Habitat fragmentation by damming threatens coexistence of stream-dwelling charr and salmon in the Fuji River, Japan

Habitat fragmentation by damming can affect the persistence of single species population and also coexistence of two or more species through intensified competition. This study examined the effects of habitat fragmentation by damming on the coexistence of two stream-dwelling salmonids: the southern form of white-spotted charr (Salvelinus leucomaenis japonicus) and the red-spotted masu salmon (Oncorhynchus masou ishikawae). We examined charr, salmon, and dam distributions in 27 streams of the Fuji River basin, central Japan. In the 1970s, there were streams with five sympatric and 22 allopatric populations (n = 13 for charr, n = 9 for salmon). However, from the 1970s to 2004, 356 impassable dams were constructed in the surveyed streams, and four of the five sympatric streams became allopatric. In the extant sympatric stream, more than 20 dams fragmented habitat. Species distributions were separated by dams (with decreasing altitude) in the following order: extirpation area, charr-dominant area, and salmon-dominant area. Within the uppermost sympatric section (i.e., situated between the dams), salmon congregated in the largest uppermost pool just below the dam; despite these conditions, salmon frequency increased in the downstream direction at the stream scale. The results suggest that habitat fragmentation threatens the coexistence of stream-dwelling charr and salmon at both the basin and stream scales. We believe that exclusion of one species by another is likely in extremely fragmented habitats with minimal gradients and little variation in physical conditions (through reduced stream gradient and increased sand sedimentation caused by damming). In addition, multiple sites of damming ensure that there are no salmonid refuges from the collapse of metapopulation structure. In such fragmented habitats, even small tributaries serve important roles, as they are used mainly by salmonid fry and juveniles. We propose that habitats of native salmonids should be maximized by reconnecting fragmented habitats as part of a broader management plan.