Results
Detection efficiencies
The probability of detection during a single downward passage event at the top antenna (Antenna #1) ranged between 25.3±9.8 and 44.9±13.9% (mean+95% confidence limits), depending on the calculation method used, specifically which antenna array chosen as the second reference antenna in the Zydlewski et al. (2006) formula. The calculated values were not significantly different (χ²-Test, p > 0.05) when using the different antennas (#2-5) for the calculations. Overall, we computed a mean detection probability of 34.8±14.2 % (Table 1). Similar calculations were performed for Antenna #2, reaching detection probabilities comparable to those of Antenna #1, ranging from 24.0% ± 9.7% to 38.7% ± 13.6%, with an overall mean of 31.4% ± 14.4% (Table 1). Again, the method used to calculate the detection efficiency had no effect on the calculated value (χ²-Test, p > 0.05).
During the study period (2021-2024), the probability of a fish entering the fish pass system and being detected at the top Antenna #1 ranged from 52.4% and 74.6%, depending on which antenna was used as the secondary antenna (see methods), with an average detection probability of 65.4%. Using this probability, a total of 106 nase were calculated to have entered the system from the Danube reservoir above the HPP (69 nase detected at Antenna #1 / 0.654), representing 55.8% of the tagged fish. For Antenna #2 the detection probability ranged from 42.9% to 67.3% with an overall mean of 55.7%. Using this probability, it was again estimated that 106 nase entered the system from the reservoir (59 fish detected at Antenna #2 / 0.557).
Fish size & detection rates
Of the 190 nase tagged in November 2020, a total of 80 fish (42 %) was detected by at least one of the 5 antennas within the fish pass. Detection rates increased with fish size: 12 % of nase < 20 cm in total length were detected, this increased to 55 % for those between 20-30 cm, and 63 % for those from 30-40 cm, before decreasing to 44 % for nase larger than 40 cm (Figure 2). Consequently, while only a small proportion (12%) of juvenile nase (< 20 cm) were detected, a much higher percentage (59%) of adult nase (> 20 cm) were detected within the fish pass.
Antenna position & daily detection- numbers & -rates
Of the 80 nase that entered the fish pass from the Danube above the HPP between 18.11.2020 and 13.06.2024, a total of 69 individuals (86 %) were detected at the uppermost Antenna #1 at the “exit” of the fish pass (Table 3). At Antenna #2, positioned in the upstream ramp, approximately 1 km downstream of Antenna #1, 58 fish were detected. Antenna #3, situated in the Aschach River, recorded 48 nase, while Antenna #4, located in the Innbach River, detected 36 fish. Finally, 21 individuals were detected at the lowermost antenna, near the “entrance” of the fish pass below the HPP.
Throughout the study period 1.918 detections were recorded on antennas 1-5. Each detection was counted once per fish per antenna and day, though the same fish was counted multiple times if detected on different antennas on the same day. Table 3 summarizes the total number of detection days, the number of fish recorded, and the mean and maximum number of detection days for the years 2021-2024.
In 2021, the number of detection days was higher across all antenna arrays compared to subsequent years. Detection rates varied among antennas, with Antennas #1 and #2 consistently recording the highest number of unique fish (69 and 59 nase, respectively). In contrast, Antennas #4 and #5, located at the lower end of the bypass, had the fewest detection days throughout the study. Antenna #3, situated in the natural tributary Aschach, consistently registered the highest total and average detection days each year, especially in 2021. That year, it recorded 577 detection days from 41 individual fish, averaging 14 detection days per fish, with a maximum of 47 days for a single fish. This indicates significant ”traffic” at this key junction between the fish pass and the natural tributary.
First & last detections per year
Table 4 summarizes the locations (antennas) of the first and last detection of nase for each year from 2021 to 2024. In each year, the first and especially the last detections predominantly occurred at the most upstream antenna, Antenna #1, located at the exit of the fish pass. In 2021, a total of 78 individual fish were detected in the fish pass. The distribution of initial detections slightly decreased across the five antennas, with 24 fish detected first at Antenna #1 and 10 at Antenna #5. However, the final detections were predominantly at the uppermost Antenna #1, where 61 of the 78 fish (78 %) were last recorded that year. Similarly, in 2022, 34 of 42 fish (81%) were last detected at Antenna #1. This indicates that the majority of nase left the fish pass via upstream exit (= Antenna #1) into the Danube impoundment and spent the autumn and winter there after initially entering the fish pass via the same route. In 2023, although Antenna #1 was not operational from 23.06.2023 to 01.02.2024, data from the next closest antenna (Antenna #2) showed a similar trend, with a higher number of fish leaving the system at the upper end of the fish pass. Notably, 41 nase (51% of fish detected) exhibited a migration pattern of leaving the bypass system via Antenna #1 (or Antenna #2 during the period when Antenna #1 was not operational) in one year and returning via the same antenna in a subsequent year.
A notable number (9 individual nase) of last detections at Antenna #5 indicate that some fish are passing completely through the 14,2 km long bypass and exiting into the Danube downstream the HPP. Four nase were observed to have left the bypass system via Antenna #5 (last detection of the year at #5) and returned via Antenna #5 (first detection of the year at #5) in subsequent years.
The total number of fish entering and leaving the bypass system has declined over the years, with 78 nase recorded in 2021, 42 specimens in 2022, 32 fish in 2023, and 15 fish entering the system by mid-June 2024 (end of data analyses).
Residence time of nase within the bypass system
Table 5 presents the minimum, maximum, and mean (+/- SE) number of days that fish remained in the bypass, calculated based on the time between the first and last detection of each nase within each year. Throughout the study, individual fish were observed to stay in the bypass system for as short as 1 day. However, the mean residence time was initially very high averaging 101 days per year in 2021. This average residence time decreased to 90 days in 2022 and 2023. The maximum recorded residence time for a fish in the bypass system was 358 days in 2022.
Seasonality & fish size
Figure 3 illustrates the timing of the first and last detections of nase categorized by size class. The data indicate that larger individuals tend to enter the fish pass earlier in the season compared to juvenile fish. Additionally, nase of all size classes entered the fish pass earlier in 2022 and 2023 compared to 2021. This shift in timing correlates with water temperature data, which show that the mean temperature in the fish pass between February and June 2021 was approximately 0.94°C lower than in 2022 and 0.24°C lower than in 2023.
Seasonal use of the bypass system
An abacus plot visualizes the antenna detections of each individual nase (grouped by size) from 2021 to 2024 (Figure 4), enabling the tracking of fish movements within the bypass system over the years.
The plot reveals multiple detections of individual nase across different years, with noticeable clusters of detections starting around spring (March to May) each year, as fish enter the bypass. Continuous detections during the summer months (June to August) show that many fish remain in the bypass for extended periods. Detections in the autumn (September to November) and winter (December to February) indicate fish exiting the bypass.
A closer examination of the abacus plot shows that the majority of first and last detections of nase occur at the upstream Antenna #1 (brown dots), indicating that most fish enter and exit the fish pass from and to the Danube above the HPP Ottensheim. The peaks in detections, particularly during spring and summer, likely correspond with spawning migration and spawning periods. The high number of detections at Antennas #2-4, and especially at Antenna #3 (orange dots) and #4 (dark green dots) located at the natural rivers Aschach and Innbach, show that many fish use the fish pass not only to overcome the HPP dam or for spawning but also to remain within the bypass system throughout the growing season, from March to October.
The movement patterns indicate that fish are not merely passing through but are also exploring or utilizing various parts of the bypass, including the natural areas within it. Detections at Antenna #5 confirm downstream passage through the bypass.
It is evident that larger nase (> 20 cm) make extensive use of the bypass system (from 71 ind. in 2021 to 15 in 2024), while only 8 smaller fish (< 20 cm) entered the bypass in 2021 and none in subsequent years. These smaller fish also spent less time in the system.
Repeated use of the bypass system
As illustrated in the abacus plot, nase repeatedly utilized the bypass system throughout the four-year study period. 33 fish were detected in the bypass system during only one year of the study period (2021-2024), 20 individuals were detected in two years, 15 fish were detected in three years, and 12 nase visited the bypass every single year. Larger fish (> 30 cm in total length) were found to use the bypass system more than twice as often as smaller fish (< 30 cm) across multiple years.