Abstract
Numerous sediment-control dams are installed in most headwater catchments in Japan. These dams cause habitat fragmentation of stream-dwelling salmonids, raising the risk of local extinction of their isolated populations. We examined such an effect of habitat fragmentation due to dams on red-spotted masu salmon (Oncorhynchus masou ishikawae) in Shikoku, southwestern Japan. We hypothesized that their local extinctions would occur from uppermost reaches (uppermost margins of their distribution range), where populations are completely isolated, and then extend downstream. Our purpose was to clarify habitat size (catchment area upstream of dams) at which local extinctions becoming apparent in the present state. Classification tree analysis using presence/absence data at 69 study sites suggested that local extinctions due to dams were apparent in stream reaches at catchment area 0.49-0.89 km2. In addition, regression tree analysis showed that masu salmon density increased upstream, suggesting that upper reaches have higher habitat quality. Estimations of stream length and population size based on catchment area indicated that stream reaches facing extinction threat are within 500-600 m from the headwater and have only 40-50 individuals of adult population size. Our results emphasized the priority of uppermost reaches in conservation and restoration of connectivity for populations of stream-resident salmonids. We proposed four management classes (critically endangered area: catchment area < 2 km2 ; least-required area: 4 km2 ; required area: 10 km2 ; target range: 40 km2) based on previous knowledge and the present results to facilitate conservation and restoration of the headwater connectivity.
