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 region's largest 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 11.0 m at BIEG07 on Egmont to 31.2 m at BIPB75 on Peros Banhos and BIVB99 on Victory Bank.

Hard coral cover averaged 37.9% over all 112 stations, and 26 held 50% or more. The northern atolls were richest: Peros Banhos averaged 46.6% (n = 20) and Salomon, Blenheim and Speakers Bank 45.1% (n = 25), with Speakers Bank alone at 51.7% (n = 7). Victory Bank and the northern Great Chagos Bank averaged 40.5% (n = 8), Egmont, Danger Island and Cauvin Bank 36.2% (n = 9), and the western Great Chagos Bank 30.7% (n = 50). The lowest station in Chagos was BIDI12, a macroalgal meadow off Danger Island at 4.2%, followed by Cauvin Bank (BICA01, 4.6%). The highest recorded value, 86.0% at BISP96 on Speakers Bank, is best read as a best-patch value, as are 72.4% at BIPB72 and 71% at BISA98; among the rest, BITB58 on the Three Brothers reached 67.0%. 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 10.3% at BISP96 to 70.0% at BICC51.

The expedition coincided with the onset of the 2015 bleaching. No Coral Health Index was scored in Chagos, but published bleaching values averaged 8.6% across the 99 stations that have one, rising from 6.7% on the March leg (n = 51) to 10.6% in April and May (n = 48). By region, the western Great Chagos Bank averaged 6.8% (n = 44) and Egmont, Danger Island and Cauvin Bank 6.2% (n = 7), against 10.7% at Peros Banhos (n = 19) and 11.0% at Salomon, Blenheim and Speakers Bank (n = 22). The highest values were 38.2% at BIPB112 on Peros Banhos, 34.0% at BISA84 and 31.5% at BISA104 on Salomon's exposed forereefs. Repeat dives at Peros Banhos tracked the event directly: BIPB69 rose from 0.0% on 16 April to 36.5% on 2 May, and BIPB72 from 10.0% to 37.5% 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. Lesioned brain and massive corals were noted at three Danger Island stations.

What stands out is the combination of very high diversity and a reef system caught mid-bleaching. BIBL87 on Blenheim carries 36 coral genera, the highest count stated for any Chagos station, and the endemic Ctenella was recorded on the Three Brothers, Victory Bank, Salomon, and Speakers Bank. An abandoned fishing net draped over live coral at BISA107, inside the no-take protected area, was the only fishing gear recorded. Sharks were seen throughout, including silvertip, gray, and whitetip reef sharks and a tawny nurse shark, along with a reef manta ray at BIPB66, hawksbill turtles and spotted eagle rays.