Conclusion
Detection efficiencies
PIT tag systems are a well-established method for studying fish migration and movement (Schmidt et al., 2016; Lucas & Baras, 2000; O’Donnell et al., 2010; Sloat et al., 2011; Hodge et al., 2015). When analysing migration patterns based on antenna detections, it’s important to account that detection efficiencies can vary due to changing abiotic factors, such as flow velocity and water depth at the antenna site. Aymes & Rives, (2009) showed that upstream movements were better detected than downstream movements. Nase, which have been tagged in the Danube downstream of the HPP Ottensheim and migrated upstream into the fish pass, showed a higher detection efficiency (approximately 60 %) (Reckendorfer, personal communication) than fish migrating downstream (24-45%, Table 1). The lower detection efficiency of fish moving downstream is probably due to the faster downstream movement of fish with the current, using the entire water column, compared to the slower, more bottom- oriented upstream movement against the current.
Fish size & detection rates
In this study, we found that detection rates increased with fish size: 12% of juvenile nase (<20 cm) were detected in the fish pass, compared to 59% of adult nase (>20 cm). Additionally, larger individuals tended to enter the fish pass earlier than juveniles. This aligns with documented behaviour of potamodromous species like nase, where larger adults typically move to spawning grounds earlier. For example, Kamler & Keckeis (2000) observed in the Austrian Fischa River that male nase (size 30-48 cm) often arrive at spawning sites before females (size 40-52 cm), occupying deep pools near the spawning area, with females arriving later and positioning upstream or downstream of the males, whereas small-sized fish were less numerous or even absent. Similarly, Penaz (1996) reported comparable behaviour. Epple et al. (2020) noted that adults were detected earlier in the year (March to April) and at lower water temperatures, while juvenile abundance peaked in August in fish bypass channels in the River Iller, Bavaria. Benitez et al. (2022) also found that juvenile nase used fish passes later in the year, from late June to early November. Apparently, juveniles use the fish pass for feeding during the growing season and enter the bypass system later in the year.
In addition to spawning migrations, nase use the fish pass at HPP Ottensheim throughout the year, depending on their size class. Meulenbroek et al. (2008) observed that nase in the 200 to 350 mm length class were underrepresented in a newly constructed fish bypass on the Danube in Vienna, while juveniles were found throughout the bypass, particularly in stagnant side arms and the pool pass. Larger individuals (>350 mm) were almost exclusively found in the meandering and straightened sections.
In our study, we found that fish repeatedly using the bypass and passing downstream were significantly larger than the average of all 190 tagged fish (p = 0.005). Previous research by Sanz-Ronda et al. (2021) showed that over 40% of tagged barbel in headwaters successfully found the exit of a slot bypass, with larger or more experienced fish showing an even higher success rate (>50%). Fish rely on spatial perception, using all senses and memory for orientation (Salena et al., 2021; Healy & Patton, 2022). This enables older and larger fish to learn and repeatedly find bypass entries (Reckendorfer et al. 2023). Kieffer & Colgan (1992) noted that fish can adapt to environmental changes and that homing behaviour is influenced by both brain development and experience. Similarly, Odling-Smee & Braithwaite (2003) suggested that fish may be predisposed to learn specific associations relevant to their navigational and migrational challenges.
During our 4-year study, larger fish (>250 mm at tagging in 2020) were detected twice as often (average 2.2 years) as smaller fish (average 1.1 years). This supports the hypothesis that larger, potentially more experienced fish may learn to use the bypass repeatedly over time. Especially considering that the upstream inlet structure, where the fish enter the fish pass from the Danube, is relatively small (2 x 5 m wide and 3.4 m high) compared to the Danube’s width (around 320 meters) and is located within a monotonous rip- rap bank at the upper end of the impoundment with a reduced flow velocity.
The total number of fish entering and leaving the bypass system has steadily decreased over the years, from 78 nase in 2021 to 31 in 2023, and just 15 fish recorded entering the system by mid-June 2024. This decline is likely caused by natural mortality. Spindler (1993) reported an average annual mortality rate of 42% for nase in the Austrian Danube and its backwaters below Vienna. Therefore, it is not surprising to see a reduction in detections four years after tagging in 2020, especially given that only a few younger nase have been detected during the study period.
The use of fish pass for downstream migration
Most research on fish passes focuses on their role in upstream migration, with many studies suggesting that fish passes are either unsuitable for downstream migration or play a minimal role in this regard (Pelicice et al., 2015, Knott et al., 2023, Larinier & Travade, 2002, Agostino et al., 2011, Pelicice & Agostinho, 2012; Bravo-Cordoba et al., 2018; Birnie-Gauvin et al., 2019; Geist, 2021, Eberstaller et al., 2001). However, recent research has begun to recognize the importance of fish passes also for downstream migration of potamodromous fishes (Reckendorfer et al., 2023, Bravo-Cordoba et al., 2023, Sanz-Ronda et al., 2021, Celestino et al., 2019, Unfer & Rauch, 2019, Petz-Glechner, 2009, Telhado et al., 2015). Especially on the Austrian Danube, with 10 large HHPs on a river length of only 350 km, the installation of fish passes allowing bi-directional movement and ideally providing habitat, is essential to benefit fish populations. In particular nase, which suffer significant habitat loss in dammed stretches, require restored migration routes alongside improved habitat conditions in the main river and its tributaries (Panchan et al., 2022).
The fish bypass system at Ottensheim-Wilhering resembles a natural river, allowing fish to swim freely in both directions. This is shown by numerous fish entering the fish pass from upstream and hundreds of detections across the five antenna arrays throughout the year. The final annual detections of some nase at the lowest antenna (#5) indicate a complete downstream migration into the Danube below the HPP. Nevertheless, nase are likely to display strong homing behaviour, also regarding their wintering habitats (Ovidio & Nzau Matondo, 2024, Panchan et al., 2022). This drives them to migrate from the bypass system back upstream into the Danube during autumn and winter, as reflected by many last annual detections at Antenna #1, situated at the upper entrance of the bypass.
Our findings demonstrate that nase, though in relatively low numbers, successfully used the fish pass for a downstream passage into the Danube below the dam. These results align with other studies conducted on the Austrian Danube by Eberstaller et al. (2001) and Meulenbroek et al. (2018).
The (repeated) use of the fish pass as a suitable habitat
During our 4-year study, 51 % of nase, tagged in the Danube above the HPP, were detected within the fish pass. By analysing the detection patterns across the 5 antenna arrays and tracking individual fish over time, we observed that while some nase used the fish pass to overcome the HPP dam within a few days, the majority spent several weeks within the bypass system (average stay >86 days per year, with a maximum of 358 days in 2022).
This 14 km long, near-natural fish bypass, with its integrated tributaries, provides suitable habitats for spawning, as shown by elevated detection rates in spring. Spawning activities in the bypass have also been reported by Zauner et al (2017), the extremely high numbers of 0+ fish and feeding traces of juvenile nase within the fish pass reported by these authors indicates the use of the bypass as fish nursery. The high “traffic” at Antenna #3, located in the Aschach River, accounted for 48% of all detections in the system, highlighting the importance of a connection to natural tributaries.
Detection data also show that after entering the fish pass in spring from the Danube above the HPP, nase utilize the various habitats within the bypass system during summer before leaving the fish pass again via the upstream end to spend winter in the Danube. Thus, near-natural bypass rivers offer much more than just longitudinal connectivity; they provide valuable habitats for numerous fish species and life stages throughout the year. These habitats are very rare in degraded large rivers like the Austrian Danube and near natural fish passes which include natural tributaries, such as at the one at HPP Ottensheim, significantly support the potamodromous fish fauna of the main river and its tributaries.
In contrast to our study, investigations of nase migration through fish-bypass channels in the Iller River, Germany, revealed that most nase entered the bypass solely for spawning and left immediately afterward to migrate downstream (Epple et al., 2020). A telemetry study by Panchan et al. (2022) on the Austrian Danube found that 100% of nase exhibited distinct homing behaviour during the spawning season, returning to specific tributaries to spawn and migrating afterwards along the entire river stretch between two hydropower plants.
In our study, 47 of 80 nase (59%) were detected in the bypass system for more than one year, suggesting repeated use and homing behaviour. This aligns with Panchan et al. (2022), who observed nase returning, sometimes multiple times, to specific sites in the river and its tributaries after months of absence. The study also highlighted that Danube nase populations have a much larger home range compared to populations in smaller waters outside the reproductive period (Baras, 1997; Huber & Kirchhofer, 1998; Benitez et al., 2018; Ovidio et al., 2016).
Results of telemetry studies on the Austrian Danube by Wagner (2010) and Eschelmüller (2009) showed that nase exhibited an average total migration of 23.11 and 33.3 km respectively and a home range of 12.1 and 14.1 km. Migration behaviour varied seasonally: Winter showed the shortest migrations (4.9 km and 7.8 km), while prespawning (1st week in February to mid of April) recorded the longest distances (up to 10.8 km). Moderate migration activities were observed in summer and autumn. Individual nase have been observed to migrate distances up to 110 km, although this is less common. Historical data show that in the 1930s, when the Danube still had richly structured river sections, the migration distances of nase were significantly shorter than in recent years (Steinmann et al., 1937).
Meulenbroek et al. (2018) observed that the migration behaviour of nase and barbel, along with their multiple spawning activities within a nature-like fish pass at the HPP Freudenau on the Austrian Danube, is comparable to behaviour observed in natural streams and tributaries of the Danube (Keckeis et al., 1997, Ovidio & Philippart 2008; Melcher & Schmutz 2010). While artificially constructed systems can provide functional spawning grounds (Pander & Geist, 2016; Meulenbroek et al., 2018), large river-like bypasses that incorporate natural tributaries offer particularly suitable habitats for the rheophilic fish fauna of the Danube.
Our data showed that nase remained within the fish pass system for extended periods, averaging about three months each year, rather than simply passing through. Given the poor quality and limited availability of habitats in both the upstream and downstream areas of the Danube, it raises an important question: why would fish migrate along the monotonous main channel of the Danube when suitable habitats are available within the bypass system? This study indicates that bypasses serve a greater purpose than just passageways, offering significant potential to support potamodromous fish species in large rivers, as supported by other research (Quigley & Harper 2006; Calles & Greenberg 2007,2009, Tamario et al. 2018, Meulenbroek et al. 2018, Panchan et al., 2022).
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