Invasive Eurasian Minnow Alters the Trophic Niche and Growth of Brown Trout in High‐Latitude Lakes

ABSTRACT Invasive species pose a major threat to aquatic ecosystems, particularly in high‐latitude lakes which are characterised by low biodiversity. In northern Europe, the Eurasian minnow ( Phoxinus phoxinus ) has colonised lakes historically dominated by salmonids, raising concerns about the impacts of invasive cyprinids on native fish populations and food webs. We compared the trophic niche, growth, and maturation of brown trout ( Salmo trutta ) in lakes with and without minnow and assessed dietary overlap between the two species using stomach content and stable isotope analyses. Stable isotope analysis revealed that in lakes with minnow, trout exhibited more pronounced ontogenetic niche shifts from pelagic to littoral feeding and towards higher trophic positions compared to lakes with only trout. The isotope data also showed that small trout overlapped in trophic niche with minnows. Stomach content analysis revealed a shift in trout prey use, with reduced consumption of Eurycercus lamellatus and Gammarus lacustris , increased use of surface insects and a transition towards partial piscivory (prevalence of piscivory 5.5%). Despite potential resource competition at early life stages and shifts in diet, when coexisting with minnow, trout grew faster and females showed a tendency to mature earlier. Overall, the presence of invasive minnow does not appear to negatively affect native trout. This is likely due to a combination of flexible resource use and the opportunistic piscivory exhibited by trout. Since our study systems were recently invaded, the findings provide new insights into how native salmonids respond to invasive species shortly after their establishment in small high‐latitude lakes.