The researchers tested their new curved-bar rack (CBR) system in a laboratory flume equipped with five immersed cameras. This setup allowed them to establish not only whether the flow signals helped the fish to find the bypass, but also how the animals behaved. “We found that different species dealt with the rack in very different ways,” says Eawag biologist Oliver Selz. For example, nase, barbel, brown trout and Atlantic salmon parr used their caudal fins to explore the spaces between the bars. However, whereas a quarter of brown trout swam through the rack after inspecting it, this was very rarely the case with salmon parr and nase.
Overall, the rack was very effective at guiding four of the six tested species – barbel, spirlin, nase and Atlantic salmon parr – to the bypass. Of the brown trout, about half responded as desired, while the eels showed no reaction at all to the rack. For rivers that are home to eels, the researchers therefore recommend that the CBR only be used in combination with bottom overlays or even electric fields – signals to which eels respond well. Electrified CBRs have so far only been trialled in an experimental setting.
The first pilot projects with adapted variants of the CBR are planned to be introduced at the power plants of Herrentöbeli (on the River Thur) and Bannwil (on the River Aare). At the same time, further laboratory experiments are underway with a view to making improvements.