Unveiling the rise of non-native fishes in eastern alpine mountain rivers: Population trends and implications.

Insufficient knowledge about the occurrence and spread of non-native fish in mountain regions has impeded effective management strategies worldwide. To address this gap, this study analysed over 1300 electrofishing surveys across 650 sites, encompassing a vast 7400 km area in the Eastern Alps. The primary objectives were to quantify the occurrence of non-native species and predict their spread in different river types. Furthermore, the study estimated population sizes and biomass trends for over 150 sites that were surveyed multiple times between 2000 and 2020. Out of the 42 fish species in the study region, 11 were identified as non-native. Notably, two invasive species of Union concern, Lepomis gibbosus and Pseudorasbora parva, increased their population sizes by 8% and 9% per year, over the past decades, supposedly supported by increasing water temperatures. Among the non-native species relevant for recreational fishing, Oncorhynchus mykiss populations showed a significant increase of approximately 7% per year, Salmo trutta populations remained stable, and Salvelinus fontinalis populations experienced a notable decline of approximately 7.4% per year. These varying population trends may be attributed to disparities in stocking intensities, with S. fontinalis receiving minimal stocking compared to the other species. This study revealed that non-native and invasive fish species are a relevant part of fish communities in mountain rivers. Non-salmonid non-natives thrive in warm rivers at lower elevations, whereas salmonid non-natives consolidate in steeper habitats. Because rising temperatures in mountain rivers will accelerate the spread and growth of these species, this first quantification of the current extent will improve fish management strategies in mountainous areas.