Déploiement du réseau d’hydrophones : Mission MD174 : TRACK-2 –OHA-SIS-BIO (VT109). 19 décembre 2009 – 24 janvier 2010, La Réunion – La Réunion. Mission MD175 : GEISEIR-2 –OHA-SIS-BIO (VT112). 25 janvier – 21 février 2010, La Réunion – La Réunion

OHA-SIS-BIO (Observatory by HydroAcoustics of the SISmicity and Biodiversity in the Indian Ocean) is a 24 to 36 months hydroacoustic monitoring program of the seismic activity and of large marine mammals in the Indian Ocean. The monitoring will be conducted with 9 autonomous hydrophones moored in the SOFAR channel between the islands of La Réunion, Crozet, Kerguelen and Amsterdam. Their deployment for two periods of 12 to 18 months each are carried out by R/V Marion Dufresne during her transits to these southern islands. This temporary network will be complemented to the north and east by the permanent hydroacoustic stations of the Comprehensive nuclear Test-Ban Treaty Organization (CTBTO). The hydrophone array is designed for a continuous monitoring of the earthquake activity associated with 3 contrasted spreading ridges (16 to 70 mm/yr) and with the intraplate deformation in the southern Central Indian Basin. This approach proved very effective for detecting and locating low magnitude (>2.5) earthquakes, which are not recorded by the land-based seismological networks, and for deciphering magmatic from tectonic events. The objectives are to characterize the seismic climate of mid-oceanic ridges with ultra-slow, slow and intermediate spreading rates, and the temporal and spatial distribution of the intraplate deformation, and to test the occurrence of low-magnitude precursors prior to large earthquakes along active sub-marine transform faults. This observatory will also monitor the vocal activity of marine mammals such as large whales. An analysis of hydroacoustic records from the CTBTO permanent station in Crozet Island (now out of order) shows that their typical acoustic calls can be used as a proxy for monitoring the abundance and seasonality of 2 subspecies of blue whales. The objective of this new experiment is to improve our general knowledge on the presence, abundance and migration pattern of large-mammal endangered species over a two year cycle in the southern Indian ocean. The two objectives have in common to use hydroacoustic records in the same low frequency-band and to require continuous time series as long as possible to be representative of the seismic regime and marine mammal presence in these remote areas. Objectives of the transits VT109 and VT112 were to deploy the Oha-Sis-Bio 9-hydrophone array in five sites, including two antennas of 3 hydrophones.