A small, gravel-loving fish that vanished from Bavaria’s Inn River for more than a century has turned up alive again, and researchers say this time it is not an isolated fluke. The Danube gudgeon, a highly endangered species barely 12 centimeters long, was documented by researchers at the Technical University of Munich in a stretch of the Inn near the town of Simbach, roughly 13 kilometers upstream from the spot where the species reappeared only a few years earlier. Together, the two sightings suggest a population once written off as gone from the river may be slowly reclaiming territory it lost generations ago.
A Century Without a Confirmed Sighting
The Danube gudgeon, known scientifically as Romanogobio uranoscopus, is a small bottom-dwelling fish with whisker-like sensory barbels around its mouth that help it navigate and find food along riverbeds. It needs gravel-bottomed, fast-flowing, well-oxygenated stretches of river to survive, conditions that make it acutely sensitive to pollution, dam construction and habitat loss. That sensitivity is also what makes the species useful to scientists: because it cannot tolerate degraded water, its presence in a river is treated as a reliable sign of good ecological health. For more than 100 years, the Danube gudgeon was considered extinct in the Inn, a casualty of the dams, channelization and water-quality declines that reshaped the river over the 20th century. The species belongs to the gudgeon genus Romanogobio, a group of small Eurasian river fish found across the wider Danube basin, and individuals caught by the Munich team have measured only about 12 centimeters long, with brownish, mottled scales that help them blend into a gravel riverbed.
A Second Sighting Roughly 13 Kilometers Upstream
The species first reappeared in the Inn only a few years ago, recorded downstream of the Ering-Frauenstein dam for the first time in more than a century. The latest detection, made by Dr. Christoffer Nagel, a researcher at the Technical University of Munich’s Chair of Aquatic Systems Biology, came as a genuine surprise even to a scientist who spends his career counting fish. Nagel found the gudgeon during routine field surveys near Simbach and has said that after observing hundreds of thousands of fish over the course of his research, rediscovering a species believed lost from the river was not something he expected to witness. After the fish was identified and measured, the team released it back into the water. Researchers see the roughly 13-kilometer gap between the first rediscovery site and the newer one near Simbach as a meaningful signal in its own right, since it suggests the gudgeon is not confined to a single lucky pocket of habitat but may be gradually working its way along a longer stretch of the river.
Twelve Fishways, 38,000 Fish, One Rare Catch
The find emerged from a long-running research project, led by Professor Jürgen Geist at the university’s Chair of Aquatic Systems Biology, that has systematically surveyed fish populations in twelve largely nature-like fishways built along the Inn River. Across that survey work, researchers have logged roughly 38,000 individual fish spanning 36 species, including protected species such as bullhead, barbel, common nase, huchen and grayling alongside the rare Danube gudgeon. The project is a joint effort with the hydropower operator VERBUND, which built the fishways to let fish move past its dams and reservoirs on the Inn, and the scale of the survey is part of what makes an isolated species like the gudgeon detectable at all. Surveying twelve separate fishways over several years, rather than checking a single site once, is what let the team notice a pattern rather than a one-off catch, since a single Danube gudgeon turning up in a spot-check could easily have been dismissed as a stray rather than evidence of a spreading population.
Why Fishways Double as Habitat, Not Just Passageways
Hydropower facilities on rivers like the Inn interrupt the migration routes that many fish species depend on throughout their life cycle, from early development through spawning. Conventional technical fishways, often built as a series of geometric concrete basins, let fish pass but otherwise offer little resemblance to a natural riverbed. Nature-like fishways are designed differently, replicating varying water depths, flow conditions and substrate through built-in wooden structures that create a range of current patterns rather than a straight channel. According to the research team, that design is what lets a species like the Danube gudgeon persist there at all, since the fishways recreate the gravel-bottomed, fast-flowing conditions the fish cannot find in the heavily engineered main channel. Researchers have also found that some species, particularly grayling, can use the fishways as a habitat for their entire life cycle, not just as a corridor to swim through.
A Shared Win for Researchers, Regulators and a Hydropower Operator
The Technical University of Munich’s account of the discovery frames it as validation of years of cooperation between scientists, the hydropower industry and conservation authorities. Johannes Wesemann, VERBUND’s project manager on the Inn River work, has said the repeated appearance of the Danube gudgeon and other protected species shows the restoration measures are not just helping isolated sites but reviving the river ecosystem more broadly. Geist, who leads the broader research program, has called the rediscovery a shared success built on years of coordination among researchers, the hydropower operator, water management and fisheries authorities, and local angling clubs. Whether the Danube gudgeon can keep expanding along the Inn remains to be seen, but for a species written off as extinct in the river for a century, two confirmed sightings in a few years already mark a reversal few expected.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
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