Assessment of the effectiveness and efficiency of two fishways with vertical slot openings in an Alpine River (Toce River, northern Italy)

Abstract

Weirs and dams impede the longitudinal connectivity of rivers and create obstacles to free fish migrations. Blue infrastructures, such as fishways, have been implemented to restore river connectivity over the last 20 years. However, after construction, issues regarding the functionality and effectiveness of fishways often remain. In this study, we investigated the effectiveness and efficiency of two step-pool fishways with vertical slot openings (i.e., Tana and Prata fishways), in the Toce River, an Alpine River tributary of Lake Maggiore. We carried out this investigation over two years by using combined technologies: PIT tag telemetry and video monitoring, taking into account the fish size and river discharge. Fishways were suitable and utilized by trout (Salmo spp.), barbels (Barbus spp.), chub (Squalius squalus) and dace (Telestes muticellus), but not for bullhead (Cottus gobio). Additionally, we PIT tagged both wild and hatchery-reared trout, to compare the use of the fishways by wild and hatchery fish. The passage efficiency was 65.4 % for the Tana fishway and 60.0 % for the Prata fishway, corresponding to values within the range found in the literature. On the other hand, attraction efficiency was very low. Our observations suggest that finding and entering the fishways was more challenging than completing the passage. Most passages occurred in autumn and spring, in relation to spawning migration and high-flow periods. Slight differences were recorded between the two fishways, with longer transit times for the Tana fishway and better performance for adult wild trout at the Prata fishway. The combination of telemetry and video monitoring provided important information to guide future river defragmentation projects in the area and for similar water basins.