 9.2% at BISP89. BISP115 has no published figure, but its photos show unmistakable bleaching. At BISA107 an abandoned fishing net was photographed draped across live coral inside the no-take marine protected area. No crown-of-thorns or disease was reported at the sites that recorded their absence. 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.