Hydromorphological Features and Refugia as Key Features in Shaping Salmo marmoratus Population Structures in Several Watercourses of Friuli Venezia Giulia (Northeastern Italy)

Among salmonids, the marble trout Salmo marmoratus is endemic to the Adriatic River Basin of northern Italy and the Adriatic River system of the Western Balkans. It is an iconic fish of angling and conservation interest but, despite this, the species is heavily threatened by habitat loss, river fragmentation, water pollution, overfishing, water diversion and hybridization with the brown trout Salmo trutta. The marble trout is therefore listed in the Habitat Directive 92/43/EEC (Annex II) and reported as ‘Critically Endangered’ in the Red List of the Italian Vertebrates, but a progressive restriction of the original distribution area occurred since the second half of the past century. Due to these reasons, many conservation projects have been raised in recent years. In the present study, we report early results of a new conservation project about marble trout, carried out in several watercourses of Friuli Venezia Giulia (Northeastern Italy), where the species is still present with representative densities. The main aim of the study was to highlight the main hydromorphological features favorable to support the marble trout populations within the investigated watercourses. Electrofishing sampling activities were carried out in 2025 within eight watercourses, while main mesohabitats and substrate types were recorded. Our observations suggest that the presence of different types of refugia appeared to play a pivotal role to support well-structured trout populations: the presence of a combination of shallow margin areas, rocks and woody debris may serve as key nursery zone whereas canopy shade, eroded banks, and boulders within main mesohabitats likely function as preferred sheltering spots for adult specimens. Conservation projects have a pivotal importance, and proper ecological analyses could provide useful information for management programs. In this context, environmental restoration activities, including the placement of rocks, large boulders and large trunks within the riverbed areas, may represent a viable, low-cost strategy to enhance habitat availability for the species.