TY - JOUR T1 - Micronekton distribution as influenced by mesoscale eddies, Madagascar shelf and shallow seamounts in the south-western Indian Ocean: An acoustic approach A1 - Annasawmy,Pavanee A1 - Ternon,Jean-Francois A1 - Lebourges-Dhaussy,Anne A1 - Roudaut,Gildas A1 - Cotel,Pascal A1 - Herbette,Steven A1 - Ménard,Frédéric A1 - Marsac,Francis AD - MARBEC, Univ. Montpellier, CNRS, Ifremer, IRD, Sète, France AD - Department of Biological Sciences and Marine Research Institute/ICEMASA, University of Cape Town, Cape Town, South Africa AD - LEMAR, IRD, UBO, CNRS, Ifremer, Plouzané, France AD - Laboratoire d’Océanographie Physique et Spatiale (LOPS), IUEM, Univ. Brest, CNRS, Ifremer, IRD, Brest, France AD - Aix Marseille Univ., Université de Toulon, CNRS, IRD, MIO, Marseille, France UR - https://doi.org/10.1016/j.dsr2.2020.104812 DO - 10.1016/j.dsr2.2020.104812 KW - Micronekton KW - Diel vertical migration KW - Mesoscale eddies KW - Madagascar shelf KW - Seamount KW - South-western indian ocean KW - Multi-frequency N2 - An investigation of the vertical and horizontal distributions of micronekton, as influenced by mesoscale eddies, the Madagascar shelf and shallow seamounts, was undertaken using acoustic data collected during two research cruises at an unnamed pinnacle (summit depth ∼240 m) thereafter named “MAD-Ridge”, and at La Pérouse seamount (∼60 m) in the south-western Indian Ocean. MAD-Ridge is located to the south of Madagascar, in an “eddy corridor”, known both for its high mesoscale activity and high primary productivity. In contrast, La Pérouse is located on the outskirts of the Indian South Subtropical Gyre (ISSG) province, characterised by low mesoscale activity and low primary productivity. During the MAD-Ridge cruise, a dipole was located in the vicinity of the seamount, with the anticyclone being almost stationary on the pinnacle. Total micronekton acoustic densities were greater at MAD-Ridge than at La Pérouse. Micronekton acoustic densities of the total water column were lower within the anticyclone than within the cyclone during MAD-Ridge. Micronekton followed the usual diel vertical migration (DVM) pattern, except within the cyclone during MAD-Ridge where greater acoustic densities were recorded in the daytime surface layer. The backscatter intensities were stronger at the 38 kHz than at the 70 and 120 kHz frequencies in the daytime surface layer at MAD-Ridge cyclonic stations. These backscatter intensities likely correspond to gas-filled swimbladders of epi- and mesopelagic fish actively swimming and feeding within the cyclone or gelatinous organisms with gas inclusions. Our findings evidenced that the distributions of micronekton and DVM patterns are complex and are influenced significantly by physical processes within mesoscale eddies. The mesoscale eddies’ effects were dominant over any potential seamount effects at the highly dynamic environment prevailing at MAD-Ridge during the cruise. No significant increase in total micronekton acoustic densities was observed over either seamount, but dense aggregations of biological scatterers were observed on their summits during both day and night. Y1 - 2020/06 PB - Elsevier BV JF - Deep-sea Research Part Ii-topical Studies In Oceanography SN - 0967-0645 VL - 176 ID - 74579 ER -