The Danube gudgeon had been considered extinct in the Inn River for more than 100 years. Only a few years ago, the highly endangered species was rediscovered there. Now, researchers at the Technical University of Munich (TUM) have documented the fish in another section of the Inn. The finding emerged from a joint research project with hydropower operator VERBUND examining the effects of nature-like fishways on fish populations and river ecology.
Andreas Heddergott / TUM In brief
- Long-lost Danube gudgeon rediscovered in the Bavarian stretch of the Inn River near Simbach
- Researchers found the species as part of a long-term study on the effects of nature-like fishways
- Nature-like fishways also serve as habitats
For Dr. Christoffer Nagel, a researcher at the Chair of Aquatic Systems Biology at TUM, the finding came as a surprise. He rediscovered the Danube gudgeon during field surveys in the River Inn: "Over the course of my research, I have seen hundreds of thousands of fish. Rediscovering such a rare species that was believed lost from the river is a truly special experience and something I never expected."
The Danube gudgeon (Romanogobio uranoscopus) is considered highly endangered and is particularly sensitive to declining water quality and changes to its natural habitat. Its presence is therefore considered an indicator of good ecological conditions in a river. Only a few years ago, the species was recorded in the Inn for the first time in more than a century downstream of the Ering-Frauenstein dam, located about 13 kilometers farther downstream. Together with the latest finding near Simbach, researchers see this as an indication that the species may have expanded its range within a larger section of the Inn.
Success of a long-term research project
The detection of the Danube gudgeon in the Inn adds to a series of positive findings from a long-term research project conducted at the Chair of Aquatic Systems Biology and led by Professor Jürgen Geist. Systematic fish surveys in twelve largely nature-like fishways along the Inn River recorded approximately 38,000 individuals representing 36 fish species. In addition to isolated records of particularly rare species such as the Danube gudgeon, researchers have regularly recorded protected species including bullhead, barbel, common nase, huchen, and grayling.
Fishways enable fish to move through river systems where migration routes would otherwise be interrupted by numerous hydropower facilities. Fish depend on these migrations throughout their life cycle, from early development and feeding to reproduction. Along the Inn, side channels have been created to provide alternative routes around barriers. These bypass channels allow fish to circumvent hydropower facilities. When designed in a nature-like way, such fishways can replicate natural habitats through varying water depths, flow conditions, and substrate structures. This distinguishes them from technical fishways, which often consist of geometric concrete basins. Nature-like fishways provide more suitable conditions for a wider range of species. According to the researchers, the Danube gudgeon also benefits from these habitats, as it depends on gravel-bottomed, fast-flowing, and well-oxygenated sections of rivers.
Johannes Wesemann, project manager of VERBUND, sees the results as a confirmation of the renaturation approach: "The discovery of the Danube gudgeon, as well as many other protected species, underlines that the measures we have taken in the reservoir areas not only aid the specific locations, but also 'revitalize' the entire river ecosystem. We thereby create the foundation for stable fish populations in the Inn River."
Not only migration corridors, but habitats
The researchers have observed additional benefits of nature-like fishways. "The fishways we studied support a high level of biodiversity. They provide important habitat functions for different life stages of aquatic organisms. For some species, particularly grayling, these systems can serve as key habitats throughout their entire life cycle. Well-designed fishways are used both for spawning and as nursery areas for juvenile fish," says Christoffer Nagel.
For Jürgen Geist , the extraordinary finding highlights the value of scientifically supported river restoration projects and broad stakeholder collaboration: "Years of close cooperation among researchers, the hydropower operator VERBUND, water management, fisheries and conservation authorities, as well as associations and local angling clubs, have created an important network through which restoration measures along the Inn have been continuously improved. The detection of the once-lost Danube gudgeon is therefore a remarkable shared success for everyone involved."
Christoffer Nagel / TUM 
Christoffer Nagel / TUM Christoffer Nagel / TUM - Prof. Jürgen Geist is Professor of Aquatic Systems Biology at the TUM School of Life Sciences .
