This cruise participated in the maintenance of the OHASISBIO network, an acoustic monitoring network in the southern Indian Ocean. The aim here was to maintain one of the sites (MAD-E), the closest to Reunion Island, with the recovery of a prototype hydrophone on mooring, deployed in August 2018 and whose autonomy was coming to an end (3 years), and its replacement by a mooring line equipped with a "classic" autonomous hydrophone. The two operations were successful.

OHA-SIS-BIO(Observatory of HydroAcousticity from SISmicity and Biodiversity in the Indian Ocean) is a long-term hydroacoustic program for monitoring the seismic activity and the vocal activity of large marine mammals in the southern Indian Ocean. This proposal requests the redeployment, in an improved geometry, of the network of autonomous hydrophones currently deployed between La Réunion, Crozet, Kerguelen and Amsterdam islands, taking advantage of the yearly journey of R/V Marion Dufresne to these southern islands (9 moorings, 10 hydrophones).

The hydrophone array is designed to continuously monitor 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 also monitors the vocal activity of large baleen whales. The analysis of our hydroacoustic records shows that their typical acoustic calls can be used as a proxy for monitoring the abundance and seasonality of 5 populations of blue whales. The objective is here to improve our general knowledge on the presence, abundance and migration pattern of large-mammal endangered species over several year cycles in the southern Indian Ocean.

Our data also display unexpected information on the sea-state and icebergs (calving, collision, dislocation). These two applications also require long time-series to validate sea-state prediction models or to monitor the evolution of sound sources (natural or man-made).

These objectives have in common to use hydroacoustic records in the same low frequency-band (< 120 Hz) and to require continuous time-series as long as possible to be representative of the seismic regime, mammal activity or ambient noise changes in these remote areas.

Figure 1 : Prototype of a long-lasting hydrophone (3-4 years) equipped with messengers (spheres).

Figure 2 : Cruise tracks of MD231 cruise with multibeam bathymetric data

Figure 3 : Example of a low-frequency acoustic recording (earthquakes, whales and seismic airgun shots)