Descriptors to characterize acoustic scattered layers: evidence of interest in three Atlantic African Large Marine Ecosystems

Hydroacoustic is now a reliable and often used tool to monitor and study marine ecosystems (Brehmer et al. 2006). This study focus on acoustic scattered layers, which are the echosounder detection of pelagic marine organism of low trophic level, important in ecosystems functioning. Data have been recorded at 38 kHz in the three Atlantic African Large Marine Ecosystems (LME). Layers have been extracted using Matecho (Perrot et al. 2018) at a threshold level of -70 dB. To describe parsimoniously ecosystems, compare them and understand the difference, two descriptors have been used. Some of them are based on already used descriptors and others are new. The aim of this study is to ensure that these descriptors are relevant to monitor and compare systems. Therefore, we first explore temporal and spatial dimension. For such purpose, we use a large acoustic database collected in Canary Current Large Marine Ecosystem ‘CCLME’ (Diogoul et al., 2020; Brehmer et al. 2018). The temporal dimension is studied by diel comparison and by analysing change over years in each system. Spatial dimension is explored by intra-LME comparison concerning CCLME, which present two systems with different functioning. Results highlight the effectiveness of these descriptors. The methodology presented in this work is innovative, introducing original new descriptors to monitor pelagic compartment of each LME and should be efficiently used for environmental monitoring in case of perturbation as overfishing, climate change or marine pollution. Indeed the acoustic scattered layer are mainly composed of macrozooplankton and ichtyoplankton, which are sensitive to environmental change.