Nikola Renić

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.

Invasive fish species pose a major threat to freshwater ecosystems by outcompeting native species and altering habitat conditions, leading to biodiversity loss. The Plitvice Lakes National Park, a UNESCO World Heritage site in Croatia, harbors a unique but species-poor native fish community, including Danube trout (Salmo labrax), Italian minnow (Phoxinus lumaireul), Italian golden loach (Sabanejewia larvata), and Italian spined loach (Cobitis bilineata). However, human-introduced species such as rainbow trout (Oncorhynchus mykiss), rudd (Scardinius erythrophthalmus), chub (Squalius cephalus), and northern pike (Esox lucius) have established thriving populations, with rudd emerging as the dominant invasive species. To mitigate their ecological impact, targeted removal efforts were implemented using casting nets and electrofishing, aiming to reduce invasive populations while minimizing harm to native species. Monitoring activities, performed during the course of the removal project, revealed significant declines in invasive fish abundance, demonstrating the effectiveness of these management interventions. Rudd abundance in Prošćansko Lake decreased by 56.9% and biomass by 69.7% over three years, while rainbow trout abundance in the B-C-M system declined by 92.7%, and its distribution shrank to a single locality, indicating strong population reduction. These findings highlight the importance of implementing active conservation measures to control invasive fish populations, thereby improving the status of native fish communities and preserving ecological balance.

The Bednja River is the longest river flowing with its full course exclusively through Croatia and one of the largest right tributaries of the Drava River, which belongs to the Danube River basin. Due to the variety of habitats present within the course of the Bednja River, as well as older literature reports, it can be expected that this river harbors a high number of freshwater fish species and would likely benefit from conservation and preservation efforts. We compiled and analyzed the existing literature data on the Bednja River fish communities and performed field investigations at various localities on this river in order to describe the current structure of its freshwater fish fauna and monitor changes induced by human activities. Our results corroborate the presence of a rich and diverse fish community. Moreover, with 36 native species, the Bednja River harbors one of the richest fish communities in Croatia as well as in the Danube basin. Unfortunately, modifications of the native fish community were evident in the form of local extinctions, reductions in the population abundance of several native species and the presence of non-native species. Habitat degradation and fragmentation were identified as the most serious threats provoking negative effects on the native fish populations, followed by predatory and competitive effects of invasive species.