Vertical distribution of the European sardine (Sardina pilchardus) larvae and its implications for their survival

This study presents results of the vertical behaviour of the European sardine (Sardina pilchardus) larvae as observed at sea off the NW Iberian coast during an oceanographic cruise conducted in May 2002. Samples were taken in a grid of 38 stations (conductivity-temperature-depth [CTD] measurements and Longhurst Hardy Plankton Recorder [LHPR] plankton hauls); a 69-h fixed station study was also performed (hourly CTD measurements and LHPR/neuston hauls every 2 h). The horizontal distribution of larvae is closely related to the circulation patterns measured by a current metre-mooring array deployed during the cruise. Larvae were mainly distributed in the upper 20-25 m of the water column, in evident association with the waters of the Western Iberia Buoyant Plume (WIBP). Large (older) larvae are found mainly in the surface layers, and larval size decreases with depth. A diel rhythm of migration to the neuston layer was observed, correlated with the inflation/deflation activity of the swim bladder. Larvae with lengths greater than 12.5 mm and inflated swim bladders were only found in this layer. Considering the near surface stratification conditions for food availability and Ekman transport in the upper few metres, even small vertical migrations of larvae can be very important for their survival and subsequent recruitment success.