Frédéric Ménard, a researcher at the IRD and a specialist in marine ecosystems, makes an initial assessment of the operations carried out at Saya de Malha. The particular situation of this bank as large as Switzerland, as well as its specific characteristics, make it an exciting study site for scientists. Saya de Malha raises interest and questions.
Saya de Malha: an ecologist's perspective.
Saya de Malha, an exotic name little known to the general public, is a shallow area of the Mascarene Plateau, an oceanic plateau located east of Madagascar in the southwestern Indian Ocean. Saya de Malha is one of the main shoals of this vast plateau which extends in an arch over 2000 km between the Seychelles in the north and Reunion in the south. These islands and shoals, despite the distances that separate them, share a common and fascinating geological history. All of them were formed by the Reunion hot spot and have undergone the expansion of oceanic plates, periods of rising and falling sea levels, and erosion, over millions of years. The Saya de Malha seabed is still poorly mapped. Based on the few expeditions that have taken place and satellite data, the overall topographic architecture of the bank is divided between shallow submerged reef ledges (around 50 m) on the eastern and northern parts of the bank, a central inland area of a mosaic of seascapes at depths of less than 150 m, and a southern plateau. All around, depths of over 3000 m are reached very quickly. Saya de Malha is surrounded by the deep ocean.
The geomorphology and geographical location of Saya de Malha make it a bank subject to complex oceanographic conditions that shape the ecosystems that develop there. In this southern part of the tropical Indian Ocean, a powerful, thick westward current, the South Equatorial Current, comes up against Saya de Malha, which rises abruptly from the deep ocean. All these singularities make Saya de Malha an exciting site for us ecologists. How does the south-equatorial current, which is particularly intense at the surface, affect the oceanographic conditions of Saya de Malha? Does the bank contain a rich and preserved fauna? Or are the ecosystems disturbed by climate change? Degraded by unscrupulous fisheries? How are the emblematic Saya de Malha sea grass beds faring? These shallow areas covered with flowering marine plants, like our Posidonia meadows in the Mediterranean. What are the links between the organisms attached to the bottom and the fauna living in the water column? What about sharks? What about marine mammals? What about seabirds?
What conclusions can we draw from the wide-ranging surveys we have carried out? At first sight, Saya de Malha does not meet our expectations. Few large fish, very few sharks, few seabirds. The macro fauna is not there. And yet, the seagrass beds are in good health, the fish are present in the coral zones, the mosaic of habitats shelters a fixed fauna that is not very abundant but diversified and so fascinating, as for example for the sponges, Our plankton nets collect an interesting diversity of organisms living in the water column, and the images brought back by the small underwater robot that we deployed on the slope breaks testify to the presence of fish, sharks, corals, gorgonians, crinoids (called “sea lilies”). … Refuge zones for macrofauna that we hardly observe on the plateau? There is much work to be done to understand the links between the singular environmental conditions of Saya de Malha and the distribution of the fauna and flora. There are many questions to be answered that fascinate our small community of Mauritian, Seychellois and French scientists on board the S.A. Agulhas II. And the conviction that the knowledge acquired here and together will allow us to better preserve the Saya de Malha site.
Frédéric Ménard is a researcher at IRD, the French Research Institute for Development. He was Director of the OCEANS, Climate and Resources Department at IRD from 2015 to 2020. He now holds the position of Scientific Advisor Overseas and participates in IRD’s scientific strategy.