ASSESSMENT OF THE SPATIAL STRUCTURE AND BIOMASS EVALUATION OF NEPHROPS NORVEGICAS (L.) POPULATIONS IN THE NORTHWESTERN MEDITERRANEAN BY GEOSTATISTICS

The spatial distribution and biomass of Nephrops norvegicus were asses sed by trawling over commercial fishing grounds (''Serola'', off Barce lona, Spain) during two surveys (spring and fall 1991), using geostati stical methods. The surveys were set 6 months apart, in order to analy se possible seasonal differences. In the present surveys, Norway lobst er was caught between 200 and 600 m depth. with peak abundance at abou t 400 m. The analysis of the structure of spatial correlation by means of semivariograms showed that densities of Nephrops norvegicus were s patially autocorrelated and lobster populations were distributed in hi gh-density patches 6 to 9 km in diameter. No spatial segregation per b iological category (size or sex) was detected. The semivariograms were consistent for all biological categories. A strong linear relationshi p between local mean and standard deviation (proportional effect) was modelled by the relative semivariogram. Relative experimental semivari ograms were fitted to a spherical model. The share of the semivariogra m, and the spatial autocorrelation structure of Norway lobster populat ions, remained stable over the two surveys. The density of Nephrops no rvegicus available to the experimental gear was mapped by point krigin g. High-density patches of different biological categories exactly con formed and remained stable over the two surveys, showing a certain int ra-annual stability. However, mean densities and overall abundance (co mputed by global kriging) decreased sharply in the fall survey. This w as accounted for by means of knowledge on the biology of the species f or the same area. The biological characteristics of Nephrops populatio ns in the area studied are similar to those of other Mediterranean and Atlantic populations, hence our results are not restricted to the stu dy area. We conclude that the geostatistical analysis approach, which rakes into consideration the spatial autocorrelation structure of the populations, is adequate for the direct biomass estimation and assessm ent of Nephrops harvestable stock.