Too many dams: habitat fragmentation threatens the future of the Japanese giant salamander

Habitat fragmentation is a major driver of global biodiversity loss, and amphibians are particularly sensitive to this threat. In riverine ecosystems, dams cause longitudinal disconnection, especially affecting fully aquatic species. Across North America, China, and Japan, giant salamanders inhabit rivers where they exert key ecological roles. As strictly aquatic species, dams may severely reduce habitat connectivity and compromise their long-term population viability. In this context, we assessed the fragmentation risk faced by Japanese giant salamander (Andrias japonicus) populations in Japan by compiling a detailed inventory of dams in eight watersheds where the species occurs. We adapted a Dam Fragmentation Index (DFI), taking into account density and height of dams, and the presence of fishways, to quantify fragmentation levels. A total of 2,434 river barriers were identified, with only five percent equipped with fishways potentially usable by giant salamanders. DFI scores indicated moderate to very high fragmentation levels. Moreover, salamander locations were on average at 587 m from barriers. These findings suggest that river fragmentation represents a major threat to the long-term viability of A. japonicus populations by disrupting movements needed for breeding and gene flow. Conservation actions are urgently needed, including the implementation of suitable passage structures such as salamander-adapted ladderways, and the integration of this species’ ecological needs into future river management plans. Our study provides a new framework to quantify barrier impact on amphibians and highlights the importance of addressing river connectivity for the conservation of freshwater biodiversity.