Bayesian Spatio-Temporal Modeling of the Atlantic Blue Crab Callinectes sapidus: Invasion Dynamics and Socioeconomic Impacts in Tunisia and Italy

The Atlantic blue crab (Callinectes sapidus, Rathbun, 1896) has established self-sustaining populations throughout the Mediterranean basin, generating radically divergent socioeconomic outcomes in different national contexts. In Italy, the 2023 outbreak in the Northern Adriatic drove clam production to zero in some months and prompted emergency government expenditure exceeding Є12.9 million (approximately USD 13.9 million). In contrast, Tunisia has converted the same biological pressure into a multimillion-dollar export industry, shipping over 7,500 tons annually by 2021 with continued growth through 2025. This study develops a rigorous Bayesian spatio-temporal framework to model invasion dynamics and their socioeconomic consequences, integrating (1) a reaction-diffusion partial differential equation for population spread driven by sea-surface temperature (SST) and salinity covariates, (2) a hierarchical Bayesian negative-binomial model for catch-per-unit-effort data with Gaussian Process spatio-temporal random effects, (3) a state-space Bayesian model for fishery revenue impacts estimated via the Ensemble Kalman Filter, and (4) a causal Differences-in-Differences panel model to isolate the net economic effect of the invasion from confounding trends. Model parameters are inferred using Hamiltonian Monte Carlo (No-U-Turn Sampler) with full posterior uncertainty quantification. Applied to openly available Food and Agriculture Organization of the United Nations, Copernicus, and published survey data (2010 to 2024), the framework developed here identifies SST as the dominant driver of northern expansion ( SST = 0.42$, 95% confidence interval: [0.31, 0.53]), quantifies the economic damage to Italian small-scale fishers at approximately Є8,200 per vessel per year (approximately USD 8,900) post-2021, and projects continued Mediterranean-wide colonization under both RCP 4.5 and RCP 8.5 climate scenarios. The cross-country comparison reveals those adaptive institutional responses—not ecological differences—are the primary determinant of net socioeconomic outcomes, with strong implications for Mediterranean fisheries governance.