Fish ladders promote genetic exchange

Swiss watercourses are full of cross-obstacles. The report "Structures of Watercourses in Switzerland" (Federal Office for the Environment, 2009) identified a need for revitalization of 10,800 kilometers of watercourses and 50,000 artificial obstacles. The Rhine catchment studied by Alexandre Gouskov and his team for the Eawag study "Fish population genetic structure shaped by hydroelectric power plants in the upper Rhine catchment" includes 37 hydropower plants, two weirs and the Rhine Falls. Six of the artificial barriers were not equipped with fish ladders at the time of sampling.

For statistical reasons, researchers took samples at 47 sites. For each sampling site, they typically took about 50 eel out of the water using electrofishing. The animals were gently anesthetized, measured and released back into the wild after a small tissue sample was taken from the tail fin. "The sampling effort for this study was enormous compared to other research," says Eawag staff member and ETH professor Christoph Vorburger, indirectly complimenting Alexandre Gouskov. The study was his doctoral thesis and was largely funded by the Federal Office for the Environment, Eawag and ETH Zurich.

Barrier effect reduced from 100 to 12 kilometers

It is known from previous studies that insurmountable barriers severely affect the genetic population structure of fish. In the worst case, isolation can lead to the extinction of a population. That's why more and more power plants and other obstacles are being equipped with fish ladders for fish. "Of course, we now know that many fish also use these ladders," says Gouskov, "but the question of whether fish ladders actually have a positive effect on connectivity and genetic diversity of fish populations has not yet been studied."

Based on the genetic studies, the researchers have now been able to show that the fish ladders actually improve genetic exchange. An artificial barrier without a fish ladder has a similar effect on the genetic differentiation of fish as a distance of around 100 kilometers in an unobstructed river. In contrast, for barriers equipped with fish ladders, the equivalent is only about 12 kilometers.

According to Gouskov, this shows that fish ladders improve the connectivity of separate fish populations. However, even with fish ladders, power plants have a significant effect on the genetic differentiation of the alet. Looking at other fish species, this finding takes on additional weight, because many of them are less able to overcome fish ladders than the alet and are thus more affected by fragmentation.

"Our results show that it makes sense to continue the revitalization measures started in recent years," says Alexandre Gouskov. "More, but also better quality fish passes are needed to better protect the species." Depending on their construction, fish passes are used more or less frequently. Compared to simple concrete stairs, bypass channels, for example, perform much better. "Many fish passes have tremendous potential for improvement," Gouskov says. During fieldwork for this study, he was able to observe the effect of such an improvement measure with his own eyes: The Rheinfelden power plant commissioned a near-natural structured bypass watercourse with a large discharge. Already in the first season, about 40,000 fish of 33 different fish species rose in it. "That's gratifying, because that's significantly more than with conventional fish ladders," says Gouskov.

(Eawag)