Abstract
Artificial barriers, such as dams, diversion weirs, culverts, and the bases of bridges, prevent aquatic animals from migrating to upstream habitats. The barrier effect is serious for freshwater fishes that must swim to migrate, and isolated populations may become locally extirpated. In this study, we assessed the effect of habitat fragmentation by artificial barriers on the habitat distribution of two stream-dwelling salmonids: white-spotted charr, Salvelinus leucomaenis, and red-spotted masu salmon, Oncorhynchus masou ishikawae. We surveyed the distributions of presumed native populations in the Fujigawa Basin, Japan, by snorkeling along the stream and using a global positioning system (GPS) to record the points farthest upstream where fish were observed, as well as the locations of artificial barriers, from June to September 2004. In total, 356 artificial barriers have been constructed in the 29 surveyed streams (12.3 barriers per stream), and none of the barriers have fish ladders. Unfortunately, some isolated populations of both species have been locally extirpated, although both had occurred throughout the headwaters during the 1970s. Moreover, the number of streams in which charr and salmon coexist has decreased from five to one over a 30-year period. Multiple logistic regression analysis was conducted to examine whether seven variables were related to the probability of the occurrence of a fish population above the artificial barrier farthest upstream. The result suggests that the probability of the occurrence of salmon was lower than that for charr, and disappearance increased with decreasing watershed area (i.e., habitat size) above the artificial barrier farthest upstream, with increasing numbers of artificial barriers in the stream. According to this model, the estimated watershed area necessary to maintain a population is 1.01km^2 for charr and 2.19km^2 for salmon, suggesting that the probability of extirpation is very high if artificial barriers are constructed upstream in these watershed areas.
