Introduced Fish Threaten Amphibians

Alpine lakes are naturally fish-poor or fish-free ecosystems that have received numerous non-native fish introductions globally in recent decades. Studies have demonstrated that alien fish have significant negative impacts on aquatic organisms, particularly amphibians, which are highly susceptible to habitat alterations. However, studies from Asia remain scarce, particularly in plateau lake ecosystems. In southwest China, fish have been extensively introduced through alien and intra-regional translocation for purposes including religious release, tourism, and aquaculture. In this study, we used environmental DNA techniques to survey fish and amphibian species composition and abundance at 63 sampling locations across southwest China between 2022 and 2023. We examined the effects of fish introductions on amphibian richness and occurrence using generalized linear models, and further evaluated the impact magnitude of fish from different sources. We then quantified the species co-occurrence using probabilistic species co-occurrence analysis. Next, we evaluated the impact of fish richness from different sources on community composition using non-metric multidimensional scaling, and assessed community similarity and assembly mechanism through the Raup-Crick index and modified stochasticity ratio. Ultimately, we explored the predation behavior of selected pairs of translocated fish and native amphibian to verify the cause of negative co-occurrence. Our results showed that non-native fish predominantly inhabit lentic habitats, with translocated species comprising a substantial proportion. Amphibian richness and occurrence were negatively correlated with fish richness. Introduced fish increased amphibian community similarity and shifted community assembly toward stochastic dominance. Translocated <i>Carassius auratus</i> exhibited strong predation on <i>Rhacophorus dugritei</i> tadpoles, significantly reducing its occurrence. These impacts were largely driven by intra-regional fish translocation. Our findings highlight the critical need to consider intra-regional fish movements in conservation strategies. This study provides key insights for managing introduced fish populations and protecting threatened and endemic amphibians in alpine lake ecosystems of southwest China and similar highland regions worldwide.