Coastal upwelling and associated retention indices derived from satellite SST. Application to Octopus vulgaris recruitment

The thermal infrared data from the satellites of the European Meteosat seri es are used to characterize the dynamics of the West African coastal upwell ing for the period 1984-1998. Sea surface temperature (SST) images are prod uced from satellite data with a spatial resolution of 5 km and a time-step of 5 days. These are used to characterize the spatial structure of the upwe lling by automatic localisation of the SST minima at each coast location an d to derive a normalised upwelling intensity index, based on SST difference s. A new index for retention potential is also proposed. The strong interan nual variability observed in these indices is quantified, mainly in terms o f intensity and seasonal lag. An objective method of space partitioning, ba sed on the variability of the proposed indices is introduced. Objective ana lysis applied to three characteristic regions shows that the spawning areas of the coastal pelagic species may be clearly delineated in terms of spati al and temporal optimisation of enrichment and retention processes. The imp ortance of the retention processes is shown for the recruitment of Sardinel la aurita Senegal and for Octopus vulgaris in Mauritania. According to the concept of Bakun's triad for upwelling areas, a precise determination of th e upwellings intensity and associated spatial processes is essential for ev aluating the balance between enrichment and its optimal use in an ecologica l sense. A Java-based software package is produced as part of this study to visualise long series of pre-processed images and to extract the above ind ices. (C) 2000 Ifremer/CNRS/IRD/Editions scientifiques ct medicales Elsevie r SAS.