Role of unstable winter dynamics shaping the fish larval distribution in the Northwestern Mediterranean

Abstract

Winter conditions in the NW Mediterranean cause instability of the water column and non geostrophic dynamics, such as vertical mixing and convection, are significant. These events contribute to renew nutrients at the photic zone thus providing conditions for phytoplankton blooms at shallow shelves. Interestingly, besides the high productivity in winter, only few fish species reproduce (~20 %): those characteristic of relatively cold water of north Atlantic biogeographic affinity. This study aims to elucidate how the winter hydrodynamics off the Catalan coast determine the spawning strategies of Micromesistius poutassou and Sardina pilchardus in February 2017 and 2018. In 2017, a surface strip of low salinity, due to heavy rain, was found in a wide area along the northern coastal area. Besides this situation played against vertical mixing, these surface waters were rich in nutrients. By contrast, 2018 was dry thus with higher surface salinity and lower vertical stability involving subduction and cascading events. In both years, the shelf-slope front was evident through the epipelagic layers. The spawning and larval distribution of S. pilchardus were associated with the coastal productive waters. The spawning of M. poutassou took place along the upper slope and even over the shelf, also at the submarine canyons, but offshore limited by the front. Larger larvae were mainly found over the highly productive shelf. The larval vertical distribution of both species showed high variability, not only related with the daily cycle or developmental stage, but also with the vertical instability. The well known hydrodynamic structures, such as anticyclonic eddies that use to retain larvae during the stratified season, become less important in winter. In that case, the shelf-slope front appears to be a key factor retaining M. poutassou larvae avoiding their offshore dispersion, even though the species is commonly considered as oceanic