Eradication attempt for an early detected 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 an eradication attempt undertaken to counteract the spread and contain the 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 activities were carried out and a biomass of 39.74 kg was removed. The presence of six cohorts (from 0+ to 5+), including the young-of-the-year (0+), attested to the rapid acclimatisation 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, suggesting the effectiveness of the removal actions. Our findings provided some evidence that early detection and eradication measures conducted on a small scale, but with high effort, represent effective management tools to limit population abundance and prevent invasive crayfish from spreading further.