Acoustics characterization of micronekton spatial distribution in Indian Ocean using scientific surveys and integrated marine observing system database: Acoustics characterization of micronekton

In this work scientific echosounder were used hull mounted onboard R/V Marion Dufresne II to scrutinize the vertical distribution of micronekton according to water masses characteristics along transect which were discriminated with spatial remote sensing data. Acoustic data were collected continuously at 38 kHz frequency during eighteen transits carried out in the South-West Indian Ocean (20-60°S, 50-80°E) between 2010 and 2014. The data set used includes scientific and opportunistic fishing surveys from Integrated Marine Observing System database. A structure in three main depth layers i.e., "surface", "intermediate" and "deep", has been found continuously all along the survey. Changes of micronektonic vertical structure were investigated along North South latitude: the surface layer acoustic density and thickness decrease by going southward. The intermediate layer is generally almost empty except between 30°S and 40°S. The deep layer acoustic density increases from North to South but its thickness does not change significantly. To assess the importance of these vertical changes, a spatially constrained clustering was applied on acoustic data and a positive correlation was established between vertical acoustic organization and oceanographic fronts' position. We conclude that spatial organization of micronekton is structured depending on water masses and confirm the interest of collecting acoustic data from fishery vessels to complete scientific surveys that are often restricted in time and space.