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.