Not every hydropower plant is equally dangerous for fish, as a comprehensive analysis now shows. According to this, the passage of hydropower plants with certain types of turbines is particularly often fatal for fish. The risk of death increases, among other things, with a high speed of the blades, small blade spacing and when the water falls at great heights. The danger is particularly high for eels, sea trout and sturgeon that migrate long distances.
In hydropower plants, the energy of falling or flowing water is converted into electricity, for example by using the water pressure to drive turbines. The high availability of the resource, the low operating costs of the systems and the emission-free generation of energy make hydropower systems particularly popular. But even if the systems contribute to climate-friendly electricity generation, they are not necessarily environmentally friendly: They often have a massive impact on local ecosystems. Often the water quality deteriorates, rivers accumulate and migration routes for fish are blocked.
What is the risk of death for fish?
A team of researchers led by Christian Wolter from the Leibniz Institute for Freshwater Ecology and Inland Fisheries (IGB) has now examined the danger to migratory fish from hydropower plants in more detail. It has long been known that fish can get into the turbines of hydropower plants and die. But so far there has been no clear assessment of the mortality risks for various hydropower plants. The scientists have now developed a new method for analyzing fish mortality – analogous to the index for the mortality risk of bats and birds on wind turbines.
In what is known as the Mortality Risk Index (MGI), the research team first defined the general risk of death – excluding hydropower – for all native fish and lamprey species that occur in freshwater. The method takes into account population-biological factors such as reproduction and population development of the animals and nature conservation aspects, e.g. the rarity and conservation status of the fish species. In a second step, the researchers finally assessed the risk of fish species being killed at hydropower plants of various sizes and types in a five-stage system.
High risk with Francis and Kaplan turbines
The evaluations showed that there is a high risk of killing by hydropower plants, especially for species that migrate long distances, such as eel, salmon or sturgeon, as well as the various char and vendace species that migrate in river systems. The likelihood of fatal injury increases with the size of the fish. “Hydroelectric power plants are particularly dangerous for fish species that have a high risk of death due to other biological and man-made factors, such as eels and sturgeons,” explains Wolter’s colleague Johannes Radinger. In addition, migratory fish that reproduce several times in their life – such as the sea trout – are also endangered.
According to the data, the mortality rates of the fish also depend on the type of turbine in the hydropower plants. According to the study, the risk of death is highest in so-called Francis and Kaplan turbines. They are the most common reaction turbines used in hydropower plants. The rotating vanes or blades work at a speed of at least 80 revolutions per minute – the following applies: the faster, the more dangerous for the fish. In comparison, Archimedes’ snails and waterwheels are generally less dangerous – for example, they have no blades and larger blade spacing. In addition, the distance between the turbine blades and the turbine housing is crucial: larger gaps also cause higher mortality risks. And systems in which the water falls from great heights also increase fish mortality.
The risk to the fish also depends on the location of the electricity generator in the water, as the scientists report: If it is located in the middle of or directly adjacent to migration areas or regions with a large fish population, the mortality rate increases. According to Wolter and his colleagues, several systems in direct succession can even kill an entire fish population.
Hydropower plants usually do not provide fish protection
According to the researchers, small hydropower plants in particular are ecologically problematic: “The around 7,000 hydropower plants in Germany with an installed capacity of less than one megawatt only produce around 14 percent of the total electricity from hydropower of 17.5 terawatt hours per year in 2019,” says Wolter. “Your contribution to the energy transition is marginal, but the damage it causes in aquatic ecosystems and fish stocks is comparatively high.”
The evaluation of various hydropower plants shows that the risk of fish being killed is only reduced if an effective fish protection system is installed. These include, for example, mechanical fish deflectors and adequately designed fish passages. The scientists therefore demand that hydropower plants be equipped with better protective measures for the fish in the future. However, this often makes the problematic, small power generators unprofitable.
Source: Leibniz Institute for Freshwater Ecology and Inland Fisheries (IGB); Technical article: Technical planning assessment of the mortality of fish at hydropower plants (PDF)