Horizon 1 1 17 ACL : Articles dans des revues avec comité de lecture répertoriées par l'AERES Journal of Marine Systems 104113 [14 ] Active acoustics Micronekton Senegal Upwelling SENEGAL ATLANTIQUE 2025 fdi:010094808 ENG Journal of Marine Systems 0924-7963 ISI:001544332800001 10.1016/j.jmarsys.2025.104113 https://www.documentation.ird.fr/hor/fdi:010094808 https://horizon.documentation.ird.fr/exl-doc/pleins_textes/2025-09/010094808.pdf 251 Horizon (IRD) The coastal area of north-west (NW) Africa is a highly productive ecosystem due to the presence of a strong upwelling. This ecosystem supports large populations of small pelagic fish, such as sardinellas, which have significant socio-economic value for local populations. In this study, we analyzed the acoustic data collected during a one-month survey along the Senegalese coast at the beginning of the upwelling season. Hierarchical clusterings were performed to classify the acoustic data from the epipelagic zone (down to 120 m-depth) separately for daytime and nighttime. The analysis identified five echo-groups during the day and six at night. The resulting echo-groups were then compared to stratified midwater trawl samplings to support hypotheses about the organisms responsible for the echoes. Additionally, a remotely operated towed vehicle (called Scan-fish) was used to monitor environmental variables down to 100 m depth. Two machine learning models were applied to link the classified echo-groups to the environmental data for both day and night. Each daytime echo group had a corresponding nighttime echo group, with also similar environmental preferences. Fish schools were mainly found in shallow coastal waters while dense sound-scattering layers detected at 38 kHz, likely composed of small fish or fish larvae, were observed in the temperature range of 17 degrees-21 degrees C for both day and night. The other echo-groups were composed of fluid-like zooplankton or gas-bearing zooplankton. The sixth night echo-group corresponded to migrant organisms and was predominant at night. Overall, the analyses of the abiotic habitats for each echo-group allow us to better understand the organism distributions during the beginning of the NW Africa upwelling season. 036 ; 082 UR195 / UR254 / UR235 / UR182