Identifying a Self-Sustaining Population, Spawning Site and Population Size of the Danube Salmon Hucho hucho Using the Citizen Science Approach

The last populations of Danube salmon (Hucho hucho) in Croatia are found in the Kupa and Una rivers. However, the size of the suitable area that could serve as the best spawning ground for the remnant Danube salmon population in these rivers is not known. This study aimed to leverage anglers' experiences to determine the sections of the Una and Kupa rivers where the last populations of Danube salmon (Hucho hucho) in Croatia are most abundant, identify potential spawning sites and assess threats that could impact the species' existence. The Kupica river emerged as a tributary of the Kupa river with a large Danube salmon population in Croatia. Experienced divers conducted three dives along a 350-m transect in the Kupica river, estimating a total of 13 Danube salmon individuals ranging from 50 to 115 cm in total length, in pool and run mesohabitat types. In the Kupa and Kupica rivers, the survey identified 25 Danube salmon spawning sites. The primary local threats identified for the Danube salmon population include small hydropower plants, the construction of a radioactive waste disposal site, water abstraction, the removal of riparian vegetation, overfishing and climate change. Based on the results of this study, we strongly recommend the implementation of protective measures against these threats and the establishment of an ichthyological reserve specifically for Danube salmon in the Kupica river. This recommendation aligns with the EU Biodiversity Strategy, emphasizing the need to identify designated areas by 2030.