Variability of larval fish assemblages under contrasting winter environmental conditions in the northwestern Mediterranean Sea

This study analyses the influence of the winter environmental conditions on the structure of larval fish assemblages off the Catalan coast (NW Mediterranean). Data were obtained during two oceanographic surveys (February 2017 and 2018) with contrasting environmental conditions. Winter 2017 was mild, the coastal zone was occupied by cold, low salinity and productive surface waters, and the water column was slightly stratified. Winter 2018 was more severe, temperatures were lower, with intense vertical mixing and deep-water formation and cascading events down the slope that enhanced shelf-slope water exchanges. Larvae of 52 fish taxa, both shelf and oceanic, were identified, with Sardina pilchardus being numerically dominant. Larvae of shelf fish species were more abundant in 2017 associated with the productive coastal waters. Larvae of oceanic species were more abundant in 2018, with a wide distribution all over the area that would be related to the vertical mixing and the shelf-slope exchange processes that year. Accordingly, the distribution of fish larvae in the water column was wider and deeper in 2018 than in 2017. The assemblages identified by the cluster analysis were determined by bottom depth, sea surface temperature and sea surface chlorophyll-a, with dynamic height being the main factor explaining the differences in assemblages between years. These differences highlighted the role of the shelf-slope water exchange processes in 2018, mainly in the submarine canyons. The results shed light on how changes in larval fish assemblages are indicators of shifts in environmental conditions. Under a climate change scenario, it is likely that the decrease in the fertilisation processes, such as deep-water formation (by convection or cascading) and vertical mixing, would have adverse effects on larval fish populations.