Objectifs :

Inventory of the benthic biodiversity

Age, isolation, surface area: all these elements support the hypothesis that Saya de Malha would be an important centre of speciation and endemism of marine fauna and flora in the Indian Ocean. And yet, our knowledge of the fauna of the Mascarene Plateau in general, and of Saya de Malha in particular, is limited. Only about thirty publications describing new taxa, i.e. about 50 marine species have been dedicated to this geographical area (mainly fish, molluscs and a few species of crustaceans - see bibliographical list in paragraph 13). This is the state of our knowledge of a tropical shoal the size of Switzerland. The inventory, which will be implemented within the framework of the Saya de Malha project, aims to enrich our knowledge of biodiversity and to discover unknown species.

It is essential to decipher the complex assemblage of benthic communities of Saya de Malha in order to be able to delimit biodiversity hotspots to be protected in case extractive activities are implemented in the future. The expected high richness of habitats on the bank and the high degree of specialisation of organisms require the use of specific sampling methods. The objective of the inventory expedition is to aim for exhaustiveness within the targeted taxonomic groups (algae, corals, molluscs, crustaceans, echinoderms, ascidians, fish) by integrating a wide range of sizes for all these taxa, with a particular focus on small species (of the order of mm) for molluscs, crustaceans and algae. These taxa, which are often small in size, have a high potential for diversity and are taxonomically challenging, are however in reality the most important contributors to the diversity and richness of marine habitats. It is through an original and naturalistic angle based on the description of these taxa often considered neglected in conservation policies, as opposed to “flagship” species (i.e. corals, teleost fish), that the data acquired during the expedition will enable us to meet management objectives for this ecological heritage.

Sampling has been carried out using two complementary approaches: diving and towed gear (diving from 0 to 58 m and towed gear between 70-1,441 m) have been maximized on the selected sectors in order to optimize all types of habitats: soft bottoms, rocky bottoms, phanerogam meadows, calcareous algae beds, bioconstructive and non-bioconstructive coral communities.

Beyond fundamental scientific questions (evolution, magnitude and richness of marine biodiversity, macro-ecology and discovery of unknown species), taxonomic data (georeferenced occurrences of species) but also data on habitats will be intended for stakeholders in the conservation of marine areas in Seychelles and Mauritius and will be used in policies related to the management of economic development activities by these two countries.

Contexte scientifique :

Monaco Explorations “Indian Ocean Expedition” is the first element of the “Monaco Explorations” project endorsed by the United Nations Decade of Ocean Sciences for Sustainable Development 2021-2030. The expedition itself was endorsed by the 2^nd International Indian Ocean Expedition. It took place between Reunion, Mauritius, and Seychelles onboard the South African oceanographic research and supply ship S.A. Agulhas II. The scientific programme was initially composed of eight research projects with three different geographic scopes:

• The Saya de Malha Joint Management Area between Mauritius and Seychelles, located beyond the Exclusive Economic Zones of the two States. The expedition aimed to gather scientific elements that could help consolidate the joint governance of a relatively unknown area with potential outstanding universal value. This area was the priority focus of the expedition.
• Two islands: Aldabra (Seychelles) and Saint-Brandon (Mauritius). The objective was to characterize the function of refuges for biodiversity and to help preserve it against the impacts of anthropic pressure and climate change.
• The region as a whole: the objective was to improve the understanding of the regional ocean dynamics.

1.1.1Multidisciplinary study of the Saya de Malha Bank

This project was designed with the objective of “science for marine governance and planning support” with the following objectives:

• To complement the existing scientific information on the marine environment of the shallow areas of the Bank and its slopes;
• To undertake an inventory of the benthic biodiversity, to assess the species richness and possible endemism, in order to map the sensitive habitats that may require specific conservation measures;
• To produce fact sheets on the habitats and communities of the Bank;
• To contribute to policy briefs for institutional stakeholders and policy makers in Seychelles and Mauritius;
• To publish the main results in international journals.

It combined open water and seabed measurements. The water column, both on the bank (ranging in depth from 15 to 120 m) and up to 35 km beyond the slopes, was sampled using vertical CTD profiles with water sampling at various depths to determine physical, chemical, and biological parameters. The project also included a survey of the benthic biodiversity (fauna and flora attached to the seabed) by means of scientific rebreather dives, enabling work to be carried out at depths of over 60 m, and by means of towed gears. Led by IRD in collaboration with the Muséum National d’Histoire Naturelle (MNHN), the project team included representatives from Mauritius (Department for Continental Shelf, Maritime Zones Administration and Exploration, Prime Minister’s Office; Ministry of Blue Economy, Marine resources, Fisheries and Shipping; Mauritius Oceanography Institute; University of Mauritius), Seychelles (Ministry of Fisheries and the Blue Economy; Seychelles Fishing Authority; Seychelles Parks and Gardens Authority; University of Seychelles; Talma Consultancy), and other institutions such as the University of Lodz, Poland and Nelson Mandela University, South Africa.

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