Patchiness and spatial structure of the early developmental stages of clupeiforms in the NW Mediterranean Sea

Abstract

We analyse the spatial structure of early developmental stages of three ecologically important small pelagic schooling fish species in the northwestern Mediterranean Sea: the anchovy Engraulis encrasicolus, the round sardinella Sardinella aurita, and the sardine Sardina pilchardus. We used data on egg and larval abundance (arranged by development stage), hydrographic data and potential prey biomass collected during two cruises in summer and autumn 2005. Our analysis is based on the computation of Lloyd's patchiness index during the ontogeny of early developmental stages, as well as the computation of spatial auto-correlograms and cross-correlograms with environmental variables. The combined analysis allowed determination of the relative role of behavioural traits and the influence of hydrographic conditions in shaping the spatial structure of the three clupeiforms. We show that the spawning and developmental strategies of the two summer-spawning species (E. encrasicolus and S. aurita) are different. Engraulis encrasicolus has relatively low patchiness for all early developmental stages, while the importance of trophic variables in determining spatial structure increases during development. Sardinella aurita spawns in a trophically favourable environment and the patchiness of its early developmental stages increases for older larvae, becoming decoupled from environmental variables. The autumn spawned S. pilchardus eggs and larvae showed a development strategy similar to S. aurita, with limited importance of environmental variables in determining their patchiness.