Data from: Invasive Eurasian minnow alters the trophic niche and growth of brown trout in high-latitude lakes

Invasive species pose a major threat to aquatic ecosystems, particularly
in high-latitude lakes which are characterised by low biodiversity. In
northern Europe, 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.