A sky-watching fish, missing from the Inn for a century, turns up near Simbach

In a new side channel dug around a hydropower dam at Simbach am Inn, the biologist Christoffer Nagel lifted a fish barely the length of a finger out of the water and recognised it at once. The Danube gudgeon had been absent from this river for more than 100 years. He noted it down, and put it back.

Romanogobio uranoscopus grows to eight or ten centimetres, rarely more. Its Greek species name means sky-watcher, for the eyes set high on the top of its head. It spawns over shallow gravel in flowing water, and it is fussy: it cannot live where the river has been straightened, silted or slowed. Germany's Red List puts it in the highest category, threatened with extinction.

That fussiness is exactly why the find matters. A gudgeon in the channel is not one more entry on a species list; it is a verdict on the water. Nagel, who chairs the Aquatic Systems Biology group at the Technical University of Munich, had seen the species once before on the Inn, about 13 kilometres downstream near the Ering-Frauenstein dam, a few years earlier. A second site further up the river suggests the fish is not merely hanging on in one refuge but moving.

A truly special experience, and something I never expected.

That was Nagel on the hundreds of thousands of fish he watched pass through these channels during the work. The survey he and his colleagues ran with the hydropower operator VERBUND covered 12 fishways and recorded roughly 38,000 individual fish across 36 species — among them the bullhead, the huchen, the schneider, and on one occasion a wels catfish measuring 1.2 metres.

The dam the channel skirts is older than most of the people studying it. Built between 1951 and 1954, it sits on a river that has been worked hard: four run-of-river plants on the lower Inn impound almost 25 kilometres of it between them. The same stretch has carried international protection since 1976, when the reservoirs were entered on the Ramsar list of wetlands, and the Council of Europe added its own designation three years later.

Protection on paper did not bring the gudgeon back. A meandering ditch full of gravel did. The bypass channels were built on the assumption that fish need a way around the barriers, and the assumption is now being tested by the fish themselves — species that tolerate nothing less than a clean, moving, gravelly river, choosing to live in something an engineer dug.

Nagel held the gudgeon long enough to be certain of what it was. Then the river took it back.