Chagos Archipelago

Between 9 March and 3 May 2015, the Khaled bin Sultan Living Oceans Foundation's Global Reef Expedition surveyed 112 stations across the Chagos Archipelago: Egmont atoll, Danger Island, and Cauvin Bank; the western Great Chagos Bank, including the Three Brothers and Eagle Island; the northern Great Chagos Bank and Victory Bank; Salomon, Blenheim Reef, and Speakers Bank; and Peros Banhos. The Great Chagos Bank is a mostly submerged atoll of about 13,000 km², the largest atoll structure in the world (Spalding et al. 2001). The archipelago formed over the trace of the Réunion hotspot at the southern end of the Chagos–Laccadive Ridge, about 500 km from the Maldives. It holds some 220 scleractinian species, including Ctenella chagius, which may be found nowhere else, and its remote, largely uninhabited atolls lie inside the British Indian Ocean Territory marine protected area. Before 1998, coral typically covered 50–80% of seaward and lagoon slopes, and that year's bleaching is estimated to have removed 80–85% of it on seaward reefs.

The survey ran in two legs. From 9 to 28 March the team worked Egmont, Danger Island, Cauvin Bank, and the western Great Chagos Bank (59 stations). From 14 April to 3 May it worked the northern atolls and banks: Peros Banhos (20 stations), the Nelson Island lagoon and Victory Bank (8), and Salomon, Blenheim, and Speakers Bank (25). Most stations are sheltered lagoon, bank, and channel reefs, with forereefs on the outer rims of the atolls and off the Brothers. Logged maximum depths ran from 4.8 m at BIDI16.5 on Danger Island to 31.2 m at BIPB75 on Peros Banhos and BIVB99 on Victory Bank.

Hard coral cover averaged 39% across the 109 stations with a value, and 26 held 50% or more. Peros Banhos averaged 49% (n = 20) and Salomon, Blenheim, and Speakers Bank 46% (n = 25), with Speakers Bank alone at 52% (n = 7). Victory Bank and the northern Great Chagos Bank averaged 41% (n = 8), Egmont, Danger Island, and Cauvin Bank 37% (n = 8), and the western Great Chagos Bank 32% (n = 48). BIDI12, a seagrass meadow off Danger Island, had 4.2%, and Cauvin Bank (BICA01) had 4.6%. Recorded values included 82% at BISP96 on Speakers Bank, 77% at BIPB72, 71% at BISA98 (a single transect), and 67% at BITB58 on the Three Brothers. Most reefs are Acropora-led, with tiered tables and staghorn in the lagoons, while Porites leads on some forereefs, blue coral (Heliopora) dominates BIVB99, and algal cover ranged from 12% at BISP96 to 70% at BICC51.

The expedition coincided with the onset of the 2015 bleaching. A Coral Health Index was scored at only eight stations (mean 3.28/5). Published bleaching values averaged 8.5% across the 100 formal survey stations that have one (Mayfield and Dempsey 2026, S1), rising from 6.6% on the March leg (n = 52) to 10.6% in April and May (n = 48). By region, the western Great Chagos Bank averaged 6.7% (n = 45) and Egmont, Danger Island, and Cauvin Bank 6.2% (n = 7), against 10.7% at Peros Banhos (n = 19), 11.0% at Salomon, Blenheim, and Speakers Bank (n = 22), and 9.1% at Victory Bank and the northern Great Chagos Bank (n = 7). The three informal '.5' stations, surveyed from photographs only rather than transects, are reported separately: BIDI16.5 19.7%, BIEA24.5 27.8%, and BITB34.5 17.1%. Published values of 30% or more were recorded at BIPB112 on Peros Banhos (38%) and at BISA84 (34%) and BISA104 (31%) on forereefs outside Salomon atoll (leeward and side-on, respectively, by their GPS positions), and on the early-May repeat dives at BIPB72, BIPB69, and BIPB70 (37%, 36.5%, and 33%). Repeat dives at Peros Banhos tracked the event directly: BIPB69 rose from 0% on 16 April to 36.5% on 2 May, and BIPB72 from 10% to 37% in 13 days, with tabular Acropora worst affected. Several published values do not match the photographs, in both directions. Crown-of-thorns starfish were rare except for an active outbreak at BINI80, with at least eleven starfish in five frames; single animals were photographed at BIEG02, BIEA52, and BITB55. A clean white lesion of unresolved cause was photographed on one brain coral at BIDI10.

Two things stand out: the number of coral genera recorded and a reef system caught mid-bleaching. BIBL87 on Blenheim carries 37 coral genera, and the endemic Ctenella was recorded on the transects at 39 of the 109 stations, on Danger Island, Eagle Island, the Three Brothers, Central Chagos, Nelson Island, Peros Banhos, Victory Bank, Salomon, and Speakers Bank. An abandoned fishing net was photographed draped over live coral at BISA107, inside the no-take protected area. Sharks were seen throughout, including silvertip, gray reef, whitetip reef, and nurse sharks. Anderson saw and filmed a whale shark at BIPB112, and manta rays were recorded at BIPB66 and BIPB61; hawksbill turtles and spotted eagle rays were also seen.