Trait-Based Stage-Structured Risk Profiling of Non-Native Freshwater Fishes Reveals the Underestimated Threat of Within-Country Translocations ^†

Introduction: Freshwater ecosystems are global biodiversity hotspots, yet they remain highly vulnerable to biological invasions. Non-native freshwater fish species (NNFS) have established self-sustaining populations across nearly all biogeographic realms, reshaping regional ichthyofaunas and driving community-level impacts through predation, competition, hybridisation and ecosystem disruption. Critically, both foreign introductions and within-country translocations (extralimital species) contribute to this process, yet the latter remain more weakly regulated and consistently under-studied in invasion risk frameworks. Objective: We developed a stage-structured profiling framework to jointly evaluate foreign and extralimital NNFS in Greece and predict three sequential invasion outcomes, establishment, spread and integration, with the goal of identifying the ecological traits and pathway variables that best explain invasion success at each stage and informing management policy. Methodology: We compiled a dataset of 63 NNFS recorded in Greek freshwaters (36 foreign, 27 extralimital), characterised by eleven ecological, biogeographic and anthropogenic attributes. Logistic and multiple regression models and classification and regression trees (CART) were fitted independently for each invasion stage, with cross-validated predictor screening to limit multicollinearity and a taxonomy-based covariate to account for phylogenetic non-independence. Results: All 27 extralimital translocations established successfully, compared with only 11 of 36 foreign introductions, underscoring the disproportionate establishment success of within-country movements. Establishment probability was positively associated with high physiological tolerance and proximity to the nearest native source, and negatively associated with maximum adult size; propagule pressure provided only weak additional support. Spread across drainage basins was driven primarily by introduction effort and physiological tolerance. Integration increased with introduction effort, while the CART identified distance from the nearest native source as the primary partition of widespread, high-abundance outcomes, with trophic level further structuring outcomes among extralimital taxa. Conclusions: Our results indicate that management frameworks focused solely on foreign NNFS substantially underestimate invasion risk from within-country translocations. A compact set of predictors, biogeographic proximity, physiological tolerance and introduction effort, offers a practical, pathway-inclusive screening tool to guide prevention, surveillance and early detection in Mediterranean river networks, addressing a recognised European policy gap where extralimital movements remain more weakly regulated than foreign introductions.