Ecology of the Lasca Protochondrostoma genei (Bonaparte 1939) in the Chiarò di Cialla Creek (Northeast Italy): New Insights from a Biodiversity Hotspot Affected by Alien Species

The lasca Protochondrostoma genei is an endemic freshwater fish of high conservation concern, listed in Annex II of the Habitat Directive and classified as Endangered in Italy. This study investigates the ecology, population structure, and habitat use of P. genei in the Chiarò di Cialla Creek (Northeast Italy), a biodiversity hotspot recently designated as a Special Area of Conservation (SAC IT3320041). Particular regard is given to the issue represented by the presence of the common nase Chondrostoma nasus, which competes with the lasca for habitats and trophic resources. Fish assemblages were surveyed by electrofishing during 2023 and 2024, coupled with detailed hydromorphological characterization of mesohabitats. Results highlighted that the lasca showed preference for glides and especially pools, characterized by moderate depths, low to moderate current velocities, high shading, and abundant shelters such as boulders, roots, and woody debris. However, P. genei extensively overlapped in habitat use with the invasive common nase C. nasus, whose density increased markedly during the study period, suggesting ongoing colonization and potential issues for the lasca conservation. Despite moderate river functionality scores and localized anthropogenic pressures, the Chiarò di Cialla Creek remains a key refuge for P. genei. These findings provide essential ecological information to support targeted conservation actions, including habitat management and invasive species control, within protected riverine systems.