Using species population structure to assist in management and decision-making in the fight against invasive species: The case of the Atlantic blue crab Callinectes sapidus

The blue crab Callinectes sapidus Rathbun, 1896 is an invasive species in the Mediterranean, with a remarkable ability to adapt to various habitats, and thereby having significant impacts on biodiversity and artisanal fishing activities. Currently, fishing is a measure being considered to control blue crab populations in invaded sites. However, it is necessary to understand the temporal structure of blue crab populations (e.g., growth, reproduction, sexual maturity) to determine specific periods when control measures need to be implemented. Considering the species' ability to adapt to various habitats, it is crucial to evaluate how temperature and salinity influence population structure. In this study, we present fishers-dependent monitoring on the temporal population structure of blue crabs in two lagoons in Corsica (France) over 12 months. Through this approach, we provide new information that can assist in decision-making for the implementation of control measures. Even though these two lagoons are geographically close, blue crab populations showed differences, particularly in terms of sexual maturity. Specimens from Biguglia mature later (males: 16.16 cm; females: 16.79 cm) than those in the Palo Lagoon (males: 14.38 cm; females: 13.86 cm). Seasonal size distribution also showed differences between the lagoons and within the same lagoon between males and females. Temperature and salinity had a significant effect on the monthly relationship between carapace width and wet weight (referred to here as growth rate) for males and females and between the lagoons. In the Biguglia Lagoon, the higher the temperature, the greater the growth rate over a wide salinity window (16–30 psu); for females, the environmental window was restricted (temperature: 20–30°C; salinities > 16 psu). The dynamics differed in the Palo Lagoon, with a more restricted high growth rate window for males, and females showed a much wider window, with high growth rates over the entire temperature and salinity range. By employing blue crab's populations monitoring along with environmental parameters, we were able to determine how the environment influenced the blue crab's population structure, thus identifying periods conducive to species control. The most effective strategy would undoubtedly be to eliminate females before their period of sexual maturity and copulation ensue. In our case, this would involve intensive control in summer for Biguglia and in spring for Palo. These recommendations have been translated into operations to be implemented as part of the "Territorial Plan for Combating the Blue Crab (Callinectes sapidus) in Corsica (2024–2027)".