Trophic niche segregation among native whitefish and invasive vendace in a north Norwegian lake system

Abstract Introductions and invasions of non‐native species alter nutrient cycling and trophic dynamics resulting in significant ecological disturbance. Stable isotope data were used to test for evidence of invader‐induced trophic niche differences in a north Norwegian lake system differentially dominated by native European whitefish ( Coregonus lavaretus ) morphotypes and invasive vendace ( Coregonus albula ). Aspects of both realised trophic niche position and trophic niche width were affected by the invader, with the effects varying by whitefish morphotype. Densely rakered pelagic whitefish demonstrated a relatively lower reliance on pelagic resources in the presence of the invader and the isotopic niche size was relatively larger in conjunction with the broadening of the prey base. Within the benthic‐dwelling sparsely rakered whitefish morphotype, the trophic impacts of invading vendace were size‐dependent, with larger individuals experiencing niche compression. Smaller sparsely rakered whitefish increased, contrary to our hypothesis, reliance on pelagic‐based energy in the face of invasion. Our findings demonstrate that the trophic ecology of invaded systems can differ in multiple and subtle ways that have consequences for community‐ and ecosystem‐level energy flows, which if persisting over time are likely to have implications for the recruitment, growth and reproductive rate of the native fishes.