## 4 Discussion  
It is not feasible to simultaneously monitor fish presence and behavior in a large alpine river using conventional methodologies, such as netting (german \"Hamen\" [53]). Therefore, the FIDET project developed a multimodal sensing approach by combining hydroacoustic observations of fish with hydrodynamic variables by measuring the time-averaged velocity and pressure turbulence. Single-beam echosounders have been successfully applied to assess and monitor fish presence at several hydroelectric sites [54; 55; 56]. We used a comparable setup in a large alpine river, which provided the first long-term dataset of this river type. In terms of practicality, it was found that the echosounder installation has the potential to detect fish under the wide range of natural conditions demanded for in situ investigations. However, it should be noted that an unfavorable ratio of signal strength to background noise can severely limit detection [57]. In addition, it is necessary to devise a deployment strategy, which allows for the quick removal of the echosounders during discharges with high debris loadings or during flood events. Therefore, it is highly recommended that the experimental plan explicitly includes a maintenance design and hands-on training for responsible personnel, as periodic cleaning and removal of the floats is a key factor enabling long-term measurements. Furthermore, the device positioning should be determined on a case-by-case decision. However, in an alpine river with strong flood events and related floating debris (wood, sediment) the proposed setup (floats that can be recovered) is the only feasible one. An upward-looking array would require installation, inspection, and safe removal by divers prior to floods. This was considered impractical at this site. The development of site-specific tracking criteria was found to provide a useful basis for future studies under similar conditions. For example, the variability in TS values in the field trials was expected to be large, as it is a single-beam system with large opening angles. The detection of an object's orientation and absolute size determination is thus at best only partially feasible. While the detection was satisfactory in spring and early summer, problems did occur due to (1) noise caused by high discharges, (2) organic material (e.g., grass clippings, leaves, wood) and (3) snow and ice related to dumping of snow into rivers. In general, the echosounder-based fish tracking system provided a cost-effective long-term monitoring system, however the mentioned limitations need to be considered. Figure 10: Cross-section plots of the bulk velocity (in profile 189) measured by the ADCP, the locations of the 24 DBox measurements are shown as white circles with a black border (**a**). The plot of the pressure turbulence by combining the differential pressure fluctuations and bulk velocity measurements following Equation (2) (**b**). Note the linear relationship between the bulk velocity and turbulence (dynamic pressure coefficient). Thus, the spatial distribution of the intensities was found to be inversely proportional for this case study. At Kirchbichl, regions with high _Kpv_ had on average, lower flow velocities. In order to assess fish behavior, a multimodal hydroacoustic and pressure measurement system was developed and implemented for the first time, taking into accounts the parallel use of three imaging sonars. The data collected present new insights using non-invasive measurement technologies on fish presence and behavior in front of a large, run-of-river hydropower intake. Specifically, the combination of spatially distributed, time-averaged flow velocities recovered from the ADCP and pressure turbulence measurements form the dynamic picture of the complex flow field experienced by fish in front of hydropower intakes. While acoustic cameras are capable of capturing underwater videos, which enable investigations of fish behavior, a long-term deployment \"in the field\" is limited due to equipment as well as operational costs.