Habitat fragmentation by damming threatens coexistence of stream-dwelling charr and salmon in the Fuji River,Japan |
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Authors: | Jun-ichi Tsuboi Shinsuke Endou Kentaro Morita |
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Institution: | (1) Yamanashi Fisheries Technology Center, 497 Ushiku, Kai 400-0121, Japan;(2) Daiichi Broiler Co. Ltd., Oroshi Center, Hachinohe 039-1121, Japan;(3) Hokkaido National Fisheries Research Institute, Fisheries Research Agency, 116 Katsurakoi, Kushiro 085-0802, Japan |
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Abstract: | 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. |
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