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The signal crayfish, Pacifastacus leniusculus , is non-native to Japan, and its invasion may have contributed to the habitat loss of the endemic Japanese crayfish, Cambaroides japonicus. We assessed the distribution of the signal crayfish and the Japanese crayfish in small streams that flow into Lake Akan (Hokkaido, Japan), which the signal crayfish inhabits. We conducted this survey with 2 methods: analysis of environmental DNA (eDNA) and hand capture. Of the 17 streams we surveyed, we detected signal crayfish DNA in 16 streams and Japanese crayfish DNA in 9 streams. In contrast, with our hand-capture survey of the same 17 streams, we found signal crayfish in 9 streams and Japanese crayfish in 4 streams. These results suggest that the signal crayfish may have invaded almost all the small streams with culverts around Lake Akan. The presence of signal crayfish was also confirmed on the upstream side of a 69-cm-high road-crossing culvert. There were only 2 culverts above which the absence of signal crayfish was supported by eDNA analysis, suggesting that culverts have limited effectiveness against signal crayfish invasion in our study site. Active species-specific removal projects may be an effective management option to minimize the negative impacts of signal crayfish where they are already present. eDNA has great potential as a cost-effective tool to monitor crayfish range expansion. [ABSTRACT FROM AUTHOR]