The aim of the RETERO project is to develop new methods to reduce and, in the long term, completely replace fish experiments for assessing the mortality risk of fish when passing turbines or pumps. General approaches are numerical models with fish surrogates and an autonomous robot fish. For both, behaviour rules for fish in hydraulic conditions as found in turbines and other passage corridors are crucial. The Institute of Hydraulic Engineering and Technical Hydromechanics (IWD) of Technische Universität Dresden and the Institute of Aquatic Ecology and Fish Biology Jena therefore carry out ethohydraulic experiments in the hydraulic laboratory of IWD. In an experimental flume the hydraulic conditions of turbines, pumps and their inlets are modeled. The fish behaviour is studied in the flume providing one zone with a high spatial velocity gradient (SVG) and a second zone with flow velocities above the maximum swim speed of the fish. Both flume zones were hydraulically characterized with Laser Doppler velocimetry and Acoustic Doppler velocimetry measurements. Behaviour analysis of Salmonids were done in the first project phase. In the ongoing second phase (RETERO II) the behaviour of perch (Percids) and chub (Cyprinids) is observed with an infrared based, three-dimensional video tracking system. The discovered behavioural patterns will be implemented by the project’s collaborative partners into an autonomous sensor probe and into software-based models by the SJE Ecohydraulic Engineering GmbH and the University Magdeburg to the development of hydronumerical prediction models for fish mortality.