Fish migration through ship locks at the Danube River, Austria

Fish migration through ship locks at the Danube River, Austria | Authorea try { document.documentElement.classList.add('js'); } catch (e) { } var _gaq = _gaq || []; _gaq.push(['_setAccount', 'G-8VDV14Y67G']); _gaq.push(['_trackPageview']); (function() { var ga = document.createElement('script'); ga.type = 'text/javascript'; ga.async = true; ga.src = ('https:' == document.location.protocol ? 'https://ssl' : 'http://www') + '.google-analytics.com/ga.js'; var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(ga, s); })(); Skip to main content Preprints Collections Wiley Open Research IET Open Research Ecological Society of Japan All Collections About About Authorea FAQs Contact Us Quick Search anywhere Search for preprint articles, keywords, etc. Search Search ADVANCED SEARCH SCROLL This is a preprint and has not been peer reviewed. Data may be preliminary. 26 February 2026 V1 Latest version Share on Fish migration through ship locks at the Danube River, Austria Authors : Walter Reckendorfer [email protected] , Barbara Grüner , Renate Degen 0000-0001-7380-9378 , Regina Petz-Glechner , and Michael Schabuss Authors Info & Affiliations https://doi.org/10.22541/au.177212158.80949517/v1 173 views 75 downloads Contents Abstract Introduction Methods Characteristics of the ship lock at the HPP Aschach PIT antenna installation and monitoring Fish tagging Direct evidence of lock passage Indirect evidence of lock passage Data analysis and statistics Results Size distribution Seasonal and circadian patterns Lock operation Indirect evidence of lock passage Discussion Conclusion & Recommendations Ethics Statement Conflicts of Interest Data Availability Statement References Information & Authors Metrics & Citations View Options References Figures Tables Media Share Abstract Longitudinal connectivity in the Austrian section of the Danube has improved over the last three decades thanks to the progressive installation of fish passage facilities at hydropower plants. However, navigation locks which are present at all barrages, remain poorly quantified with respect to their role in fish migration. This study used long-term PIT tag monitoring to evaluate the impact of navigation locks on the upstream migration of fish at the Aschach hydropower plant in Upper Austria over a 16-month period (March 2019 to June 2020). Additionally, indirect evidence of ship lock passage was obtained by analysing long-distance movements across dams lacking fishways. A substantial number of fish, representing a wide range of species and body sizes, were detected passing through ship locks. The repeated detection of individual fish and their passage through multiple consecutive locks indicates a consistent and deliberate pattern of use rather than an incidental occurrence. Lock passage showed pronounced seasonal peaks in spring and autumn, and detections were predominantly diurnal. These temporal patterns were not significantly correlated with the frequency of lock operation. Nevertheless, an effect of lock operations could not be ruled out. Our results demonstrate that navigation locks can effectively serve as supplementary migration routes in large, regulated rivers. They also provide novel insights into species composition and size structure, as well as seasonal and diurnal patterns of lock use. Introduction Potamodromous species depend on a variety of habitats throughout their life cycle. For example, they spawn on gravel bars in swift currents, nurse in shallow littoral zones, and grow in deeper, faster areas. These distinct habitats must be accessible when needed. Thus, connectivity between these habitats is a prerequisite for sustaining fish populations. In many large rivers, connectivity is constrained by dams and other transverse structures associated with hydropower generation and navigation (Lucas & Baras, 2001; Schmutz & Mielach, 2015). To mitigate these impacts, purpose-built fishways have become the primary measure to restore connectivity at hydropower plants (Lucas & Baras, 2001; Bunt et al., 2012). While their performance has been extensively studied, alternative passage routes associated with navigation infrastructure, such as ship locks, have received comparatively little scientific attention (Travade & Larinier, 2002). Although primarily designed for navigation, several studies have demonstrated that fish can pass dams via navigation locks, including both anadromous and potamodromous species across a wide range of body sizes (Nichols & Louder, 1970; Schwevers & Gumpinger, 1998; Schubert et al., 1998, Bailey et al., 2004). In some systems, reported passage efficiencies through navigation locks equal or even exceed those of adjacent technical fishways, although lock usage can be highly variable and strongly influenced by operational protocols and environmental conditions (Moser et al., 2000; Bailey et al., 2004). Collectively, these studies highlight the potential of navigation locks as migration routes for a broad spectrum of species. Consequently, non-navigational lock operation strategies have been proposed as a potential means to enhance fish passage, particularly where conventional solutions are absent or incomplete (Garrone-Neto et al., 2014; Lin et al., 2013; Lucas & Baras, 2001; Moser et al., 2000; Travade, 2002). Despite this evidence, robust quantitative assessments of fish migration through navigation locks remain scarce in large European rivers. Existing studies are often limited to short monitoring periods, single locations, or indirect observations, and long-term datasets collected under real operational conditions are rare (Schwevers & Gumpinger, 1998; Travade & Larinier, 2002; Schmutz & Mielach, 2015). This knowledge gap is particularly relevant for highly regulated river systems, where navigation locks are ubiquitous but their contribution to longitudinal connectivity remains poorly understood. The Austrian Danube exemplifies this situation. The river is regulated by a cascade of ten hydropower plants and classified as an international Class VIb waterway. Thus, all hydropower plants are equipped with navigation locks that allow large ship convoys to overcome the head difference at each dam. Although these locks are operated exclusively for navigation purposes, they may also provide migration corridors for fish. In this study, we assess the role of a navigation lock as an upstream migration route for fish in the Austrian Danube using long-term PIT-tag monitoring. Specifically, we (1) quantify the contribution of a ship lock to upstream fish passage at a hydropower plant, (2) identify the species and size classes utilizing this route, and (3) analyse seasonal and diel patterns of lock passage in relation to lock operation. In addition, we provide indirect, system-scale evidence of navigation lock passage by analysing long-distance upstream movements of fish that must have crossed one or more dams lacking fishways. By focusing on a large, regulated river under real operational conditions, this study aims to improve the understanding of navigation locks as a functional element of river connectivity and to inform adaptive management strategies in large river systems. Methods The Danube River in Austria The Danube flows through Austria for approximately 350 kilometres, descending more than 150 metres in elevation along this stretch. This energy potential is harnessed by ten hydropower plants (HPPs) with average heads ranging from 8.6 to 15.3 metres (Fig. 1). The oldest facility, HPP Jochenstein, was completed in 1956. At the time of their construction, none of the Danube power plants—except for the most recent, HPP Freudenau, commissioned in 1996—were equipped with fish migration aids. Since the retrofitting of a fishway at the Melk hydropower station in 2007, a gradual installation of fish passage facilities has taken place at other Danube power plants (Table 1). Currently, seven power stations are equipped with fishways, while fish passage infrastructure is being implemented at the remaining three as part of the LIFE project “LIFE Bluebelt Danube-Inn,” aiming to restore migratory continuity along the river. Table 1 The Danube hydro power plants Jochenstein 2203,3 2050 9,78 1956 in preparation Aschach 2162,7 2480 15,3 1964 in preparation Ottensheim-Wilhering 2146,1 2250 10,5 1974 2016 Abwinden-Asten 2119,5 2475 9,3 1979 2020 Wallsee-Mitterkirchen 2094,5 2700 10,8 1968 2014 Ybbs-Persenbeug 2060,4 2650 10,9 1959 in preparation Melk 2038,2 2700 9,6 1982 2007 Altenwörth 1980,5 2700 15 1976 2021 Greifenstein 1949,2 3150 12,6 1985 2017 Freudenau 1921,1 3000 8,6 1998 1998 As an internationally classified Class VIb waterway (according to ECE and CEMT standards), all hydropower plants along the Austrian Danube are equipped with ship locks. These locks enable ship convoys of up to 230 meters in length and 23 meters in width to overcome the elevation difference. Lock operation is managed by Via Donau, while Verbund Hydro Power GmbH owns the facilities and is responsible for the maintenance and operation of the lock installations. Longitudinal arrangement of hydropower stations at the Danube with operation level Characteristics of the ship lock at the HPP Aschach The main study was conducted at the navigation lock of the Aschach hydropower plant (HPP Aschach) on the Austrian Danube (Fig. 2). The lock is representative of ship locks along this river section and consists of two parallel lock chambers, each 255 m long and 24 m wide. The head difference at the facility is approximately 15 m, and the filling and emptying system is located beneath the lock chambers. Under regular operating conditions, filling of a lock chamber takes approximately 13 min. In 2020, about 1,550 upstream lockages were recorded at HPP Aschach, with a similar number of downstream passages, reflecting continuous navigation use throughout the year. Areal foto of HPP Aschach with the two lock chambers in the foreground. PIT antenna installation and monitoring Fish movements through the ship lock were monitored using a stationary PIT antenna system (FDX system by Biomark®, USA) installed during scheduled lock maintenance in February and March 2019. The system’s high scanning rate (up to 30 reads s⁻¹) ensures high detection efficiency, particularly for schooling and fast-moving potamodromous species (Nagel et al., 2023). It comprised two antenna arrays and a technical unit housing the power supply, battery backup, reader, and modem (Fig. 3). One antenna array was installed within the lock chamber and consisted of six flat, 4-m long FDX antennas (Fig. 4). A second array comprising four 4-m antennas was installed in the filling channel beneath the lock chamber (Fig. 5). To reduce electromagnetic interference between antenna arrays, they were arranged in a staggered configuration, with the filling channel array offset by 35 meters relative to the lock chamber antennas. The antenna system operated continuously from 10 March 2019 to 10 June 2020. Schematic diagram of the antenna system at the shiplock Aschach The lock chamber with the antennas (upper antennas in Fig. 3) during installation The filling chamber with the antennas (lower antennas in Fig.3) during installation Fish tagging Fish were tagged with passive integrated transponders (12.5-mm PIT tags, 2.1 mm diameter) in the tailwaters and fishways of several hydropower plants located downstream of HPP Aschach: HPP Ottensheim-Wilhering (river km 2146.7), HPP Greifenstein (river km 1949.2), HPP Nussdorf (1933,5) and HPP Freudenau (1921,0). No fish were tagged directly in the tailwater of HPP Aschach (river km 2162.7). Direct evidence of lock passage For the statistical analysis of the ship lock data, the reference population (“statistical population”) was defined as all fish detected at the most upstream antenna of the Ottensheim-Wilhering fishway during the time the Aschach arrays were fully operational (10 March 2019 to 10 June 2020). This population comprised 742 individuals representing 31 species, dominated by nase ( Chondrostoma nasus ), roach ( Rutilus rutilus ), and chub ( Squalius cephalus ) (Table 1). Indirect evidence of lock passage Fish detected at antenna arrays in Aschach, Ottensheim and Abwinden, which have been tagged below HPP Ybbs-Persenbeug must have passed upstream at least one HPP without a fishway. These detections are interpreted as indirect evidence of lock passage. The total number of these fish accounted for 73 individuals representing 10 species (Figure 11). Data analysis and statistics All statistical analyses and visualizations were performed in R (R Core Team, 2025) using the packages tidyverse (Wickham et al., 2019), lubridate (Grolemund & Wickham, 2011), and ggplot2 (Wickham, 2016). Differences in body size between fish detected at the Aschach ship lock and the statistical population were tested using a Wilcoxon rank-sum test. Differences in species composition (relative abundances) between both groups were quantified using Bray–Curtis dissimilarity and evaluated with a Monte Carlo permutation test. Differences in ecological trait composition were assessed using Fisher’s exact test, and effect sizes were expressed as Cramér’s V. Temporal patterns of fish detections were analysed using abacus plots illustrating tagging dates and subsequent detections at PIT antenna arrays. Differences in the number of lock operations across seasons and between day and night were assessed using a Kruskal–Wallis rank-sum test and chi-squared test, respectively. Daytime was defined as the period between the monthly average times of sunrise and sunset for Austria. Relationships between seasonal and diel patterns of fish detections and lock operation were evaluated using Spearman’s rank correlation. The relationship between lock activity and fish detections at the shiplock was further examined using linear models and AIC based model comparison. Data on lock operation during the study period were provided by viadonau (https://www.viadonau.org). Results Fish passage through the ship lock Between March 2019 and June 2020, a total of 77 fish out of the reference population of 742 fish (approximately 10.4%) were detected at the PIT antenna system installed in the ship lock (Table 2). Almost all fish detected at the ship lock had been tagged in the fishway of HPP Ottensheim-Wilhering, located approximately 4 km downstream. Two exceptions were recorded: one white-eye bream ( Ballerus sapa ) tagged downstream of HPP Greifenstein and one barbel tagged downstream of HPP Ottensheim-Wilhering. All but one fish were detected exclusively within the lock chamber, only a single small perch ( Perca fluviatilis ) was repeatedly recorded in the filling channel. Five individuals were detected in the lock in more than one month, one barbel (Barbus barbus), one perch (Perca fluviatilis), two roach (Rutilus rutilus), and one schrätser (Gymnocephalus schraetser). Three individuals (two roach Rutilus rutilus and one barbel Barbus barbus) were detected in the ship lock in two consecutive migration seasons, with records in autumn 2019 and again in spring 2020. Species composition and ecological traits In total out of the 31 species recorded exiting the Ottensheim fishway, 13 species were subsequently detected at the Aschach ship lock (Table 2). The relative species composition of fish detected in the ship lock differed significantly from that of the statistical population at the Ottensheim fishway (Bray–Curtis dissimilarity = 0.226, Monte Carlo permutation test, p = 0.0041; Table 2). Eurytopic species were dominant among ship lock detections, accounting for 71% of all individuals, whereas stenotopic species represented 29%. When classified according to flow preference, the ship lock community consisted of 73% eurytopic, 17% oligorheophilic, and 10% rheophilic species. The distribution of flow preference traits differed significantly between fish detected in the ship lock and the statistical population at the Ottensheim fishway (Fisher’s exact test, p < 0.0001), with a small to moderate effect size (bias-corrected Cramér’s V = 0.153). Although eurytopic species also dominated the statistical population, their relative proportion there was lower (56%). Several planktivorous species, including bream (Abramis brama) and bleak ( Alburnus alburnus ), were underrepresented or absent in ship lock detections (Table 2). Table 2 Statistical population of tagged fish for the analysis (detected at the exit of the fishway) and fish detected in the shiplock (95% confidence interval, lower limit (LL) and upper limit (UL)). Classification according to Zauner & Eberstaller (1999), with additions. Abramis brama Eurytop eurypar 28 1 3,57% 1,30% 2,3% 4,9% Alburnoides bipunctatus Rheophil rheopar 9 1 11,11% 6,84% 4,3% 18,0% Alburnus alburnus Eurytop eurypar 47 0 Babka gymnotrachelus Eurytop eurypar 6 0 Ballerus sapa Oligorheophil rheopar 26 1 3,85% 1,45% 2,4% 5,3% Barbus barbus Rheophil rheopar 43 2 4,65% 0,96% 3,7% 5,6% Blicca bjoerkna Eurytop eurypar 3 0 Carassius gibelio Eurytop limnopar 1 0 Chondrostoma nasus Rheophil rheopar 165 5 3,03% 0,20% 2,8% 3,2% Cyprinus carpio Eurytop limnopar 8 1 12,50% 8,10% 4,4% 20,6% Gobio gobio Rheophil rheopar 9 0 Gymnocephalus schraetser Oligorheophil rheopar 10 2 20,00% 7,84% 12,2% 27,8% Hucho hucho Rheophil rheopar 1 0 Leuciscus aspius Eurytop rheopar 4 0 Leuciscus idus Eurytop eurypar 5 0 Leuciscus leuciscus Eurytop rheopar 8 0 Lota lota Eurytop eurypar 9 1 11,11% 6,84% 4,3% 18,0% Neogobius melanostomus Eurytop eurypar 5 0 Perca fluviatilis Eurytop eurypar 17 4 23,53% 4,89% 18,6% 28,4% Ponticola kessleri Eurytop eurypar 5 0 Romanogobio kesslerii Rheophil rheopar 1 0 Rutilus meidingeri Eurytop rheopar 2 0 Rutilus rutilus Eurytop eurypar 145 43 29,66% 0,62% 29,0% 30,3% Salmo trutta fario Rheophil rheopar 4 0 Scardinius erythrophthalmus limnophil limnopar 2 0 Silurus glanis Eurytop eurypar 1 0 Squalius cephalus Eurytop eurypar 141 6 4,26% 0,28% 4,0% 4,5% Thymallus thymallus Rheophil rheopar 9 0 Vimba vimba Oligorheophil rheopar 4 2 50,00% 24,50% 25,5% 74,5% Zingel streber Rheophil rheopar 1 0 Zingel zingel Oligorheophil rheopar 23 8 34,78% 4,06% 30,7% 38,8% Flow Rheophil 77 3 3,90% 0,49% 3,40% 4,39% Oligorheophil 63 13 20,63% 1,26% 19,38% 21,89% indifferent 435 56 12,87% 0,15% 12,72% 13,02% limnophil 2 0 Spawning rheopar 293 20 6,83% 0,17% 6,66% 6,99% eurypar 412 55 13,35% 0,16% 13,19% 13,51% limnopar 11 1 9,09% 5,12% 3,97% 14,21% Total 742 77 10,38% 0,08% 10,30% 10,46% Size distribution Fish detected at the ship lock ranged in total length (TL) from 11 to 75 cm, encompassing small benthic species such as zingel ( Zingel zingel ) and schrätzer ( Gymnocephalus schraetser ), both listed in Annex II of the Habitats Directive, as well as large pelagic fishes like a carp ( Cyprinus carpio ) measuring 75 cm. Compared to the statistical population, a higher proportion of individuals smaller than 30 cm TL was observed among ship lock detections (12.5% vs. 3.5%; Fig. 6). However, overall size distributions did not differ significantly between fish detected at the ship lock and the statistical population (Wilcoxon rank-sum test, p = 0.092). Relative size composition of the statistical population (red, n=742) and fish detected in the shiplock (blue, n=77) Seasonal and circadian patterns The temporal distribution of detections across the study period is illustrated in the abacus plot. Abacus-Plot showing the detection dates and sites (Aschach shiplock, red dots), near natural fish bypass of HPP Ottensheim-Wilhering (greenish & bluish dots) of the 77 fish detected in the ship lock Fish detections at the ship lock showed a pronounced seasonal pattern, with most detections occurring during spring and a secondary peak in autumn (Fig. 7 (red dots), Fig. 8). Very few detections were recorded during winter (October–February), and only sporadic detections occurred in midsummer (June–August). Seasonal pattern of detections in the Aschach shiplock Initial detections at the ship lock antennas occurred predominantly during daylight hours between 10:00 and 19:00, with a peak around midday (Figure 9). Circadian trend: most detections during day; between 10 am and 7 pm Lock operation The frequency of lock operations varied significantly across seasons (Kruskal–Wallis rank-sum test, p 1,750 operations) and the lowest in February (<500 operations). However, no statistically significant correlation was detected between monthly fish detections at the ship lock and the seasonal pattern of lock operations (Spearman’s ρ = 0.525, p = 0.0796), although a weak positive trend was apparent (Figure 11). Including monthly lock operations in a linear model did not improve model performance compared to the null model (ΔAIC = 0.07; AIC weight M1 = 0.49). Number of lock operations per hour over the study period Relationship between seasonal fish detections and lock operations. Lock operations showed a daytime maximum over the study period (7146 vs. 6232 lock operations; Chi-squared = 83.35, p < 0.0001). No significant correlation was found between the diel distribution of fish detections and the timing of lock operations (Spearman’s ρ = 0.283, p = 0.18). Although hourly detections increased slightly with the number of lock operations, the linear model including lock frequency performed only marginally better than the null model (ΔAIC = −0.46; AIC weight = 0.57), and the effect was statistically non‑significant (β = 0.006, p = 0.138). Thus, lock operations explained little of the variation in hourly detection rates. Relationship between hourly fish detections and locking. Indirect evidence of lock passage Of all fish tagged downstream of the HPP Ybbs-Persenbeug, where a fishway is currently under construction, 73 individuals representing 10 species were detected at a PIT antenna located upstream of this hydropower plant (Figure 12, light and dark blue dots). These detections confirm that fish passed through at least one shiplock during upstream migration. The longitudinal distances travelled by these fish ranged from 112 to 273 km. Total length of fish varied between 11 cm and 130 cm (mean = 30 cm, SD = ± 17 cm), with the largest individual being a European catfish ( Silurus glanis ). Species composition was dominated by nase ( Chondrostoma nasus ; 44%) and bleak ( Alburnus lburnus ; 21%), followed by asp ( Aspius aspius ; 7%) and ide ( Leuciscus idus ; 7%). Rheophilic species accounted for more than half of all detections. Repeated passage through multiple navigation locks was documented for several individuals. In total, 14 fish belonging to four species (nase, bleak, asp, and white-eye bream) were detected upstream of at least three consecutive ship locks during their upstream movements (Fig. 12). Abacus-Plot showing the detection dates and sites of fish that were tagged mostly in the tailwaters and fishways of HPPs Altenwörth and Greifenstein. Sites are fishways of HPPs, the number in bracket indicates the number of antenna arrays per fishway. In both the legend and along the y-axis of the plot, sites are arranged sequentially from upstream to downstream. Discussion All detections at the Aschach ship lock represent voluntary upstream movements beyond at least one previously passed migration barrier, indicating active use of this structure during upstream migration. Ship locks as upstream migration routes in a large regulated river This study provides quantitative evidence that navigation locks on the Austrian Danube are regularly used as upstream migration routes by at least 19 species. Thirteen species were directly detected at the Aschach ship lock, and indirect evidence demonstrated successful upstream passage through multiple navigation locks by six additional species. Repeated detections of individual fish within and across migration seasons indicate that lock usage is not incidental but represents a consistent component of upstream movement behaviour. Based on detections at a single lock chamber, approximately 10% of the statistical population passed through the ship lock. Assuming similar usage of the second chamber, overall upstream passage through both chambers could involve at least 20% of the fish population. This is a conservative measure, as detection probability of a swim-over antenna is well below 100%. Swim-over antennas, particularly under deepwater conditions or when fish pass higher in the water column, typically perform less effectively than pass-through systems (Connolly 2010; Zentner et al. 2021, Schabuss et al. 2026). The observed passage rates place ship locks in the Austrian Danube at the upper range of efficiencies reported elsewhere and overlap with, or exceed, passage rates reported for anadromous species at some low-head dams (Moser et al., 2000; Bailey et al., 2004). Our findings challenge the assumption that fish passage through ship locks is negligible in large rivers (Schmutz & Mielach, 2015) and underline their potential role as functional, albeit unintended, elements of longitudinal connectivity. Species composition and ecological traits Species composition at the ship lock differed significantly from that of the reference population exiting the Ottensheim fishway. Passage through the ship lock was dominated by eurytopic species with broad habitat and flow tolerances, a pattern consistent with previous studies on navigation locks in European rivers (Schwevers & Gumpinger, 1998; Schubert et al., 1998). Oligorheophilic species such as Vimba vimba and Zingel zingel were also recorded in notable proportions, whereas strictly rheophilic species were comparatively underrepresented among direct ship lock detections. Benthic-oriented species appeared more frequently in lock detections than pelagic species. This pattern should be interpreted with caution, as pelagic species may be under-detected by swim-over PIT antenna systems. Supporting this methodological explanation, trap-based studies at the Lahn River reported bleak and bream among the most abundant species ascending ship locks (Schwevers & Gumpinger, 1998), whereas both species were rare or absent in the present lock detections despite being abundant in the statistical population. Together, these findings indicate that observed differences in species composition at ship locks reflect a combination of ecological traits and methodological detection constraints rather than absolute passage limitations. Rheophilic species and long-distance migration In contrast to the direct ship lock detections, the analysis of long-distance migrants—fish that must have passed at least one navigation lock—revealed a dominance of rheophilic species, particularly nase ( Chondrostoma nasus ). This result is noteworthy given the common assumption that ship locks are poorly passable for rheophilic species due to limited or diffuse attraction flow (Wagner, 2010). Our results show that rheophilic potamodromous species can successfully pass navigation locks over large spatial scales, including repeated passage across multiple barriers. This discrepancy indicates that attraction flow may play a less dominant role for potamodromous species than for anadromous migrants, whose orientation is often guided by directed flow cues. Potamodromous fishes may therefore rely more on exploratory behaviour and broad-scale movement tendencies when using navigation locks as passage routes (Schabuss et al., 2025). Fish size and passage capability Fish size did not constitute a limiting factor for ship lock passage. Individuals ranging from small-bodied benthic species to large pelagic fish exceeding 1 m in length were recorded either directly at the ship lock or indirectly through long-distance detections. This is consistent with previous studies reporting successful lock passage across a wide size spectrum, from small cyprinids to large-bodied species such as paddlefish and potamotrygonid stingrays (Schwevers & Gumpinger, 1998; Simcox et al., 2015; Garrone-Neto et al., 2014). The absence of size selectivity highlights a key difference between navigation locks and many technical fishways, where hydraulic conditions may restrict passage of particular size classes. Seasonal & circadian patterns of lock usage The seasonal patterns of ship lock passage closely matched typical migration dynamics in temperate rivers, with pronounced peaks in spring and a secondary peak in autumn (Waidbacher & Haidvogl 1998). Spring movements are commonly associated with spawning migrations, whereas autumn movements are often linked to foraging or habitat shifts (Lucas et al., 2001; Hladík & Kubečka, 2003). No significant relationship was found between the seasonal frequency of lock operations and fish passage, suggesting that migration timing was primarily biologically driven rather than controlled by navigation intensity. Similarly, most fish detections occurred during daylight hours, with a peak around midday. Diel migration patterns appear to reflect species-specific behavioural strategies, which vary widely among potamodromous fishes, ranging from predominantly diurnal to nocturnal or arrhythmic activity (Benítez et al., 2018; Mameri et al., 2019; Angelov et al., 2020; Rato et al., 2024). Again, no significant correlation between diel lock usage and fish passage was observed, although the effect of lock operation could not be ruled out. Implications for genetic connectivity & exchange Although genetic analyses were beyond the scope of this study, the observed passage rates and migration distances have important implications for genetic connectivity in the Danube. Previous studies indicate weak genetic differentiation among impoundments for several species (Baranyi et al., 1997; Reinartz et al., 2011; Friedrich et al., 2022). Modelling studies suggest that even low levels of migration (~1%) may be sufficient to prevent long-term genetic isolation (Ruzich et al., 2019). The passage rates documented here exceed this threshold, indicating that navigation locks, together with other dispersal processes, may contribute meaningfully to maintaining genetic connectivity in regulated large rivers. Conclusion & Recommendations This study demonstrates that navigation locks effectively facilitate the upstream passage of a wide range of fish species and size classes in the Austrian Danube, enabling migration over distances of at least 273 km. The observed passage rates and repeated use of ship locks by individual fish suggest that this behaviour is consistent and deliberate rather than accidental. At the population level, the scale of movement through ship locks documented in this study is likely sufficient to facilitate genetic exchange and reduce the risk of population isolation in the Upper Danube. Currently, fish passage through navigation locks is restricted to periods of active shipping. While the underlying mechanisms require further investigation, our findings point toward the possibility of improving the ecological function of ship locks by making targeted operational adjustments. For example, the frequency of non-navigational lock operations could be increased during critical spawning periods, and lock operation schedules could be aligned with species-specific diel migration patterns. Crucially, navigation locks should not be considered a substitute for dedicated fish passage facilities, but rather as a complementary element within an integrated connectivity framework. Ethics Statement The care and use of experimental animals complied with Austrian animal welfare laws, guidelines, and policies as approved by Amt der Oberösterreichischen Landesregierung, Abt. Gesundheit, Ges-2016- 445528/50-Bit and Ges-2016-445528/63-Bit. Conflicts of Interest The authors declare no conflicts of interest. Walter Reckendorfer, Barbara Grüner, and Renate Degen are employed by VERBUND Hydro Power GmbH as aquatic ecologists. The views and opinions expressed in this manuscript are those of the authors and do not necessarily reflect the positions of VERBUND Hydro Power GmbH. Data Availability Statement The data that support the findings of this study are available from the corresponding author upon reasonable request. References 1. Angelov, M. V., Zhelev, Z. M., Popgeorgiev, G. S., & Apostolos, A. I. (2020). Dynamics of 24-hour upstream fish migration in the lower course of Tundzha River, Southern Bulgaria, Based on a Fish-Pass Video Monitoring. Acta Zool Bulg, 72(2), 289-296. Google Scholar 2. Bailey, M. M., Isely, J. J., & Bridges Jr, W. C. (2004). Movement and population size of American Shad near a low‐head lock and dam. Transactions of the American Fisheries Society, 133(2), 300-308. Google Scholar 3. Baranyi, C., Gollmann, G., & Bobin, M. (1997). 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Keywords navigation lock potamodromous fish sluice telemetry Authors Affiliations Walter Reckendorfer [email protected] VERBUND Hydropower GmbH View all articles by this author Barbara Grüner VERBUND Hydropower GmbH View all articles by this author Renate Degen 0000-0001-7380-9378 VERBUND Hydropower GmbH View all articles by this author Regina Petz-Glechner Umweltgutachten Petz OEG View all articles by this author Michael Schabuss PROFISCH OG View all articles by this author Metrics & Citations Metrics Article Usage 173 views 75 downloads .FvxKWukQNSOunydq8rnd { width: 100px; } Citations Download citation Walter Reckendorfer, Barbara Grüner, Renate Degen, et al. Fish migration through ship locks at the Danube River, Austria. Authorea . 26 February 2026. DOI: https://doi.org/10.22541/au.177212158.80949517/v1 If you have the appropriate software installed, you can download article citation data to the citation manager of your choice. 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