Abstract

Flood-driven hydrological connectivity in managed floodplain wetlands can create transient opportunities for fish dispersal, yet direct field observations of such events remain limited. In Lake Izunuma–Uchinuma, Japan, a rapid water-level rise and temporary backflow from a downstream river were recorded during a flood event, followed by the first detection of the bagrid catfish Tachysurus nudiceps. Its occurrence outside its natural range may pose significant conservation concerns, particularly habitat competition with native congeneric species. The temporal coincidence between the flood and species detection is consistent with potential passive transport into the lake via temporary backflow. Subsequent routine surveys, including stationary net sampling and environmental DNA (eDNA) analysis, did not detect the species, suggesting either a transient occurrence or an abundance below the detection threshold of conventional monitoring. These findings indicate that short-lived occurrences or low-abundance inflow events may escape conventional, pre-scheduled monitoring, potentially undermining local conservation efforts. We therefore propose that invasion surveillance in managed wetlands should include hydrological information and event-based sampling, especially after extreme rainfall, to enhance conservation strategies and prevent early-stage biological invasions.

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