Regional Mountain Conference

High-mountain lakes represent hotspot of prey subsidies that support several native insectivores. These include terrestrial taxa such as ground-dwelling invertebrates (e.g., spiders and predatory ground beetles), adult stages of amphibians, birds, and bats, which feed along shorelines or on the wing, primarily on emerging insects. More specialized semi-aquatic predators, such as shrews and dippers, directly exploit aquatic resources by diving and feeding on aquatic macroinvertebrates.

High-mountain lakes are naturally fishless, but introductions of predatory fish is a widespread threat to mountain biota such as aquatic invertebrates. In particular, fish introductions alter macroinvertebrate community composition and reduce the abundance of conspicuous benthic and nektonic taxa (e.g., Coleoptera, Trichoptera, Plecoptera), which represent the main prey for native semi-aquatic insectivores: the European water shrew Neomys fodiens, and the White-throated dipper Cinclus cinclus. We hypothesize that they may compete with introduced fish for aquatic macroinvertebrate prey, potentially leading to their competitive exclusion.

This study develops within the context of the LIFE RESQUE ALPYR project (LIFE20 NAT/ES/000369, 2022-2026), which aims to restore >20 lakes from four protected areas from the Alps and Pyrenees through mechanical fish eradication. More than 50 lakes contrasting in fish status (naturally fishless, containing non-native fish, or undergoing restoration) were monitored for prey availability and for water shrew and dipper occurrence and abundance. Monitoring was complemented with bycatch data from lakes undergoing eradication, which, although unfortunate, provided valuable insights into the recovery of native insectivore communities.

Results highlight that introduced fish can affect not only fully aquatic organisms but also semi-aquatic insectivores, which are traditionally excluded from impact assessments. This emphasizes the pervasive Impact of fish in mountain lakes and underscores the importance of conserving these ecosystems fishless.