INDAGINI SULL’HABITAT DI SALMO MARMORATUS (CUVIER, 1829) IN UN TRATTO DEL FIUME BRENTA (NORDEST ITALIA) ANALYSES OF THE HABITAT OF SALMO MARMORATUS (CUVIER, 1829) WITHIN A BRENTA RIVER STRETCH (NORTHEAST ITALY)

The marble trout Salmo marmoratus has been analyzed in a section of the Brenta River, where its populations show constant decrease despite many restocking operations carried out in the area. The study was carried out from October 2016 to December 2017 within three stream stretches placed from the border between Veneto and Trentino Alto Adige regions to the area near the Cismon del Grappa town. Analyses regarded fish communities, habitat availability for Salmo marmoratus, ecological status, fluvial functionality and chemical-physical features. Results highlighted several critical issues for the presence of well-structured marble trout populations, such as hybridization with brown trout Salmo trutta, habitat alteration and water pollution. Within the investigated area, pure Salmo marmoratus specimens (frequencies equal to 4-25%) and hybrids with Salmo trutta (5-27%) were observed. Brown trout was largely present in the area (8-60%). Available habitat ranged from 0 to 21% of the river bed for marble trout, which competes with brown trout for spaces and trophic 37 resources. Ecological state analyses lead to a judgement equal to sufficient for all the stretches due to alteration of the trophic condition, which as highlighted by the macrophytic index RQE_IBMR and by the nutrient concentrations. This is probably related to a reduction of the river functionality, due to the anthropogenic impacts which were detected in the area.