D’Arros Island and St Joseph Atoll are situated in the remote Outer Islands of the Seychelles. They form part of a small chain of islands that comprise the Amirantes Group, named for the bank on which they are situated. Mahé and the other Inner Islands of the Seychelles lie 250 kilometres north-east of the Amirantes, while Zanzibar is 1,400 kilometres due west. Because of their inaccessibility, infertile soils and lack of fresh water, the Outer Islands have long been regarded as unsuitable for permanent habitation. Nevertheless, over the past two centuries small communities have lived temporarily on some of the islands to grow coconuts, collect guano and catch fish. Life was difficult and dangerous for these Seychellois, but many individuals stayed for such a long time that they became local legends. Today conservation and tourism are the main activities on the Outer Islands.
Although D’Arros Island and St Joseph Atoll are separated by a channel one kilometre wide and 70 metres deep, they are considered a single ecological unit as their ecosystems are inextricably linked. D’Arros and the 16 islands that comprise the St Joseph Atoll are sand cays that, at about 15,000 years old, are relatively young. A sand cay is formed when ocean waves and tidal currents transport loose sediment across the surface of a shallow reef flat to a point where the currents slow or converge and deposit the sediment in a pile that rises above the high-tide mark. Over time, ocean currents, wind and birds deposit seeds on the exposed sand to give rise to plants, and eventually insects, crabs, reptiles and even larger animals arrive through dispersal by wind, ocean or human intervention. On the surrounding reefs, nutrient run-off from the land promotes coral and algal growth, which in turn attracts reef fishes, rays, sharks, turtles and other marine life. Thus, sand cay formation is an important geological process because it creates biodiversity and biomass hotspots in an otherwise empty expanse of ocean.
D’Arros Island is oval shaped and about two kilometres at its widest axis, which means you can walk around its uninterrupted sandy beach in less than two hours. St Joseph Atoll consists of a circle of 16 islands that have a combined landmass just less than that of D’Arros. These islands are situated on a shallow reef flat that encircles a central lagoon of 3.5 kilometres in length and three metres in average depth. At low tide, marine organisms are trapped in the lagoon because the reef flat is entirely exposed. As the tide rises again, water spills back over the reef flat to eventually lap against the beach crests of the islands. Most of the rain falls during the north-west monsoon (mean annual rainfall of 1,490 millimetres) between December and March. For the remainder of the year the island is subjected to moderate south-easterly winds and relatively dry weather. Air temperature varies between 25 and 35 °C and the sea cools to 24 °C in winter and warms to 31 °C in summer.