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

Abstract

The thermal infrared data from the satellites of the European Meteosat series are used to characterize the dynamics of the West African coastal upwelling for the period 1984-1998. Sea surface temperature (SST) images are produced 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 upwelling by automatic localisation of the SST minima at each coast location and to derive a normalised upwelling intensity index, based on SST differences. A new index for retention potential is also proposed. The strong interannual variability observed in these indices is quantified, mainly in terms of intensity and seasonal lag. An objective method of space partitioning, based on the variability of the proposed indices is introduced. Objective analysis applied to three characteristic regions shows that the spawning areas of the coastal pelagic species may be clearly delineated in terms of spatial and temporal optimisation of enrichment and retention processes. The importance of the retention processes is shown for the recruitment of Sardinella aurita in Senegal and for Octopus vulgaris in Mauritania. According to the concept of Bakun's triad for upwelling areas, a precise determination of the upwellings intensity and associated spatial processes is essential for evaluating the balance between enrichment and its optimal use in an ecological 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 indices. (C) 2000 Ifremer/CNRS/IRD/Editions scientifiques et medicales Elsevier SAS.