Control planning for invasive crayfish: the case of Pacifastacus leniusculus (Decapoda, Astacidae) in the Clitunno River (Central Italy).

The signal crayfish Pacifastacus leniusculus (Dana, 1852) is one of the most ecologically impactful decapod crustaceans introduced in Europe. As a species of Union concern, early detection and rapid eradication measures are required to prevent its establishment and spread. We aimed to: i) test the effectiveness of the control plan prepared within the LIFE IMAGINE IPE/IT/000015 project to contrast the diffusion and contain demographic growth of P. leniusculus in the Clitunno River basin (Central Italy), where the species has been detected in 2020; ii) assess distribution, age structure and growth of P. leniusculus in this invaded area. The removal actions were conducted biweekly, from June 2022 to December 2024, using both traps and electrofishing. Biometric parameters were individually recorded, and the demographic features and growth of P. leniusculus population were assessed. In total, 259 removal actions were carried out and a biomass of 39.74 Kg has been removed. Maximum size (Total Length = 14.8 cm) was consistent with what has been reported for another Italian population, but greater than some native Californian ones. The presence of six cohorts (from 0+ to 5+), including the young-of-the-year (0+), attested the quick acclimatization of P. leniusculus , which gave rise to a self-sustaining population in a short time. The greatest removal efforts have been concentrated on a small tributary, named Fosso Vecchio, where the average values of Catch Per Unit Effort (CPUE) showed a significant decreasing trend over time, arguing in favour of the removal actions' effectiveness. Our findings provided some evidence that early detection and control measures conducted at a small scale, applying strong catching efforts, represent effective management tools to limit population abundance and prevent invasive crayfish from spreading further.