## 1 Introduction  
European research on downstream migration of fish began some 30 years ago [1; 2; 3; 4], and has increased substantially in the last decade, especially in Germany [5; 6; 7]. The majority of studies are focused on diadromous fish, such as Atlantic salmon [8; 9] and European eel [8; 10; 11]. Most studies have been limited to small hydropower plants [12; 13]. Research on large hydropower plants is therefore needed to develop suitable measures for the full spectrum of European hydropower facilities. Currently, the ongoing discussion on fish protection and downstream migration focuses on three key aspects: (1) The catchment area and its representative target species; for example, the Danube versus the Rhine, where Atlantic salmon and European eel naturally occur only in the Rhine catchment. (2) The biocoenotic region (rithral versus potamal) as alpine waters that have only a few species differ widely from lowland waters, which commonly have >25 species. (3) Differences in the hydropower plant design, especially the turbine size and type. Based on these three aspects, it has been established that a case-by-case examination is necessary to determine the balance between ecological need and economic utility at each hydropower facility [14; 15; 16]. Currently, there is no best practice concerning the provision of facilities for downstream migration of fish and/or fish protection in Austria [17]. To address this, three research projects are underway. The first is a project on downstream migration at small hydropower plants [18]. Secondly, the project \"Downstream fish migration on medium-sized rivers in Austria: population basics and implications for fish protection and fish descent\" [19]. And finally, the project \"Fish detection and fish behavior: hydroacoustic investigations for the assessment of presence and behavior of fish in front of the trash rack of-run of-river plants\" (FIDET). This paper reports the main findings of the third project, FIDET. The major challenge of in-situ research is to develop practical field measurement methods capable of addressing knowledge gaps. This can be achieved by combining multiple measurement technologies, both in the lab and in the field [20; 21; 22]. Currently, there are no published datasets on fish presence and behavior in the vicinity of trash racks of large HPPs in the Alpine region, with its potamodromous fish fauna. Thus, the FIDET project was initiated to establish new methodological approaches to assess the presence and behavior of fish in front of trash racks [15; 23]. Further, it is important to assess the local flow conditions directly upstream of physical barriers to compare fish presence with the spatial distribution of the time-averaged velocity and turbulent fluctuations. Indeed, it is well-established that the local flow environment can have a strong influence on fish swimming behavior, orientation of movement, act as flow cues, and be strongly related to the swimming speed of fish [24]. While the presence and intensity of flow turbulence is known to affect fish swimming and hydraulic preference [25; 26], there is a lack of suitable measuring methodology capable of providing turbulence metrics for field studies. The \"Differential Pressure Box\" (DBox) was specially developed as part of the FIDET project, and presents a new type of turbulent flow sensing device. Due to the challenging conditions encountered in the field, we provide one of the few in situ investigations of local hydrodynamic conditions upstream of hydropower intakes [18; 27]. We show the potential of combining hydroacoustic and flow field turbulence measurement methods to detect periods with increased fish occurrence as well as to analyze spatial distributions of fish at mechanical barriers at the individual level.