Role of mesoscale eddies in shaping the spatial distribution of the coexisting Engraulis encrasicolus and Sardinella aurita larvae in the northwestern Mediterranean

In recent years a northward expansion of Sardinella aurita has been reported in the Western Mediterranean. Spawning of this species takes place in summer coinciding with Engraulis encrasicolus, the dominant species in the region. Since both species inhabit the continental shelf, their larvae coexist during this period of the year. The circulation in the region is characterized by the Northern current, flowing along the continental slope. This current displays strong mesoscale activity presenting a meandering path and series of anticyclonic eddies over the shelf. These eddies are fed from offshore by the Northern current and by coastal waters from inshore thus merging and concentrating water from their both sides. The study analyses the role of mesoscale eddies shaping the distribution of S. aurita and E. encrasicolus larvae along the Catalan coast. Data covering the whole shelf and slope were obtained in 3 oceanographic cruises conducted in the summers of 2003 and 2004. E. encrasicolus larvae were found scattered on the whole continental shelf being particularly abundant over the shelf break and in the northern third of the region, associated with the intrusion of the Northern current. S. aurita larvae showed a more coastal distribution, being almost absent at the shelf break and under the influence of the Northern current. The observed spatial distribution of larval sizes evidenced the spawning preferences of each species and the transport and concentration of larvae of both species inside the eddies, as they entrain water from its two sides. This behaviour helps to ensure better survival conditions due to higher food availability brought from the offshore side but in an environment without dispersion.

► Anticyclonic eddies over the shelf fed by offshore and inshore waters. ► Eddies entrain anchovy and round sardinella larvae from distinct spawning grounds. ► Eddies, as structures of concentration and enrichment, favour survival of the species.