Kriging density estimates for the bluemouth rockfish, Helicolenus dactylopterus (Scorpaenidae) off the southern Portuguese coast

The objectives of this work were to monitor the spatial distribution of Helicolenus dactylopterus and to determine if the species density of the population off the southern Portuguese coast was declining. The data were collected during four summer research surveys. For the surveys in 1997 and 1998 we used a random stratified design, whereas in 2000 and 2001 we conducted surveys using a regular grid. All surveys were conducted on board the RV “Noruega”. We used the kriging estimator to compute density estimates of H. dactylopterus in the survey area. In a first approach, we determined a spatial structure for each year, but directional semi-variograms could not be computed due to the scarce number of points. Thus, herein we present a new approach to finding the spatial structure when data are scarce but repeated in time. With this approach, data for various years are assumed to be realizations of similar processes; this assumption allows directional semi-variograms within sectors with the same topography to be computed. The kriging density estimates obtained assuming isotropy and the ones obtained assuming anisotropy were very similar, for the set of data analysed here, but in the second case a greater detail can be achieved. Our data show that the density estimates decreased by more than 50% from 1995 to 2000 and that the population began to recuperate from 2000 to 2002.