Status Assessments for Two Endangered Populations of Vendace ( <scp> <i>Coregonus albula</i> </scp> ) and Evidence That Their Abundance Is Modulated by Percid Fishes

ABSTRACT The study of low‐latitude populations can inform our understanding of how cold‐adapted species might respond to environmental change. The vendace ( Coregonus albula ) is a lacustrine fish that is distributed across northern Europe. In the UK, there are two extant native vendace populations. These populations are at the south‐western edge of the species' global distribution and both are exposed to a range of pressures including eutrophication, rising water temperatures and introduced fishes. These populations were assessed in 2024 using a combination of quantitative hydroacoustics and targeted gill netting. These data were then placed in the context of population assessments spanning the preceding three decades. Finally, the time series of population assessments was analysed to identify factors that were associated with low abundances of vendace. In both lakes, the abundance of post‐juvenile vendace was higher in 2024 than expectations derived from a reference baseline. The balanced age structure of the vendace captured by gill net indicated regular recruitment to the post‐juvenile age groups and there was no evidence that the lakes lacked the deep water refugia that vendace require to avoid thermal stress. High abundances of native perch ( Perca fluviatilis ) and introduced ruffe ( Gymnocephalus cernua ) in the natal years of the dominant post‐juvenile vendace age groups were associated with markedly low vendace population sizes. As ruffe are not native to the studied lakes, this demonstrates one of the risks associated with the translocation of fish. However, despite the threats they face, the UK's two endangered populations of vendace appear to be persisting.