A good summer has passed, a warm autumn follows. Fine for summer vacationers, less so for fish. It was too hot for them. On August 24th it was a good 23 degrees in the Traun below the Theresienthal power plant (near Gmunden). For grayling and brown trout this means: higher heart rate, greater need for oxygen, stress. Two degrees more and life is in danger. Of course, fishing was stopped so as not to further stress the animals.
The sun heats the upper water layer of the (reservoir) lakes, and almost only surface water flows into the river. “We have our backs to the wall,” says Albert Pesendorfer, chairman of the “Friends of the Gmundner Traun” association, describing the situation. Are there solutions?
People on the Rhine have been dealing with this problem for a long time. Since thousands of fish died due to heat in 2018, the authorities have been working on emergency concepts. They say, for example, that agriculture is no longer allowed to withdraw water for field irrigation and that industry is not allowed to withdraw water for cooling purposes or that bathing areas are closed – where cooler streams flow into the Rhine and form refuges for heat-stressed fish. Even the wooden jetty at the Rhine Falls was closed because fish desperately needed the shade underneath and would otherwise have been driven back into the warmer water by tourists.
There is an urgent need to provide better bank shading. The installation of deep scours where cooler water is held, as well as river construction measures that bring more oxygen into the water, must be implemented as quickly as possible. The Austrian Federal Forests are currently considering the idea of feeding cold deep water into the lake outlet. A pilot study will soon begin where the Ischl flows from Lake Wolfgangsee in order to obtain initial assessments of how additional amounts of cold water affect a river.
Perhaps such a maneuver will also save Ager and Traun over the hot summers. According to estimates, a pipe with a diameter of around 1.4 meters would supply enough cool lake water to the Traun near Gmunden to support brown trout and grayling during the hot season. How such water withdrawal affects Lake Traunsee must first be investigated.
