FIThydro has prepared a report that guides the hydropower operators and the environmental managers to select the right mitigating measure. The report classifies the measures according to the problem at hand, describes how they work, what fish communities they are targeted at and how certain it is that they work as intended.

FIThydro carries out research on environmental impacts from hydropower installations and aims at finding solutions to increase power production and to support development of self-sustainable fish populations. Hydropower may alter flow and morphology in downstream and bypassed rivers, cause direct mortality of fish, change fish habitat conditions and introduce barriers for fish movement and migration. Hydropower is, however, a very site-specific technology developed where the topography and hydrology are favourable, and the magnitude and scale of these changes vary a lot.

Fish using the areas identified by red colour will have a risk of stranding when the water level drops rapidly (Photo: Tor Haakon Bakken)

More than 200 freshwater fish species are abundant in European rivers and they might have different requirements for habitat that also vary across life stages. Fish populations might have different needs and capability to move and migrate, and the aquatic ecosystem is adapted to the natural flow regime. Understanding the behaviour and needs of each of the species and how they are affected by the hydropower installations are essential in designing efficient mitigation measures. As such, finding the optimum measure for a certain location is a challenging task that requires detailed, local knowledge.

Riparian zone vegetation along Stjørdalselva and one of its tributaries in central Norway. The riparian is on most section in place, providing shelter and reducing runoff of particles and nutrients from the agriculture (source:

The measures presented and described in the report are categorised according to the main problems they are supposed to mitigate:

  • environmental flows
  • habitat
  • sediment management
  • upstream migration
  • downstream migration

The description of each measure is accompanied by photos and illustrations to explain how they function, and key scientific publications documenting their performance are given. The measures are also classified with respect to which fish species they are aimed at, what part of the regulated system they can be implemented and which physical factors they mitigate. Their technology readiness level (TRL) is also indicated. This is useful information for a first sorting of relevant measures.

Horizontally inclined mesh rack and guidance of fish through slots in the rack (arrow A) to a tube back to the river to the right (arrow B) in the intake canal of Las Rives HPP in Ariege River, France (Photo: Atle Harby).

In order to plan, construct, maintain and monitor the performance of mitigating measures, different methods, tools and devices can be useful to apply. As an example, computer-based simulation tools assessing the habitat conditions can be used to evaluate the effect on the habitat prior to construction. Model tools allow testing of different configurations and fine-tuning of the design, and how the measures function during different flow regimes. In order to evaluate the effect of for instance measures to improve upstream migration, a set of different devices are relevant to install to monitor fish movements. This can be carried out with radio or acoustic telemetry techniques, where receiver antennas in strategic places register the position of tagged fish.

Our ambition is that this publication can be the first step in assessing what measures that might be good solutions to reduce the impacts on the fish population from hydropower. The publication will guide the readers to identify potential suitable measures. Further work the coming years of FIThydro will include a more comprehensive list of mitigation measures and linkages to new and further developed methods, tools and devices, as well as approaches to identify the most cost-efficient set of mitigating measures

For more information download the full report (6MB) or or view all technical deliverables.
Additional information about some of the solutions, methods, tools and devices can also be found in FIThydro Deliverable D2.1.

Author: Tor Haakon Bakken (SINTEF)