Resolving the controversy about hydropower is only possible based on reliable data on its ecological effects, particularly fish welfare.

Herein, we propose a comprehensive assessment of conventional and innovative hydropower using a data set of 52 250 fish.

The effects of hydropower on fish were most harmful at sites with Kaplan turbines, showing £ 83% mortality. Innovative hydropower, often termed ‘fish-friendly’, caused £ 64% mortality.

Our findings suggested that the runner peripheral speed, number of turbine blades, turbulence and gas oversaturation at turbine outlets were the most important factors.

Synthesis and Applications: To reduce the impact of hydropower on fish, site-specific characteristics such as head drop, bypass options and river-specific species composition need to be more intensively considered in optimal turbine technologies and operation modes.

These data were generated in a large field experiment to investigate the effects of innovative and conventional hydropower turbines on fish. The data contain information on external and internal injuries of individual fish (Spreadsheet 1) and
physical parameters for each net emptying interval (Spreadsheet 2).

External and internal fish injuries caused by hydropower turbine passage were classified following the protocols published in

M. Mueller, J. Pander, J. Geist, Evaluation of external fish injury caused by hydropower plants based on a novel field-based protocol. Fish. Manag. Ecol. 24, 240–255 (2017).
M. Mueller, K. Sternecker, S. Milz, J. Geist, Assessing turbine passage effects on internal fish injury and delayed mortality using X-ray imaging. PeerJ 8, e9977 (2020).