Introduction:
Potamodromous fish spend their entire life cycles within freshwater environments and play a crucial role in maintaining the biodiversity and ecological balance of river ecosystems (Pennuto et al., 2018, Baras & Lucas, 2001). Their migratory behaviour is influenced by intrinsic factors such as age, life history stage, sexual maturity, or energy content, as well as extrinsic factors like temperature, season, presence of congeners (Castro-Santos & Haro, 2010; Lucas & Baras, 2000) and the availability of suitable habitats, including gravel banks and flood plain systems (Jungwirth et al., 1998, Reckendorfer, 2019).
The Danube River, one of Europe’s most important watercourses, with its complex network of tributaries and variously connected flood plains provides a mosaic of diverse habitats for about 114 fish species along its entire river course (Sommerwerk et al., 2009, Schiemer et al., 2004, Schmutz & Jungwirth, 2022). However, since the 19th century, the Austrian section of the Danube has been heavily modified, with extensive straightening and disconnection from many of its natural floodplains and tributaries (Hohensinner et al., 2008, 2009 & 2013). The construction of hydroelectric power plants in the 20th century further disrupted the natural migratory patterns of Danube fishes (Kowall et al., 2024). Currently, 273 km, or 80 % of the Austrian Danube is impacted by 10 hydro power plants (Zauner et al. 2017). As a result of this regulation and damming, shallow gravel bank zones with bay structures, which are crucial habitats for rheophilic fishes, have been greatly reduced compared to the original river landscape of the Danube (Schiemer et al. 1991; Zauner & Schiemer, 1992; Keckeis et al., 1997). In recent decades, deficits in the fish fauna of the Austrian Danube have been documented, including, the absence or dysfunctional population structure of dominant species and an overall low fish abundance and biomass (Zauner et al., 2015, Zauner et al., 2017). These deficits cannot be addressed solely by restoring upstream longitudinal connectivity; they also require the creation of essential habitats, such as spawning grounds and nurseries, to support the Danube fish community (Schmutz, 2012, Koller-Kreimel 2017). According to a recent study, existing fish passes in Austria have improved passability by an average of 20% to 24%. However, longitudinal connectivity in the Austrian Danube system remains significantly disrupted (Kowall et al., 2024). To address this issue, the design of new fish passes should ideally incorporate not only longitudinal connectivity but also lateral connectivity and habitat availability (Kowall et al., 2024; Silva et al., 2017).
Traditionally, fish passes or fishways have been constructed to facilitate upstream migration, allowing fish to bypass barriers such as dams and weirs (Castro- Santos & Haro, 2010, Silva et al., 2017, Porcher & Travade, 2002) and it was assumed that they are unsuitable or quantitatively insignificant for downstream migration (Pelicice et al., 2015, 2020, Pelicice & Agostinho, 2012, Knott et al., 2023, Larinier & Travade, 2002, Eberstaller et al., 2001). Recent studies have begun to challenge this assumption, suggesting that fish passes may also support bidirectional movement, enabling potamodromous fish to migrate downstream (Reckendorfer et al., 2023, Petz-Glechner, 2009, Telhado et al., 2015, Unfer & Rauch, 2019, Sanz-Ronda et al., 2021, Celestino et al., 2019, Calles & Greenberg 2007).
Our study contributes to the evolving discourse on potamodromous fish migration by investigating the use of fish passes for downstream movement and the potential of bypass systems to serve as spawning and feeding habitats. Focusing on a specific case study at a run-of-river hydro power plant at the Austrian Danube, we employed PIT tags and detection arrays at the fish pass of the Hydro Power Plant Ottensheim-Wilhering. We used the common nase, Chondrostoma nasus , as an example for a frequently occurring, rheophilic potamodromous fish species in the Austrian Danube to examine if and how fish utilize the fish pass for downstream migration and to investigate their use of the additional habitats provided by the integration of two natural tributaries within the fish pass system.