Introduced fish reduce the occurrence of shrews in alpine lakes

Widespread fish introductions into originally fishless mountain lakes have had severe consequences for native biota, including aquatic macroinvertebrates, which provide important food subsidies for terrestrial and semiaquatic insectivores like shrews (Fam. Soricidae). Since both fish and shrews rely on aquatic macroinvertebrates as food, whether in their larval or imaginal stage, we investigated if fish presence had adverse effects on shrews. Baited tubes were deployed to monitor the presence/absence of shrews by collecting their scats in lakes with and without fish in the western Italian Alps. Only two species, the Valais shrew (Sorex antinorii) and the Eurasian water shrew (Neomys fodiens), were found inhabiting the lakes' edges, where they fed on aquatic insect subsidies. The results indicate a significant pattern of exclusion between shrews and introduced fish. This negative association was especially evident in the presence of large-bodied fish (i.e., salmonids), but also of small fish (i.e., cyprinids). Consistently, compared to naturally fishless lakes, those with fish exhibit a lower availability of aquatic prey, representing a significant portion of the diet of both shrew species. Overall, our findings suggest that the impact on shrews may be mediated by a complex interplay of competition and predation between fish and shrews. Fish impacts may extend beyond the lakes to insectivorous mammals in surrounding areas. We recommend that the potential benefits to species and habitats reliant on aquatic subsidies be considered and integrated into conservation and restoration plans, and that these findings be communicated to the public to foster greater support for restoration efforts.