The fish migration aid at the Ottensheim-Wilhering power station sits enthroned against a gloriously blue sky.

Fish monitoring at

12 March 2019

It's a cool morning at the weir at the mouth of the Innbach stream. Regardless of sun, rain, wind or snow: aquatic ecologist Barbara Missbauer and her team are at work. First, she starts the power generator to raise the basket of the fish trap. Then she and a colleague load the previous night's catch into water-filled tubs and scan them with a reader. This is followed by determining the species, measuring, weighing, chipping, entering into the computer and releasing again. Almost 7,000 fish are under observation at the VERBUND fish migration aid at the Ottensheim-Wilhering power plant

Europe's longest fish migration aid

At the Danube power plant in Upper Austria, VERBUND and its project partners built the longest fish migration aid in Europe over a two-year construction period as part of the LIFE+ program. However, this is not a simple bypass stream. The mouth of the Innbach stream, the Aschach flood relief channel and the Brandstätter Danube arm were integrated into the new habitat, which is around 14 kilometers long. Upon completion in 2017, the experts from Umweltgutachten Petz took over the ecological analysis of the newly created stretch of water for VERBUND. 

Until fall 2019, they will measure how well the migration aid is accepted by the fish. To this end, a weir with a fish trap was installed at the inlet in front of the power plant and at the mouth of the bypass channel. This means that every single fish - above a certain size - can be documented when it wants to swim out of or into the Danube. Five further measuring points record the movements within the habitat.

Some dedicated people are carrying out fish monitoring at the fish migration aid.

Great biodiversity in the water

"Since the start of the project, we have identified 52 fish species during fishing and fish trap monitoring," explains project team member Philip Feldmüller. "The most common species are burbot, roach, aitel and nase, but other species such as barbel, tench, perch and gudgeon are also common." They are all identified, measured and weighed. "Juvenile fish or small fish species under 10 centimetres in length are not marked," says Feldmüller. "But there are also very common species such as the burbot, which we have caught so often that we stopped tagging them last year," adds the scientific head of monitoring, Regina Petz-Glechner. 

In addition to counting and documenting the two fish traps, it is also necessary to fish in the channel, the two rivers and the Danube. "So that we can better assess the data of the fish in the fish traps," says Petz-Glechner. The team also records the routes of the fish at five antenna sites, each located at crossing points in the water system. The chip is scanned - similar to the barcode of a product at the supermarket checkout.

Gnawing beavers and hungry pike

The number of fish documented each day depends heavily on the time of year and the weather. "If there are 300 fish in a fish trap on peak days, the marking takes a few hours," says project manager Barbara Missbauer, describing her work. There were complications last fall when a beaver repeatedly bit a hole in the fish trap over a period of weeks. "The work is interesting and fun. You see new species of fish every day." 

And sometimes strange things happen: "During a fishing trip, we caught a 1-meter pike in the Innbach and kept it under the reader," says Missbauer. "But the computer showed us a different species of fish: a 53-centimetre-long nase that the pike had eaten shortly before." The largest fish in the fish trap so far was a catfish measuring 1.12 meters, the smallest a juvenile nase measuring 2 centimetres.

This much can already be said during the ongoing project: the bypass channel at the VERBUND Danube power plant in Ottensheim-Wilhering is being well received by the new inhabitants as a habitat. The connections to the Danube are heavily frequented. We look forward to further exciting findings before the end of the project in fall 2019.