Estimation de la précision des campagnes acoustiques au Sénégal par la méthode géostatistique transitive à une dimension

This study presents an application of the geostatistical one-dimensional (1D) transitive method for estimating the precision of acoustic pelagic abundance estimates in Senegal. In applying the method, the major problem encountered was how to take into account the temporal variability in the computation of the estimation variance. In acoustic surveys, echointegration is performed continuously along the ship's sailing track. For a survey design made of parallel transects, each transect can thus be considered as a sampling unit. By summing the density values along the transects to obtain biomass values per transect, the 2D estimation is simply reduced to a 1D estimation procedure. If the transects are equidistant, the geostatistical transitive method applied in 1D on the biomass values per transect gives a correct and simple solution to the estimation of the survey precision. The method has been applied to the Senegalese acoustic surveys which were performed with the same sampling design made of regularly spaced parallel transects oriented East-West. The densities were summed along the transects. The latitudinal variation of the biomass per transect was presented as a 1D profile. Biomass, spatial distribution and its structure (autocovariance) showed great differences between surveys. Profiles were grouped in 4 categories which presented different structural characteristics. In each category, the average structure was estimated and modelled and the average relative estimation error was estimated. The relative estimation error varied with the differences in structure between categories but also with model hypothesis about the structure at small distance. Two hypothesis were considered, continuity of the distribution gt small distance (absence of nugget effect), or discontinuity (presence of nugget effect). The maximum relative error estimated was 31%. The major problem in modelling the structure of the 1D profile was to fit or not to fit a nugget effect. The interpretation of such nugget could be temporal variability at the scale of the transect.