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.

Methods
We sampled six subarctic lakes in Abisko, northern Sweden, comprising three allopatric trout lakes and three sympatric lakes with trout and minnow. Fish were caught using standardised multi-mesh gillnets set at multiple depths to cover all major habitats.  In the laboratory, trout were measured, sexed, aged (from otoliths), and assessed for maturity. Maturity was treated as a binary variable. Females were classified as mature if the gonads occupied at least three-quarters of the abdominal cavity and the eggs had a minimum diameter of 3 mm. Males were defined as mature if minimum half the length of the gonads appeared visibly swollen. Muscle tissue and stomachs were dissected from all fish for stable isotope (SIA) and stomach content (SCA) analyses.

Baseline samples for SIA were collected from each lake: littoral benthic invertebrates using kick net and benthic sledge, and pelagic zooplankton using vertical hauls with a plankton net. Water samples were collected for chemical analyses of TOC, TP and TN. All isotope samples were freeze-dried, homogenised, and analysed using isotope-ratio mass spectrometry, with δ¹³C and δ¹⁵N values used to infer trophic position (TP) and littoral reliance (LR) via baseline-corrected two source mixing models (Post et al., 2002).

SCA was conducted on 2023 samples using the relative fullness method (see Amundsen & Sánchez-Hernández, 2019). Prey items were identified to the lowest feasible taxonomic level.

References:

Amundsen, P.-A., & Sánchez-Hernández, J. (2019). Feeding studies take guts – critical review and recommendations of methods for stomach contents analysis in fish. Journal of Fish Biology, 95(6), 1364–1373. https://doi.org/10.1111/jfb.14151

Post, D. M. (2002). Using stable isotopes to estimate trophic position: models, methods, and assumptions. Ecology, 83(3), 703–718. https://doi.org/10.1890/0012-9658(2002)083[0703:USITET]2.0.CO;2