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

This study presents results of the vertical behaviour of theEuropean sardine (Sardina pilchardus) larvae as observed atsea off the NW Iberian coast during an oceanographic cruiseconducted in May 2002. Samples were taken in a grid of 38 stations(conductivity-temperature-depth [CTD] measurements and LonghurstHardy Plankton Recorder [LHPR] plankton hauls); a 69-h fixedstation study was also performed (hourly CTD measurements andLHPR/neuston hauls every 2 h). The horizontal distribution oflarvae is closely related to the circulation patterns measuredby a current metre-mooring array deployed during the cruise.Larvae were mainly distributed in the upper 20–25 m ofthe water column, in evident association with the waters ofthe Western Iberia Buoyant Plume (WIBP). Large (older) larvaeare found mainly in the surface layers, and larval size decreaseswith depth. A diel rhythm of migration to the neuston layerwas observed, correlated with the inflation/deflation activityof the swim bladder. Larvae with lengths greater than 12.5 mmand inflated swim bladders were only found in this layer. Consideringthe near surface stratification conditions for food availabilityand Ekman transport in the upper few metres, even small verticalmigrations of larvae can be very important for their survivaland subsequent recruitment success.