LAUSR

Abstract Converging water masses and mesoscale eddies shape habitats for larval fish assemblages in upwelling ecosystems. In the Canary Current Upwelling Ecosystem, two water masses, the North Atlantic Central Water (NACW) and the South Atlantic Central Water (SACW) converge. The resulting Cape Verde Frontal Zone (CVFZ) is located off the Banc d'Arguin, Mauritania, an important nursery ground for fishes. We investigated the impact of two converging water masses and a mesoscale anticyclonic eddy on the composition and distribution of larval fish assemblages during an upwelling event. Two distinct larval fish assemblages allowed for the reconstruction of water mass distributions along the CVFZ. While the distribution pattern of Dicologlossa cuneata covered the shelf area in the North by NACW characteristics, Helicolenus dactylopterus, Bathylagoides argyrogaster, Hygophum macrochir, and Notoscopelus resplendens were almost exclusively observed in the offshore area in the South with SACW characteristics. This emergence of a dichotomy in larval fish assemblages was facilitated by the CVFZ and the mesoscale anticyclonic eddy, which retained fish larvae in SACW offshore. Coastal geostrophic currents on the other hand transported fish larvae from the North southward in NACW towards the Banc d'Arguin. The coastal geostrophic currents abetted an onshore surface water flow of upwelled water masses. This process facilitated a phytoplankton bloom on the Banc d'Arguin and possibly enabled a larval transport towards suitable nurseries. Our results have profound implications regarding the comprehension of hydrographical processes that govern the dispersal and retention of fish larvae. Our findings further shed light on the establishment of assemblages and the habitat expansion of fish larvae in upwelling ecosystems.