Comparative study of spatial distribution patterns of the early stages of anchovy and pilchard in the NW Mediterranean Sea

Abstract

Anchovy and pilchard are the most abundant pelagic species in the NW Mediterranean Sea. They spawn in different seasons, subject to different environmental conditions: anchovy in summer, when the water column is stratified; pilchard in winter, when the water column is vertically homogeneous. The spatial distribution patterns of eggs and larvae of these 2 species are compared in relation to the main productive features in the region during their respective spawning seasons. The study was performed on the continental shelf off the Ebro River (NW Mediterranean) during June (for anchovy) and February (for pilchard). Sampling comprised a horizontal survey, designed to locate patches of eggs or larvae, followed by a Lagrangian experiment using stratified hauls to study the vertical distributions at different times of day in the water parcel tracked. While pelagic eggs and larvae of both species were present in the upper 70 m of the water column, we recorded differences in the preferential vertical distribution and migration patterns. Maximum concentrations of anchovy eggs and larvae were located in the upper 20 m; pilchard concentrations extended down to 10-40 m. Vertical displacements by the larger larvae at night occurred in both species, but with opposite patterns: anchovy larvae tended to aggregate in the upper 10 m, pilchard larvae exhibited greater dispersal at night, with a preference for levels below 30 m.