The density dependent interspecific competition between nonnative salmonids, rainbow trout and brown trout
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Interspecific interactions play a key role in determining species assemblages when nonnative species are introduced. In the Chitose River system, Hokkaido, northern Japan, nonnative brown trout Salmo trutta may have replaced nonnative rainbow trout Oncorhynchus mykiss due to interspecific competition. The outcome of this competition is predicted to be influenced by the density of both species. Then, I conducted an enclosure experiment to test whether density-dependent interspecific competition between YOY (young-of-the-year) of these species may have explained the species replacement. Rainbow trout decreased their stomach contents in the high density treatment sympatry with brown trout, and growth rate of rainbow trout decreased in the high density treatments both allopatry and sympatry with brown trout. Stomach contents and growth of brown trout were not affected by competitor species (rainbow trout) or total fish density. These results imply that brown trout are competitively superior to rainbow trout. Therefore, there is the potential for the replacement of rainbow trout by brown trout through interspecific competition.
KeywordsDensity dependence Foraging Interference competition SGR Stream
I thank Hirotaka Katahira, Takeaki Oiso, Yudai Hiraka, Nobuo Ishiyama, Kohei Shimomura, and Kengo Suzuki for helping with the enclosure experiment and Toshiaki Yamamoto for his valuable comments on an early draft of this manuscript. This study respected the bylaws of Fisheries Research Agency and Hokkaido National Fisheries Research Institute for animal welfare. This study was financially supported by the Japan Society for the Promotion of Science KAKENHI (Grant Number 24780196).
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