A biodiversity hotspot at risk: Fish communities reveal a spread of non-native species in a karst river

Freshwater ecosystems and fish fauna are experiencing severe anthropogenic pressures and declines worldwide, with non-native species as a major contributing factor. Decades of stocking with non-native species, dam construction, urbanisation and water extraction may have had major negative effects on native (endemic) fish fauna in Dalmatian rivers, a biodiversity hotspot in the Western Balkans. Therefore, the aim of the study was to investigate potential spatial and temporal changes in the fish community of the Krka River in Croatia, while exploring functional traits and taxonomic diversity to assess the potential impact of non-native species on the ecosystem. Data spanning a 20-year period was assessed to understand the composition and distribution of the fish communities. These analyses, in combination with ordination approaches, indicated a clear spatial and temporal change in the fish community of the Krka River, with native species being more abundant in the past (2004–2008) and non-native taxa becoming more dominant recently (2020–2024). This change was obvious in the investigated guilds, specifically in status (increase in non-native taxa), reproduction (dominance of macrophyte-related species) and feeding habits (prevalence of omnivores/invertivores). Here, we demonstrate that 20 years is sufficient for a shift in the structure and distribution of freshwater fish communities in a karst river in the Mediterranean region.