An extensive study by a research team at the Technical University of Munich (TUM) has found modern hydroelectric power plants do not always protect fish better than conventional ones. In addition to the technologies employed, the specific location of the plant and the fish species being present at that location also play a role in fish protection, the research suggests.

The team has systematically studied how different types of power plants affect various fish species and their habitats, with the detailed findings expected to help improve future planning as well as retrofitting of existing plants.

Jürgen Geist, Professor of Aquatic Systems Biology at the TUM, and his team have been investigating the effects of hydroelectric power plants on downstream-moving fish as well as their impact on aquatic habitats since 2014. So far, the research team has analyzed seven power plants in Bavaria, which utilize four different technologies, both modern and traditional.

In order to find out whether fish are being harmed by hydroelectric plants, Professor Geist’s team set up nets downstream from the turbines, overflows, and fish passes in which more than 70,000 fish were caught. The researchers investigated whether and how fish were injured and their mortality rates resulting from such injuries. Approximately 8,500 fish were additionally x-rayed to obtain indications of internal injuries.

Preparation of the nets. A. Heddergott / TUM 

In addition, Professor Geist’s team also investigated the composition of the aquatic biocoenoses (fish, invertebrates, aquatic plants, algae) and environmental factors including temperature, dissolved oxygen and pH, in the river upstream and downstream from the hydropower plants.

“The mortality rates at plants with modern technologies were not always lower than at the conventional plants,” reports Professor Geist in the results of his extensive study. In a report on the study, the authors say that it had been assumed that modern plants, such as hydroelectric power screws or power plants which use very low head (VLH) turbines would protect fish better because these turbines rotate slowly, allowing fish to swim through them, and were thus considered “fish compatible”.

However, the study shows that there is no plant and turbine technology that protects fish better than others. Instead, animal welfare depends on how the technologies are used, which species are present, and which conditions can be found at the respective locations.

VLH turbines, such as those installed at the research site Au at the Iller river, caused “relatively little damage”, says Professor Geist. However, it should be noted that fish survival was significantly reduced with this type of turbine at a different site that has a drop height of four meters.

Whether the plant is running at full load or partial load also effects the welfare of the fish. The empirical studies have shown that grayling and brown trout are more likely to pass the power plant safely when the VLH turbine is operated at high load rather than at low load.

The majority of the fish population, especially small fish, followed the main current and were not prevented from passing through the turbine rooms by fish protection rakes. The bypasses created at the power plants, where fish can swim around the turbines as they migrate upstream, are not used by most of the animals for the descent.

Employees of the TUM recover nets at the powerplant Baiersdorf/Wellerstadt.  A. Heddergott / TUM 

The studies also found that eel tubes are rarely used. To protect the eel in its natural distribution area in Bavaria (complete Main catchment area), the scientists suggest opening the flushing gates in the weirs, which let branches or larger alluvial deposits pass through, by ten to 20 centimeters during the main migration periods at night in the fall. According to observations, eels clearly prefer to take advantage of this opportunity to move downstream.

The scientists also took a close look at the ecological water conditions around the plant. “The negative effects of the interruption of the flowing water by the transverse structures should be compensated for in the best possible way. We recommend that replacement habitats for fish and other aquatic creatures be created and that areas should be restored to nature whenever possible”, said Geist. “The results of the study were discussed on site with all parties involved at each study site. As a result, improvements have already been worked out.”