The aim of the project is to identify the main environmental and biological factors supporting acquisition of local parasites by non-native fish hosts and assess the impact of fish introductions on native fish fauna through mechanisms related to new host-parasite interactions. To achieve this goal, we will combine field and experimental approach using one of the most successful fish invaders in Europe (the pumpkinseed Lepomis gibbosus, Centrarchidae). Susceptibility to local generalist parasites will be tested under experimental conditions. Environmental factors affecting parasite fauna will be evaluated using fragmented pumpkinseed populations resulting from recent expansion, a unique model system with low genetic and life-history trait variance. Understanding the importance of these factors appears essential for predicting and managing disease emergence and potential for spillback to wildlife or aquaculture.