Mesohabitat suitability criteria for the spawning of Twaite shad (Alosa fallax, Lacépède 1803)

Twaite shad (Alosa fallax, Lacépède 1803) is an anadromous fish that relies on European rivers for spawning. However, populations are declining due to river damming and hydro-morphological alterations, which reduce and degrade suitable spawning grounds. This study presents the first meso-scale habitat suitability criteria (HSC) in the literature for the spawning period of A. fallax. Using the MesoHABitat SImulation Model (MesoHABSIM) approach, we identified the geomorphic units (GUs) and associated habitat attributes preferred for mating purpose by A. fallax in the Tagliamento River (NE Italy). Habitat characterization was conducted using Uncrewed Aerial Systems (UAS), a two-dimensional (2D) hydrodynamic model, and field surveys performed during a monitoring period of 15 months. A classification Random Forest (RF) model, with high accuracy (98.8%) and true skill statistic (97.6%), was employed to identify key meso-habitat features for the reproduction of A. fallax. The developed HSC revealed that A. fallax prefers glides and riffles to spawn, with shallow depths (0.15-0.45 m), moderate velocities (0.30-0.75 m/s), and small sediment sizes (0.2-6 cm). Additionally, we employed a nonintrusive monitoring method using infrared cameras, which successfully recorded 72 surface mating events over two nights. This method proved effective since it was possible to precisely define the location, the duration of each mating event and the number of fish involved. The findings provide valuable insights for developing management strategies aimed at preserving or restoring A. fallax spawning habitats