Can Invasive Fish Populations Be Controlled? Insights from Prošćansko Lake and Its Tributaries (Plitvice Lakes, Croatia)

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.