Banner Image: Reef life, BIOT Copyright: Stewart McPherson www.britainstreasureislands.com


The British Indian Ocean Territory (BIOT) is located about 1500 km from the southern tip of India, 3400 km east of Africa and 3000 km west of Indonesia. The territory comprises the 55 islands of the Chagos Archipelago. It consists of five atolls, including The Great Chagos Bank – the largest atoll in the world. While the land area is only 44 km2, below the territorial seas lie over 60,000km2 of coral reefs, which comprises over 10% of Indian Ocean reef cover, and the cleanest seawater ever recorded.

Additionally there are vast deep-sea plains and limestone platforms, as well as 86 sea mounts and 243 deep sea-knolls. The territorial waters (640,000 km2) form one the world’s largest ‘no-take’ marine protected areas, encompassing almost the entire Exclusive Economic Zone. The archipelago is very diverse, with a range of coral atolls, banks and reefs. The reefs are rich in benthic and pelagic life and represent some of the least disturbed reefs in the Indian Ocean basin.

IYORBank_CoralReefs_Nelson Island_Chagos TheOceanAgency XL Catlin Seaview Survey
Coral reef in the Chagos Archipelago: Copyright: TheOceanAgency XL Catlin Seaview Survey IYOR2018

BIOT once had an indigenous population but today the islands are uninhabited, except the largest, Diego Garcia (16 km2), which is used by the US and UK for military purposes and is administered by a Commissioner from the UK. Access is restricted to the islands and a permit is required in advance of travel from the Administrator.

In 2015, UKOTCF, together with many partner organisations in the UKOTs, helped Stewart McPherson with his project Britain's Treasure Islands. As part of this, we provided information and images for, and helped to fund, a series of mini documentaries on the premise that they would be open source so that everyone could enjoy them. BIOT's mini documentary can be seen here (several other videos focusing on natural history are found below):

The Great Chagos Bank is the largest coral atoll in the world, partly submerged, but with some very shallow features, including small islands on its northern and western rim. The archipelago possesses an exceptionally low level of pollution and provides a standard for measuring the impact of human pressures on other reef systems. Most of the islands are classic coralline, composed of coral rock and sand. However, in southern Peros Banhos and north-western Great Chagos Bank, there are a few small islands which have undergone minor uplift to heights of about 6m above sea level. On Eagle Island, there is an unusual feature of a peat deposit on a coral atoll. Many of the islands have lost their native vegetation as a result of conversion to coconut plantations. Now abandoned, these have remained, although patches of native hardwood remain, and The Brothers have very small, but almost entirely undisturbed, coral-island-hardwood forests.

Cover of book which goes into depth about the unfortunate history of the Chagos

It is probably less than 4000 years since the islands had sufficient soil to support certain flora. No endemic species of plant has been recognised on the islands of BIOT. Its native species consists of around 41 species of flowering plants, 4 ferns, and a variety of bryophytes, fungi and cyanobacteria. Some settled when spores and seeds were brought to the islands by sea, wind or attached to passing seabirds, while humans have introduced others – either deliberately or accidentally. As many as 280 species of plants and ferns can now be found on the islands. Some of these are invasive and have become a threat to the native ecosystem. Some native forests were felled to make way for coconut palms used for copra oil production. In the mid-1800s, so much copra oil was being produced for export that Chagos became known as the ‘Oil Islands’. Introduced rats remain the other big challenge, having significant negative impacts on the native flora and fauna, in particular birds.

The waters around Chagos are breeding and feeding grounds for marine turtles, cetaceans and as many as 800 species of fish, including manta rays, skates and more than 50 different types of shark. They have approximately 300 species of corals, including the endemic brain coral and thick strands of branching staghorn coral. It is a vital natural laboratory for scientists to learn how a healthy reef lives, and key to replenishing ecosystems in other threatened locations. The seas around the territory contain eight times more reef fish biomass than anywhere else in the Indian Ocean. Whilst some fish, such as the endemic Chagos clownfish, linger near the shores of the islands, there are some open-ocean fish such as large wrasse and grouper that cannot be found in other reefs in the region, as well as skipjack, yellowfin and bigeye tuna. In the past, as many as 25,000 tonnes of these species were caught by international fishing fleets as they passed through the islands' waters, as well as 10,000 sharks caught each year as accidental bycatch around the archipelago. The establishment of the marine protected area has halted this, though threats from illegal and unregulated fishing remain. So methods such as satellite surveillance are being trialled. Protection of the seas around the Chagos Archipelago has provided a sanctuary where marine species can rebuild natural populations.

In 2017, the Bertarelli Foundation announced its new programme for marine science in BIOT. The programme builds on a series of expeditions, workshops and strategic plans to create a vision for the BIOT marine reserve as a global exemplar of science and conservation activities working to support effective management. From 2017 to 2021 the programme will be coordinated on behalf of the foundation by ZSL.

The islands are home to large colonies of seabirds, with 10 recognised Important Bird Areas (IBAs) that support > 50 bird species, 18 of which breed in numbers of around 175,000 pairs on the atolls. Restoring and conserving the seabird nesting habitats are a key aim of the Chagos Conservation Trust. Among the most notable species to which these islands are home are the sooty tern, the brown and lesser noddies, the wedge-tailed shearwater and the red-footed booby. The islands are home to the largest land-living arthropod, the coconut crab, with a leg-span of over a metre and weighing as much as 4 kg. On the islands of the Chagos Archipelago, there are large populations of these crabs, because of their protection. Elsewhere, they are threatened by over-collection for food and often being used as fish-trap bait or as ornaments for sale to tourists. The sandy beaches of the atolls provide nesting sites, protected by local legislation, for green turtles and the Endangered hawksbill turtles.

The Chagos Information Portal (ChIP) is a reference library of science, research and conservation work conducted in the Chagos Archipelago. It is an open source website, with information on research papers, scientific expedition reports, photos, videos, and an interactive map. ChIP was created by the Chagos Conservation Trust which works to promote and conduct scientific and historical research and environmental conservation work in BIOT. CCT works to advance international understanding of the global environmental importance of the BIOT to ensure its protection for the wider public benefit.

A virtual tour (opens in new window) for BIOT, outlining its historical and cultural importance is available.