Regional Mountain Conference

Introductions of non-native fish into originally fishless alpine lakes represent one of the most severe threats to native biodiversity in mountain freshwaters. Stocking and fishing bans can prevent further introductions, but eradication is required to restore natural conditions. Here, we describe the dynamics of mechanical fish eradications conducted in 16 high-mountain carried out within two LIFE projects: BIOAQUAE (2013–2017) and RESQUE ALPYR (2022–2026).

The study lakes are located in Gran Paradiso National Park and Mont Avic Natural Park (Western Italian Alps) and differ in size, depth, habitat complexity, and fish assemblages. Eradication was achieved through a combination of mechanical removal methods (i.e., gillnets, fyke-nets, electrofishing). The choice and intensity of methods were tailored to lake characteristics and target-species.

Any eradication action requires at least two years, with effort intensity varying according to lake and target-species characteristics. Salmonids, typically occurring at population sizes ranging from tens to a few thousand individuals per lake, were efficiently eradicated using gillnets alone or in combination with electrofishing, often within relatively short timeframes and with a contained annual effort. In contrast, cyprinids were two orders of magnitude more abundant, and required sustained multi-year efforts, invariably involving the combined use of gillnets, fyke-nets, and intensive electrofishing.

Eradication was considered successful by the absence of fish captures following one year from the capture of the last fish. All lakes treated within the LIFE BIOAQUAE are currently fish-free, as are two lakes from the LIFE RESQUE ALPYR. In all remaining lakes, Capture Per Unit Effort has already reached zero, and these systems are expected to be formally declared fishless by the end of the summer 2026 or by early summer 2027.

Our results demonstrate that the eradication of alien fish from alpine lakes is realistic. However, the time required and the intensity of effort depend on environmental complexity and species-specific traits. By documenting successful eradication trajectories across multiple lakes and contexts, this study provides valuable guidance for planning, and scaling restoration actions in alpine lakes and represents an important step toward the formalization of practical, evidence-based guidelines expected to support practitioners in undertaking similar restoration actions.