## 2 Materials and Methods
### Detailed Investigation of Fish Behavior  
DIDSON (Dual-Frequency Identification [33]) and ARIS (Adaptive Resolution Identification [34]) sonars, which produce video-identical image sequences, represent the state of the art technology in the field of visualization sonars and are often referred to as \"acoustic cameras\" [35]. The technology is most commonly used in dark and turbid waters where optical systems cannot capture meaningful data. Their applications are diverse and allow for the direct observation and recording of fish migration, fish behavior near migratory barriers, cooling water withdrawals, hydroelectric plants, bypass systems, spawning grounds, natural and artificial structures and fishing equipment [9; 36; 37; 38; 39; 40; 41]. The imaging properties of DIDSON and ARIS are principally determined by their operating frequencies, which range from 0.7 to 3.0 MHz [42]. The multibeam opening angle is typically 14\({}^{\circ}\) (vertical) and 29\({}^{\circ}\) (horizontal) and the number of single beams forming the array is a function of the operating frequency, varying between 48 and 128 single beams. The range of the projected sound cone depends on the frequency and varies from a few meters to >20 m for long range applications [43]. This allows for the two-dimensional detection of fish and other underwater objects. Depending on the application and the recording settings, the multibeam sonar used in this work can capture up to 16 images per second. This high rate of recording facilitates the recording of sonar video. The frame rate depends on the device frequency and the site-specific acoustic conditions. In this study a range of 8-12 frames per second (fps) for the DIDSON and 10-16 fps for the ARIS were achieved. The raw data files (.ddf or.aris, respectively) were stored on mobile hard drives for further transfer and processing. To record the sonar data in front of the trash rack of the HPP Kirchhoff, an innovative setup was developed, taking into accounts the simultaneous use of three imaging sonars (Figure 5a). Two DIDSON sonars (DIDSON 300 with 1.8 MHz) were positioned in front of the trash rack on the left and right bank by means of a rail mounting for depth adjustment (Figure 5b). The ARIS (ARIS 3000 with 1.8 MHz) was mounted on the provided scrape cleaner and/or jib and could thus move along the trash rack at six horizontal positions (Figure 5c). The use of both sonar systems mainly had two advantages, i.e., practical setup performance but furthermore the chance for higher resolution data. De-mounting one of the DIDSONs for the trash rack setup was unnecessary due to availability of the ARIS and thus both systems could be left at their mountings during the whole survey. While the resolution of the DIDSON is always limited to 512 range bins per frame, the ARIS range resolution is adaptive and delivered 2000 samples for the given range setting. Gathering such high-resolution data to evaluate potential of the ARIS in this novel setup was a crucial technical aspect of this study. Figure 4: Results (box plots: line = median, black dots = outliers) from the fish detection test for each of the three channels (Ch01 = right, Ch02 = middle position, Ch03 = left). (**a**) target strength TS [dB]; (**b**) number of single echoes per track [n]; (e) maximal distance of single echoes [m]. The first data collection took place from 6-10 September 2016 (>170 h sonar data). Despite favorable acoustic conditions, no fish contacts were recorded during this period. The second data collection took place in the period 4-8 April 2017, corresponding to earlier long-term monitoring results that indicated increased fish presence in the spring [15; 23]. Overall, the data from spring 2017 covers a total period of >170 h sonar data (Table S1): The data collection in the horizons 1-3 with a 90\({}^{\circ}\) angle from the shore and a window length of 10 m (Figure 5d) were carried out in parallel with two devices, and twice a day for 15 min per horizon during three consecutive days (9 h sonar data).