Mesohabitat Spawning Preference of the Anadromous Twaite Shad (Alosa fallax, Lacépède 1803) in the Tagliamento River (Italy)

Alosa fallax (Lacépède, 1803) is an anadromous fish which utilizes European rivers for spawning. As many anadromous species, Twaite shad populations are declining due to river damming and hydromorphological alterations, which impact their spawning sites. In this study, we developed mesohabitat suitability criteria for the spawning period of A. fallax by analysing the geomorphic units (GUs), with their local habitat attribute, in which the fish prefers to spawn. The study was conducted in the Tagliamento River (NE Italy). Habitat depiction was performed following the MesoHABitat SImulation Model (MesoHABSIM) approach. High-resolution spatial information from Uncrewed Aerial Systems (UAS), a two-dimensional (2D) hydrodynamic model and field data collected during the spawning period were utilized for habitat attribute evaluation. The association between spawning sites and GUs characteristics was explored by training a classification random forest (RF) model. The final parsimonious RF model demonstrated high accuracy (98.8%) and true skill statistic (97.6%), indicating that A. fallax prefers glides and riffles with shallow depths (0.15–0.45 m), moderate current velocities (0.30–0.75 m/s) and small-sized sediment (diameter 0.2–6 cm) for spawning. Using an infrared camera, 72 surface mating events were distinctly recorded between 11.30 PM and 02.15 AM over two nights, demonstrating the technique's suitability for observing shad mating activity. The video analysis revealed that the monitored A. fallax population exhibited similar mating behaviour to other European shads (e.g., Alosa alosa). This study provides useful insights to develop novel management approaches for preserving or restoring the spawning habitat of the A. fallax, supporting its conservation.