Sampling locations, information from past distribution records, and results of the field surveys.

Sites where the rake toss method was used are shown with “○”. For the past distribution and visual observation from this study, “+” indicates that Hydrilla verticillata was confirmed visually, and “–” indicates that it was not. eDNA detection shows the number of positives in eight PCR replicates. An asterisk indicates a site where eDNA detection was checked and confirmed in the next year. The eDNA concentration at each site was estimated by quantitative real-time PCR.