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The investigated fish pass is a part of а low-head hydrostatic pressure machine (HPM) located on an existing weir in the upper Iskar River. Monitoring of the hydraulic features of the fish pass was performed under different hydraulic conditions. Diversity and abundance of the local fish fauna was investigated with the aim to verify target fish species. A major limiting factor for using of the fish pass during flood periods was high water velocity (both inside the fish pass and at the fish pass downstream entrance), while during the low water discharge periods it was the insufficient water depth in some pools and the significant differences between the water levels in adjacent pools. The difference between the water levels at the entrance and exit of the fish pass reduces the chances for access of small-sized fish species. Some measures for improving fish pass functionality were developed. Several re-construction solutions were proposed in an attempt to help solving the identified problems.

Fish Pass Functionality in Relation to the Dynamics of Hydrological Conditions in the Upper Course of the River Iskar (Case Study)