Invasive alien fishes have detrimental ecological effects on aquatic ecosystems and the services they provide. Impacts from an invasion in a single ecosystem may differ across space and time due to variability in prey availability and environmental conditions. We hypothesize that such variability can be profound, even within a single ecosystem. Stable isotopes analysis (SIA) is commonly used to quantitatively describe the trophic niche of a species. However, spatial and temporal variability in occupied niches are often not incorporated into management strategies and policy options. Here, we used long-term monitoring data to investigate the invasion stage as well as SIA to analyse the trophic niche of the invasive channel catfish Ictalurus punctatus in Lake Kasumigaura (Japan), a long-term ecological research site (LTER), across distant sampling sites and years. We found a significant spatio-temporal variability in relative growth and isotopic niche occupation. Moreover, we defined a new index, the Isotopic Plasticity Index (IPI), which is the ratio between core and total home range of an occupied isotopic niche, to be used as a proxy for the trophic niche stretch or density. We found that this IPI varied considerably, confirming the spatio-temporal variability in trophic niches, suggesting the IPI to be an adequate new isotopic metric. Our results further provide evidence for the existence of variation across invaded landscapes, implying heterogeneous impacts on recipient native communities. Therefore, our work emphasizes the importance of exploring trophic plasticity in feeding ecology and growth as such information enables a better understanding of impacts and can inform the design and implementation of effective management responses.

Center for Environmental Biology and Ecosystem Studies National Institute for Environmental Studies Tsukuba 305 8506 Japan

Department of Biology University of Florence Via Madonna del Piano 6 50019 Sesto Fiorentino Italy

Muğla Sıtkı Koçman University Faculty of Fisheries Muğla Turkey; University of Łódź Department of Ecology and Vertebrate Zoology Faculty of Biology and Environmental Protection Łódź Poland

Senckenberg Research Institute and Natural History Museum Frankfurt Department of River Ecology and Conservation Clamecystrasse 12 63571 Gelnhausen Germany; University of Duisburg Essen Faculty of Biology Universitätsstraße 5 45141 Essen Germany

Senckenberg Research Institute and Natural History Museum Frankfurt Department of River Ecology and Conservation Clamecystrasse 12 63571 Gelnhausen Germany; University of South Bohemia in České Budějovice Faculty of Fisheries and Protection of Waters South Bohemian Research Center of Aquaculture and Biodiversity of Hydrocenoses Zátiší 728 2 389 25 Vodňany Czech Republic

Woods Hole Oceanographic Institution Marine Policy Center Woods Hole MA 02543 USA; Hellenic Center for Marine Research Institute of Marine Biological Resources and Inland Waters Athens 164 52 Greece; University of Southern Denmark Department of Sociology Environmental and Business Economics 6705 Esbjerg Ø Denmark

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