Robust Bayesian inference of carapace width–weight scaling in the invasive blue crab ( Callinectes sapidus )

Abstract

The Mediterranean basin is a region where both natural and anthropogenic factors have contributed to the successful spread of invasive alien species. The Atlantic blue crab ( Callinectes sapidus ), long established and commercially harvested along the Levantine coast of Türkiye, is now expanding northward along the Aegean coastline, where our study area is located. Strong relationships between morphometric data and biomass provide valuable information for mitigating the ecological and economic impacts of this species. In this study, conducted in Azmak Stream (İzmir, Türkiye) using standardized traps, the carapace widths (CWs) and total weights of 197 individuals were measured. Four competing Bayesian models were constructed, comparing Gaussian and Student’s t -error distributions with and without sex interaction, and evaluated using the Pareto-smoothed importance sampling leave-one-out cross-validation. Model M3 (elpd_loo = 101.11194), which incorporated a sex-factored Student’s t -distribution, outperformed the Gaussian counterpart (M2; elpd_loo = 97.34773). Furthermore, the degrees-of-freedom parameter (ν = 8.0) indicated that biological variation was more appropriately modelled using a Student’s t -distribution. In M3, the scaling exponent for males ( b _M ) was 2.70 (95% credible interval [CrI]: 2.40–3.00), while the female–male exponent difference was −0.52 (95% CrI: −0.87 to −0.17), corresponding to a female exponent of b _F  ≈ 2.18. Although both sexes exhibited negative allometric growth ( b  < 3), growth strategies clearly differed between the sexes. These findings demonstrate that uniform models may mislead blue crab stock assessments and highlight the importance of using sex-specific parameters in management strategies.