Putting the Western Indian Ocean on the Map

Article excerpt

Despite having been sailed by merchants for millennia, the western Indian Ocean wasn't properly explored until a century ago and it's only in the past decade that scientists have come to grips with this unique and fragile environment. Mark Spalding of the UNEP World Conservation Monitoring Centre explains the importance of this vast shallow sea to the east of Madagascar

Suppose you had to navigate with a map that read, "This mountain may lie three kilometres farther east than it is drawn." The world's best nautical charts of the more remote parts of many oceans are still littered with equivalent statements. The sea monsters are gone, but many marine charts still lack some vital information.

The western Indian Ocean is one such mare incognitum. Although traders from Arabia and East Africa sailed the coastal waters for millennia, few ventured into the open ocean.

The first efforts to explore these waters took place almost 100 years ago. The Percy Sladen Trust Expedition, led by J Stanley Gardiner, on a British hydrographic vessel, HMS Sealark. Over a period of seven months it covered a vast area, producing maps and marine charts (many of which are still the best available) and describing the flora and fauna.

In the years that followed, there was little else, except some small-scale scientific work, until the 1990s, when the Royal Geographical Society (with IBG) proposed the Shoals of Capricorn Programme, a comprehensive multidisciplinary research project that aimed to provide information about the region's environment and a plan for its management. In January 2004, the Royal Society hosted a meeting at which some of the more than 200 scientists who took part in the six-year programme presented their findings.

The area of interest stands out quite clearly on marine charts. It is some 90,000 square kilometres of shallow waters, known as the Mascarene Ridge or Plateau, bound by Madagascar to the east and an arc of islands from the Seychelles in the north to Mauritius in the south. Ten thousand years ago, at the end of the ice ages and the dawn of human civilisations, it would have been dry land, partly enclosing the Mascarene Basin as a sea. Today, unconnected to any continental shores, with only a few tiny islands, it is unique among the world's oceans.

One of the first tasks in exploration is the development, or improvement, of maps. Bathymetric charts of the oceans have been created and refined since the ages of European discovery. Back then, the relative simplicity of the available navigational and depth-sounding tools meant that gathering even single data points in the deep ocean was very laborious. Nevertheless, over the centuries, a broad picture of the sea floor gradually developed.

Modern technology has revolutionised this process. Satellite positioning, along with broad-beam and side-scan sonar, produces detailed, textured maps of the sea floor. Satellite altimetry has enabled researchers to map comprehensively the sea floor by measuring differences in the height of the sea's surface. The gravitational pull exerted by variations in the topography of the sea floor affects the height of the water, causing it to be higher, perhaps by just a few tens of centimetres, over an undersea mountain than over a deep trench. When the influence of waves and tides is averaged out, scientists can use these differences to calculate the sea floor's topography.

But there is still a long way to go. Satellite altimetry is low resolution and can't be used to develop fine-scale navigational charts, because it only picks up features of ten kilometres width or more. Developing more accurate charts using sonar systems still requires boats, time and resources. Information has poured in from the shipping lanes, but elsewhere the maps are bare.

The Mascarene Ridge is one such area. But the Shoals programme has precipitated great advances. In 1999 two naval survey vessels, HMS Scott and HMS Beagle, gathered detailed information on the deeper slopes to either side of the ridge. …