## 4 Discussion
During the deployment of the DIDSON and ARIS systems, personnel had to be present on site in order to avoid damage caused by floating debris and to adapt the setup due to operational requirements (e.g., different depth horizons). Also, the manual post-processing is time-consuming; i.e., a factor of 1:3 between field work and post-processing has to be considered. We used the sonar data for individual fish observation, but the data could also support the analyses of swimming paths of individual fish [58]. However, acoustic cameras could either be used in a certain case study (as presented herein) or possibly be installed at a safe-site for long term monitoring (e.g., within a chamber of a by-pass system). There are limitations due to orientation and range, but acoustic cameras have a high potential for underwater investigations, including technical inspections as well as biological surveys. Acoustic cameras have been sporadically used to detect and investigate fish and their behavior in close vicinity of hydroelectric sites in recent years [39; 59]. ARIS was used by [PERSON] et al. [60] to investigate activity of adult lake trout in the headrace channel of a hydropower plant in Switzerland. In this regard, our setup of an acoustic camera triplet in front of an alpine run-off river power plant is a novel application to assess the temporal and distribution pattern of fish. The majority of fish contacts were found to occur in the border area (ARIS position 1; Figure 9b) between turbulent and recirculating flow. Thus, our data suggest that different flow fields in the border area between turbulent and recirculating flow are preferred by fish. Considering the evaluation of the data from the horizontal setup this difference is significant and formed the basis for detailed design of a bypass. Confirmed by visual contacts and expert opinion, the majority of the fish are rainbow trout. The spatial analysis of the fish contact numbers (taking into accounts the sonar locations, alignments, ranges, and water depths) with the flow field, indicate that the majority of fish contacts was made at the interface between turbulent and calm water on the orographic left side. This is an important key for the design of a sustainable mitigation measure, in order to guide the fish into a bypass system. Further, it was shown that the use of a new differential pressure-based field turbulence measurement device can be correlated to ADCP measurements of the time-averaged flow field, revealing turbulent flow regions that may guide or repel fish. These regions could be more attractive to larger fish because studies on trout swimming in such eddies have shown that fish can use such eddies to reduce their energy consumption [61]. Based on this hypothesis, it could be the case that adult fish are observed on the leftmost 10 m of the hydropower intake and are less likely to be present in high-velocity regions with an overall lower turbulence parameter (Figure 10b). These patterns were confirmed by the analyses using DIDSON sonar or ARIS (Figure 9c), indicating the potential of the proposed turbulence parameter for predicting fish behavior in front of hydropower intakes. The combined time-averaged velocity and pressure turbulence measurements were chosen based on literature studies of fish sensory capabilities, where it has been experimentally determined that fish respond to bulk fluid forcing in conjunction with local fluctuations in the pressure field [62]. Within a turbulent flow field, fish experience these fluctuations at a rate directly proportional to the local flow speed, and thus the ratio of the fluctuations to the local flow speed may prove to be a useful indicator of fish behavior considering in situ flow environments [63]. Further studies should compare the distribution of flow turbulence using the proposed methodology with additional field observations to determine hydraulic preferences relevant to fish. It is also important to consider plant-specific characteristics including the turbine type and discharge conditions.