Hydropower: the mortality risk for fish at turbines

Although hydropower is a renewable energy source, it is not necessarily environmentally friendly: the plants have a strong structural and operational impact on the river ecosystems in which they are built. In particular, the high turbine-induced mortality of fish is a source of conflict in planning and permit procedures. This is due to the fact that there are as yet no standardised, objective approaches to assessing mortality risks.

In a project funded by the Federal Agency for Nature Conservation (BfN) with funds from the Federal Ministry for the Environment, Nature Conservation and Nuclear Safety (BMU), the research team has developed an assessment index for the mortality risk of fish at hydropower plants - analogous to the assessment indices that BfN has already developed for other wildlife, such as the mortality risk of bats and birds at wind power plants. "Our scientific assessment basis and work aid can support practice in standardized planning and evaluation of hydropower plants and increase the legal certainty of procedures," says Dr. Christian Wolter, researcher at IGB, who headed the study.

The new procedure can be used, for example, for assessments under the Water Framework Directive or the Flora-Fauna-Habitat Directive.

The mortality risk index as a general mortality risk, independent of hydropower

In the first step, the researchers defined the general mortality risk for all native fish and lamprey species found in freshwater – the mortality risk index (MGI) with six different classes. The MGI takes into account biological (such as reproduction and population development) and nature conservation factors (such as rarity and conservation status).

Large and migratory fish such as sturgeon, salmon, huchen, Rhine houting and allis shad, as well as the various species of char and vendace that migrate in river systems, are at high mortality risk. The endangered European eel is also represented in this class.

The risk of mortality at hydropower plants

In a second step, the researchers evaluated the project-type-specific risk of killing fish species at hydropower plants in a five-stage system.

A high project-specific risk of mortality by hydroelectric power plants exists for species that travel long distances for their behaviour and reproduction. Downward migrating fish (e.g. eel) and upward migrating fish that reproduce several times in their lifetime (e.g. sea trout) are particularly at risk. Upward-migrating fish that reproduce only once in their life are at risk as juvenile fish. They are particularly at risk because they are only protected from the turbine passage by very narrow rakes. Fish which migrate over long distances within a water system (e.g. common nase) are also at high risk of killing.

In the case of turbine passage, the probability of fatal injury to migrating fish species increases with body size. However, mortality rates also depend on the type of turbine; they are generally higher for Francis and Kaplan turbines compared to Archimedean screws and water wheels.

Determine the risk of mortality depending on the location and type of turbine

The researchers combined the general mortality index (MGI) with the project-type-specific mortality risk and the concrete construction and site details in order to be able to individually evaluate plants. Plants differ, for example, in factors such as turbine type and operating parameters, dam height, water volume, operating mode, but also the location in the water body and the presence and functionality of fish migration aids and mechanical fish protection.

Conclusion: Low risk of mortality only with extensive fish protection – EEG subsidies must be linked to fish protection measures

The researchers compiled and evaluated the various technologies and measures for preventing and reducing mortality and barrier effects of the plants. Their conclusion: a reduced risk of mortality at hydropower plants is actually only possible if effective fish protection is installed. This includes, for example, mechanical fish deflectors and adequately installed fish ladders and descent aids, the functionality of which must also be continuously checked and ensured.

"The approximately 7,000 hydropower plants in Germany with an installed capacity of less than one megawatt produce only about 14 percent of the total electricity from hydropower of 17.5 terawatt hours per year in 2019. Their contribution to a sustainable energy supply is thus marginal, but the damage they cause to freshwater ecosystems and fish stocks is comparatively high. The fact that hydropower is nevertheless promoted under the EEG is a fundamental problem that also stands in the way of the implementation of other directives, such as the Fauna-Flora-Habitat Directive or the European Water Framework Directive. At the very least, the EEG would have to be amended in such a way that hydropower plants that do not provide adequat fish protection would no longer be subsidised," concludes Christian Wolter.

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Funding:

Funded by the Federal Agency for Nature Conservation with funds from the Federal Ministry for the Environment, Nature Conservation and Nuclear Safety (BMU) (FKZ no. 3515 82 3200).