Co-occurrence of North American crayfish in north-western Italian lakes

Invasive crayfish are widespread in the Italian lakes, but their distribution and impact remain poorly understood. We carried out a multi-year study to assess the coexistence of Faxonius limosus, Pacifastacus leniusculus and Procambarus clarkii in lakes Maggiore, Mergozzo and Orta. We caught crayfish using traps across different littoral habitats (piers, beaches, reed beds) to better depict their spread. Morphological traits (body length and weight, cephalotorax length/width), Catches Per Unit Effort (CPUE), sex ratio and how they differ among the species and between the lakes were discussed. Faxonius limosus is so far diffused in the three lakes: it coexists with P. clarkii and P. leniusculus in Lake Maggiore, and with P. clarkii in Lake Orta, but it is still the unique in lake Mergozzo. While F.limosus dominated in Lake Maggiore, P. clarkii is the most abundant in lake Orta. Both, F.limosus and P.clarkii manifested highest body length and weights in lake Maggiore in respect to other lakes. At present, P. leniusculus was observed only in the Swiss part of Lake Maggiore, overlapping with the other crayfish species. Despite the steady spreading in northwestern Italian lakes, the invasive crayfish suffered with the unprecedented heatwaves in 2023 and with most likely pathogen threats.