Small-scale non-linear geostatistical analysis of Liocarcinus depurator (Crustacea: Brachyura) abundance and size structure in a western Mediterranean population

Abstract

The small-scale spatial distribution of density and biomass of the crab Liocarcinus depu- rator off the Ebro delta was analysed using universal kriging (non-linear geostatistics), considering depth as an external trend. In order to understand the spatial distribution of crab size, a third variable was created from the residuals of the non-linear regression between biomass and density. Variogram analysis of crab density and residuals of the biomass/density relationship presented a quadratic rela- tionship with depth, while biomass showed a linear relationship with depth. All experimental vario- grams were fitted to spherical models. The patch size (range) found was 10.64 km for density, 14.53 km for biomass and 20.49 km for the residual variables. The spatial distribution of 8 sediment variables (temperature, redox potential at 1 and 6 cm depth (R1 and R6), carbonate content (CO3), median grain size (ϕ), sediment sorting (IGSD: inclusive graphic standard deviation) and organic matter content (OM)) was studied. The maps obtained were compared with those calculated for the biological variables of L. depurator using Mantel and partial Mantel tests. Distribution characteristics of this species were significantly affected by most sediment variables considered, particularly by tem- perature, organic matter content and redox potential, although overall the correlation level was low, coinciding with the generalist ecological role and opportunistic behaviour of the species. Density and biomass, as well as residuals, presented a negative correlation with the sorting coefficient (IGSD) and with carbonates. In relation to IGSD, high densities of L. depurator were found in homogeneous sed- iments. Residuals (size structure of the population) were significantly correlated with grain size (ϕ), with larger individuals occurring in finer sediments, while juveniles were preferably found in areas with very fine sand sediments. Both density and biomass presented significant positive correlations with the redox potential, indicating that the species favoured well-oxygenated sediments with a low organic content.