Population dynamics and preliminary stock assessment of the blue crab Portunus segnis (Forskål, 1775) in Bizerte Lagoon, Tunisia

The Lessepsian migrant crab, Portunus segnis , has successfully colonised Bizerte Lagoon in Tunisia, raising ecological concerns while supporting an emerging fishery. This study provides a preliminary, data-limited assessment using two models: the catch-based CMSY++ (Catch-Maximum Sustainable Yield) model and the length-based LBB (Length-Based Bayesian Biomass) model. Catch data (2019-2025) and length-frequency data (1014 specimens, 2021-2024) were analysed. The population structure revealed a significant increase in the mean carapace length, from 48.1 mm in 2021 to 59.8 mm in 2024, with females consistently larger. Landings increased sharply from 74 kg in 2019 to 7.34 tonnes in 2022, then stabilised at 11.5-12.2 t/year, a trend partly attributable to increasing fishing effort. CMSY++ estimated the stock to be fully to lightly overexploited ( ; ), with Maximum Sustainable Yield (MSY = 9.9 t/year) and Carrying Capacity ( ). Retrospective analysis revealed a moderate bias (Mohn’s ), though this should be interpreted cautiously given the short time series. LBB broadly supported these findings ( ; ), indicating that the length at first capture (60.3 mm) exceeds the optimal length at first capture (Lc opt = 57 mm). Eighty-eight per cent of the landed individuals were mature, indicating that the current level of selectivity spares a large proportion of the spawning stock. The overall consistency between the models offers exploratory indicators that may inform future management, subject to validation, including setting catch limits near MSY (approximately 10 t/year), enforcing a minimum carapace length ( CL ) of ⩾60 mm, and strengthening monitoring. The broader impacts of ecosystem-level invasion require separate assessment.