Abstract: Applying various conventional and modern monitoring methods a function control of downstream migration facilities and turbine passage related mortality was conducted at the Kostheim Hydro Plant (4,9 MW; two Kaplan turbines; Δh max. 3,74 m, at MQ 2,36 m) at the river Main (km 3,2). Furthermore, the perceptibility and passability of a nature-like bypass for upstream migration was examined. Results showed a size selectivity of the bypass caused by an unsuitable entrance position 40 m downstream of the outlet of the turbines. The effectiveness of the downstream migration facilities proved to be low, leading to high mortalities at the trashrack and during turbine passage. The vertical bar trashrack with a bar spacing of 20 mm showed little repellent effect and was passed by most fish up to a size of 20-25 cm total length, causing scale loss and haematoma in Atlantic salmon smolts and cyprinid species. Eels smaller than 65 cm were unable to pass the trashrack and the majority was killed by the trashrack cleaning device. Particularly, smaller eels experienced high mortalities at the turbine passage.