Recurrence of the Spatial Structure of Summer Larval Fish Assemblages Linked to Hydrodynamics in the NW Mediterranean

In this study, we aim to investigate the contribution of various sources of variability, environmental, spatial and interannual on the larval fish assemblages (LFA) in summer off the Catalan coast, an area with a wide array of environmental conditions. The hydrodynamics of the area is controlled by the Northern Current, flowing southwestwards along the continental slope. This current displays high mesoscale variability presenting a meandering path and a series of anticyclonic eddies over the shelf. The study was based on three ichthyoplankton surveys carried out in July 2003, 2004 and 2012. The number of identified taxa, belonging to shelf and oceanic fish species, was quite similar among cruises, around 80, with the small pelagics Engraulis encrasicolus and Sardinella aurita being the dominant species. A high spatial heterogeneity in LFA was detected throughout the studied area. Most of this heterogeneity was related to the spatial variability of environmental factors, such as the continental shelf structure, latitudinal difference in surface temperature, chlorophyll-a concentration and stratification level. Hydrodynamic structures, mainly instabilities of the Northern Current and anticyclonic eddies, and the thermal front across the shelf, also played an important role in the configuration of the LFA. Although these showed certain interannual variability, mainly related to the 2003 heat wave and the low surface productivity of 2012, the recurrence of hydrodynamic structures, together with the spawning habits of adults, means that the spatial structure of LFA is maintained in different years.