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.