High resolution 3-D shapes of fish schools: A new method to use the water column backscatter from hydrographic MultiBeam Echo Sounders

The Multi-Beam Echo Sounder (MBES) is a tool that allows getting information about bathymetric, morphological and compositional characteristics of the seabed surface. Although this is still one of the most important features in hydrography, MBES can now also discriminate the acoustic imaging of the water mass by recording sampled reflectivity measurements along each beam. In this paper, two MBES, namely the Kongsberg Simrad EM3002D (300 kHz) and the Reson Seabed8125-H (455 kHz), which are generally employed to study seafloor characteristics, have been used for the identification of fish schools in the water column at very high resolution. In particular, a methodology to identify and analyse fish schools through the Water Column Backscatter (WCB) is presented. This methodology made it possible to obtain a wide variety of fish schools shape in 3-D (in our case, anchovies and sardines), which allowed a first study on the ecological behaviour of the schools according to their shape. All data were collected along with Kongsberg Simrad EK60 scientific echo sounder, allowing also a comparison of different approaches to fish schools identification. The data presented in this paper were collected during two oceanographic surveys within the project MEDIAS (MEDiteranean International Acoustic Survey), Ancheva 2010 and Ancheva 2011. Beyond the mere visualization of school shapes, we also attempted to gain numerical values of the school volumes and to realize a more detailed study of anchovies and sardines school shapes. In line with the current literature, this study showed that there are three main factors influencing the shape of a school, namely the depth of the bottom, the time of the day when data are acquired and the density of individuals in the investigated area.