Habitat-use flexibility may promote the spread of Asian weatherloaches in Europe

Asian weatherloaches have expanded their range across Europe, potentially increasing competitive pressure on the conservation-dependent native European weatherfish, Misgurnus fossilis . Because their interspecific interactions are mediated by benthic habitat use, substrate choice was quantified for native European ( n = 5) and invasive Asian ( n = 11) weatherloaches in a replicated laboratory two-way choice experiment. In this way, the experiment tested whether native M. fossilis acts as a habitat specialist, while invasive Asian weatherloaches exhibit greater plasticity according to the adaptive flexibility hypothesis. Individual fish were tested in three sediment combinations (gravel/sand, gravel/mud, and sand/mud), with four replicates of 60 min per treatment and individual fish. Habitat use was quantified as the proportional time spent on each substrate and analysed using a linear mixed-effects model. Habitat use differed significantly between invasive and native loach species. Post hoc comparisons and effect sizes (Hedges’ g ) indicated a pronounced divergence for muddy substrate, with native M. fossilis showing substantially higher mud use (74.4 ± 23.3%) than Asian weatherloaches (36.0 ± 20.3%), while differences in selection for gravel and sand were small. These findings support the adaptive flexibility hypothesis by indicating a more generalist substrate-use pattern in invasive Asian weatherloaches relative to the mud-specialised European weatherfish. From a management perspective, conserving and restoring structurally complex muddy floodplain habitats may provide a competitive refuge for European weatherfish and should be prioritised alongside targeted monitoring of Asian weatherloach spread and habitat overlap.