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 assessed by trawling over commercial fishing grounds ("Serola", off Barcelona, Spain) during two surveys (spring and fall 1991), using geostatistical methods. The surveys were set 6 months apart, in order to analyse possible seasonal differences. In the present surveys, Norway lobster was caught between 200 and 600 m depth. with peak abundance at about 400 m. The analysis of the structure of spatial correlation by means of semivariograms showed that densities of Nephrops norvegicus were spatially autocorrelated and lobster populations were distributed in high-density patches 6 to 9 km in diameter. No spatial segregation per biological category (size or sex) was detected. The semivariograms were consistent for all biological categories. A strong linear relationship between local mean and standard deviation (proportional effect) was modelled by the relative semivariogram. Relative experimental semivariograms were fitted to a spherical model. The share of the semivariogram, and the spatial autocorrelation structure of Norway lobster populations, remained stable over the two surveys. The density of Nephrops norvegicus available to the experimental gear was mapped by point kriging. High-density patches of different biological categories exactly conformed and remained stable over the two surveys, showing a certain intra-annual stability. However, mean densities and overall abundance (computed by global kriging) decreased sharply in the fall survey. This was accounted for by means of knowledge on the biology of the species for the same area. The biological characteristics of Nephrops populations in the area studied are similar to those of other Mediterranean and Atlantic populations, hence our results are not restricted to the study 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 assessment of Nephrops harvestable stock.