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 interspecific interactions are potentially mediated by benthic habitat use, we quantified substrate choice of native European and invasive Asian weatherloaches in a replicated laboratory two-way choice experiment. Individual fish were tested in three sediment combinations (gravel/sand, gravel/mud, sand/mud) with four 60-min replicates 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 Misgurnus fossilis showing substantially higher mud use than Asian weatherloaches, 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.