Non-native fish species expand tacitly but rapidly toward upstream oxbow lakes along the longitudinal gradient

The introduction of non-native fish species poses a threat to native assemblages in floodplain systems. Establishing oxbow lake-based conservation areas has been proposed as a practical approach to mitigate this threat. Previous studies have extensively discussed the effects of lateral connectivity between oxbow lakes and main channels on fish communities but overlooked the spatial longitudinal gradients of oxbow lakes on a broader spatial scale. Over the course of about ten years, the temporal changes in fish community diversity were estimated in 28 oxbow lakes along the Ishikari River in Hokkaido, Japan. The associations between these temporal variations and the locations of the lakes were then explored along a longitudinal spatial gradient. The results showed that upstream oxbow lakes underwent mild alterations to their species composition and maintained a high level of native uniqueness. In contrast, downstream oxbow lakes experienced intense species turnover and increased non-native fish diversity. Additionally, the dominance of non-native fish in downstream areas rapidly expanded upstream to a location 110 km from the source of the Ishikari River over the decade. It is emphasized that the lakes in the upper reaches were the only remaining refuges for native fish and should be prioritized in conservation efforts. Our study proposes the application of location and species contributions to regional community heterogeneity as indicators for highlighting native fish conservation areas.