Danube Fish Monitoring Reveals Healthy, Diverse Populations and Rare Species

This year, researchers once again conducted a scientific study of the fish population in the main branch of the Danube. Based on the results, the fish population of the Danube is diverse and in good overall health. In the deep riverbed, the protected pale-spotted stickleback and the non-native white-finned gudgeons occurred in the largest numbers, while in the coastal zone, in addition to the white-finned gudgeons, the number of wind-driven minnows was high.

As part of their public monitoring program, staff from the HUN-REN Institute of Aquatic Ecology, in collaboration with the Danube-Drava National Park Directorate and the HUN-REN Veterinary Medical Research Institute, conducted research fishing expeditions at Gönyű in northwestern Hungary, Nyergesújfalu in northwestern Hungary, Sződliget in north-central Hungary, Kisapostag in central Hungary, Paks in south-central Hungary, and Mohács in southern Hungary, near the Croatian border.

They collected data in two habitats at each location: the coastal zone and the deep riverbed far from the shore.

The experts identified a total of 34 fish species. Similar to observations in previous years, the number of fish species caught in the deep riverbed (21) was lower than that caught in the coastal habitat (33 species). In the deep riverbed, the protected pale-spotted stickleback and the non-native white-finned gudgeon occurred in the largest numbers. In the coastal zone, in addition to the white-finned gudgeon, the number of wind-driven minnows was high.

“The most encouraging finding of this year’s survey is that new specimens of the protected and long-thought-extinct stickleback were found near Gönyű and Nyergesújfalu,” said Péter Sály and his colleagues. Another positive result is that researchers also caught stellate sturgeon and pygmy loach near Mohács. The larger than usual number of young specimens of balin and the highly protected German nase suggest that this year was favorable for the reproduction of these species.

The bighead carp caught near the Paks shore was an unexpected catch, as these species typically live in the upper and middle layers of open water far from the shore.

The researchers also examined some of the fish caught for the presence of fish rot. “Fish rot is a secondary pathogen, the absence or presence of which can be used to infer the general health of fish,” said Edit Eszterbauer and her colleagues. However, based on the occurrence data, it appears that fish rot is typically rare in the Danube fish population: only one positive sample was found among 27 individuals.

Via HUN-REN press release; Featured photo: Pixabay

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