Materials & Methods
Fish pass
As part of the LIFE project ”LIFE+10 NAT/AT/016 - Netzwerk Donau”, a
near-natural bypass river was constructed at the Ottensheim-Wilhering
power plant at the Austrian Danube between 2015 and 2016. This fish
pass, stretching 14.2 km and designed to resemble a small river, has a
discharge capacity of up to 20 m³/s, with a bed width varying between 10
and 25 meters. It was integrated into the existing Aschach and Innbach
river systems, which were also restored within the project, thereby
reconnecting the Danube with its tributaries (Figure 1). The bypass
river enables fish to overcome a height difference of about 12 m, while
simultaneously providing key habitats, now largely missing in the
Danube, such as shallow shorelines & gravel banks, riffles and pools.
The discharge in the bypass system follows that of the Danube, an
upstream inlet structure ensures that the required volumes of water are
maintained under varying flow conditions in the Danube.
During the construction of the bypass river, the focus extended beyond
merely ensuring the passability of the hydropower plant (HPP) dam. A
significant emphasis was placed on creating a natural gradient and
restoring an original, furcating river landscape with its characteristic
flow patterns and substrate conditions. The bypass system thus improves
the habitat quality and availability, especially for rheophilic fish
species of the Danube. Fish entering the bypass river from either
upstream or downstream of the HPP can now utilize the restored sections
of the Aschach and Innbach rivers. Additionally, the newly created
sections of the bypass river provide migration routes and habitats for
spawning, feeding, and juvenile nurseries.
Pit Tags & Antenna arrays
PIT tags (Passive Integrated Transponders) have been used for the
individual tagging of fish since 1987 (McKenzie et al., 2012). As they
are battery-free passive transmitters, they have an unlimited service
life, allowing for the unique identification of individual fish
throughout their entire lifespan (Connolly et al., 2008). This makes PIT
tags particularly suitable for tracking fish individually over extended
periods, ideal for large-scale studies (Greenberg & Giller, 2000;
Roussel et al., 2004). For the investigation at the Ottensheim-Wilhering
fish pass we used the FDX system from Biomark®, which is known for its
high read rate of 30 scans per second, which enhances detection
efficiency, especially for shoal-migrating species such as the
potamodromous nase (Chondrostoma nasus ) (Nagel et al., 2023). The
tags (12.5 mm x 2.1 mm) were carefully implanted into the fish using
syringes.
The PIT tags were read automatically and contactless using five
permanently installed antenna arrays (Figure 1). These antennas were
placed at strategic points in the bypass system: antenna #1 at the
upper end and antenna #5 at the lower end of the bypass, covering the
entire width of the watercourse. Antenna #2 was positioned in the
upstream ramp connecting the bypass system with the Danube,
approximately 1 km downstream of antenna #1, Antenna #3 was installed
in the Aschach River, 125 m upstream of its confluence with the bypass
channel, and antenna #4 in the Innbach River, 430 m upstream of its
confluence with the bypass.
We analysed the antenna array data, including the date, time, and
location (antenna #) of each fish detection from 18.11.2020 to
13.06.2024 to study the downstream migration and habitat use of the
common nase (Chondrostoma nasus ). Due to technical difficulties,
Antenna #1 was not operational between 23.06.2023 and 01.02.2024.
To calculate the detection efficiency of the PIT Tag antenna arrays, it
is necessary to know both the number of detected fish on an array and
the number of fish present above or below this array, requiring data
from at least two antenna arrays. The detection efficiency can then be
determined using the method outlined by Zydlewski et al. (2006):
E1 = Dcommon to 1+2 / (Dunique to 2 +
Dcommon to 1+2)
E1 = Detection efficiency of antenna array #1
D = number of detected tags
The assumptions or necessary conditions for the calculation are as
follows:
• The probability of detection on the first array is independent of the
probability of detection on the second array
• The fish detected at the first array continues to swim in the
direction of the next array
Detection efficiency can be calculated individually for Antennas #1 and
#2. However, with additional arrays (#3-5) available to act as a
secondary antenna array, there are alternative methods to calculate the
overall efficiency.
Two different efficiencies were calculated: (i) the probability of
detection during a single downstream passage over the Antennas #1 or
#2, and (ii) the probability of being detected on Antennas #1 or #2
throughout the entire study period (2021-2024).
Binomial confidence intervals (CIs, 95%) for detection efficiencies
were determined using the normal approximation or Wald method:
CI95 = 1.96 * SQRT((E1*(1-E1)/N), with N = Dunique to
2+Dcommon to 1+2.
We used the chi-square test (χ²) to test whether differences in
detection efficiency were related to the antenna arrays used in the
calculation.
Fish
In our study, we used the common nase, Chondrostoma nasus , a
potamodromous and highly specialized fish with distinct ecological and
ontogenetic trophic niches (Reckendorfer et al. 2001, Hudson et al,
2014, Ovidio & Nzau Matondo, 2024). This species served as a surrogate
for other common rheophilic fish species of the Austrian Danube such as
Barbel (Barbus barbus ) or ide (Leuciscus idus ).
Potamodromous fishes exhibit a cyclic sequence of migrations for
feeding, refuge and reproduction across four habitat types (feeding,
winter refuge, non-winter refuge and spawning) and show habitat-use
patterns integrated with their life stages and body sizes (Thurow, 2016,
Northcote 1984; Keckeis et al., 1997). In this section of the Austrian
Danube, nase is a dominant species (“Leitart”) and was very abundant
until the beginning of the 20th century. Populations declined sharply in
the 20th century, but in recent years a recovery has also been observed
in restored sections of the Danube (Zauner et al., 2015). In spring,
nase migrate to their spawning grounds (Penaz 1996), with spawning
occurring between mid-March and late April at water temperatures from
9.6 to 10.8° C (Huber & Kirchhofer 1998; Melcher & Schmutz 2010).
Suitable spawning habitats are shallow, fast flowing and gravelly
streams (Panchan et al., 2022, Hauer et al., 2007, Kamler & Keckeis
2000). The larvae and juveniles of nase require different habitats than
adults, favouring shallow, slow-flowing areas along the river shore
(Keckeis et al. 1997, Kamler & Keckeis 2000, Penaz, 1996).
To investigate the migration through the fish pass and the movements
within it, we captured a total of 190 nase (Chondrostoma nasus )
in the impoundment above the HPP using boat electrofishing. The fish
were tagged and subsequently released on November 18, 2020 into the
Danube impoundment. Electrofishing, tagging, and handling of the fish
were conducted in accordance with the Austrian Animal Welfare Guideline.
Data Analysis
The detection data were aggregated to the number of detections per day
and analysed to monitor the movement patterns of individual fish over
time. The primary analyses included: i) calculating the number of
detections per fish ID per year to identify repeated entries; (ii)
conducting a seasonal analysis of detections to determine the timing of
migrations and habitat use; (iii) evaluating the duration spent in the
bypass to assess its role as a habitat; and (iv) comparing detection
locations to understand the utilization of different sections of the
bypass.
For data collection, analysis, and presentation, we utilized MS Access®,
MS Excel®, and Sigma Plot® software.
We used abacus or calendar plots to analyse the Pit Tag data to estimate
the general locations of each nase. The frequency and distribution of
detections over time provided insights into how often each individual
was detected, how long it remained in the study area, and their movement
patterns between antenna arrays. In an abacus plot, the x-axis
represents time, while the y-axis lists tagged fish on individual lines,
with detections displayed as dots.