Impact of a winter upwelling event on the distribution and transport of sardine (Sardina pilchardus) eggs and larvae off western Iberia: a retention mechanism

A hydrographic and ichthyoplanktonic survey was conducted on the western Iberia Peninsula shelf-slope, with the objective of assessing the influence of wind-driven flow on the transport and survival of sardine early life history stages. Measurements were taken during a 2-week period of an ‘upwelling-favourable’ wind event. An offshore displacement of the surface low salinity Western Iberia Buoyant Plume (WIBP) was observed well off the shelf break. Below the plume, the current was southward over the mid- and inner-shelf. Offshore, the flow was poleward in the northern part of the survey area and equatorward in the south. A convergence zone was detected along the outer shelf and upper slope where the buoyant plume was apparently trapped and deepened. In general, eggs and larvae were distributed according to the spreading of the low salinity surface waters, and also within the convergent band. Older larvae were retained within the latter feature. The role of wind forcing on the plume and its interaction with the slope flow were analysed. A mechanism of larvae retention was formulated based on simplified models of the observed shelf-slope circulation.