Throughout their life cycle, the aquatic organisms migrate along the river stream to fulfil their essential needs. Seeking for food, reproduction, seeking for shelter against predators and seeking for favorable living conditions are some of the main reasons for migrations. Transverse hydraulic structures in watercourses present a barrier for these organisms which cannot be passed without existence of appropriately designed and maintained fish passes. Therefore, these objects need to be designed and built according to the characteristics of fish and aquatic invertebrates living at the section of river in focus. To allow passing the Cajhen weir on the Sava Bohinjka river, just some hundred meters upstream of the confluence with the Sava Dolinka river, a technical fish pass with vertical slots was constructed in 70's. However, due to the inappropriate design it does not allow passing of the weir to any of aquatic organisms. It is also poorly maintained (backfilled with alluvium deposits in the bottom part, branches and timber logs stuck between the vertical slots).
During the periods of low water in the watercourse, also migration down the stream is not possible due to the lack of water in the stilling basin in the spillway of weir. Goal of the presented thesis was to familiarize with the different types of fish passes, with target fish species at the area, their swimming characteristics and to define the most appropriate type of fish pass for the Cajhen weir. Three different solutions were analysed: fish ramp with boulders, bottom ramp and bypass channel. According to the characteristics of the area, the latter is considered to be the most appropriate solution.