## 2 Materials and Methods
### Long-Term Assessment Using Echosounders  
In order to analyze seasonal patterns of fish presence in the headrace channel and identify optimal deployment times for the multibeam systems, fixed location echosounders were used for long term assessment. Data collection was carried out over 13 months from February 2015 to March 2016. Three Simrad EK15 single-beam echo sounders (Simrad A/S, Horten, Norway) with a frequency of 200 kHz and an opening angle of 28\({}^{\circ}\)[31], were mounted on floats to detect temporal occurrence of fish in the headrace channel [32]. The floats were manufactured polyester-coated styrofoam bodies with mounting space for the transducer and mounting points. The three units were placed approximately 200 m upstream of the trash rack, covering the majority of the channel width. This location was selected due to suitable flow conditions; i.e., the low acoustic noise level. The floats were held in place using a double rope assembly. An upstream cable assembly was used for deployment and to fix the position of the floats. A second cable, located some six meters downstream was used to lift the floats from the surface during times of high water, in the presence of large floating debris and drifting ice. The use of the three parallel echosounders enabled a large volume of the channel to be sampled with only the embankment zones missed (Figure 2). Despite the occurrence of some blind spots (unsampled  
Figure 1: Overview on the study area, including information on the ongoing extension of the HPP Kirchbichl. regions), the setup ensured a constant sampling volume. The three echosounders were connected to each other via the EK15 software and controlled sequentially every 50 ms, which corresponds to approx. 7 pings per second per channel. This was done to exclude mutual interference or \"cross-talk\". Raw data was collected with a pulse length of 80 us at a threshold of \(-\)70 dB. The systems were calibrated separately \"on-axis\" using the procedure provided by the manufacturer (Simrad EK15, Reference manual / Interactive version, Release 1.2.4, 2014). Exact information on the near-field of the 200-28 CM-transducer was not provided in the manual. However, the first meter of the water column remained unconsidered for analysis to avoid potential near-field effects. Remote access to the echosounders allowed a continuously monitored data acquisition via LAN connection. Figure 2: Overview of the headrace channel with the installation of the EK15 sounders indicated, the power station itself and the tailwater (**a**) Plan view using an orthoimage of the site (photo: droneproject.at). (**b**) Schematic cross-section through the headrace channel, showing the setup of the EK15 echosounders. (**c**) Image of the installation, facing into flow direction. The echosounder data was post-processed using the software Sonar 5 Pro (Cage Eye A/S, Oslo, Norway). The raw data collected in the field are first converted (Figure 3a). The amplitude (AMP) echograms display the digitized sample data for each ping, whereas the single echo echograms (SED) contain only the individual echoes that were detected. From individual echoes (based on threshold parameters) a fish echo track can be generated (Figure 3b). To determine the threshold parameters, detection experiments were first carried out using stocked rainbow trout of two known size classes (15-18 and 25-30 cm). The fish were released in close range to the echo sounder in small groups over a period of three hours. Tracking tests formed the basis for all echosounder data analyses: all 112 manually detected and evaluated fish echoes or tracks had target measures in the range from \(-40\) to \(-56\) dB with an average of \(-48.2\) dB. The number of individual echoes per track was at least ten and was a maximum of 37 echoes per track (Figure 4).