Evaluation of the influence of an alien salmonid, Salmo trutta, on the native freshwater fish community

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The main objectives of the present study were to elucidate the life-history pattern, migration, feeding ecology of an alien salmonid, Salmo trutta, and the interspecific oompetition with native or alien salmonld species in the sympatry, to evaluate tbe influence on the native fish community. The field survey was carried out in both the Hekiriji River of southern Hokkaido and the Lake Shikotsu of central Hokkaido, and following results were obtained:1. In the Hekiriji River, Salmo trutta was found to be abundantly stocked in the dam reservoir which was located about 7 km upstream from the mouth, and that both age 2-year females (>275 mm FL) and males (>172 mm FL) reached to mature, suggesting their participation into the spawnings.2. A few individuals of S.trutta metamorphosed into smolts when they grew up to 210-270 mm FL in the autumn, suggesting that they should become sea trout.3. Salmo turtta individuals of 200-300 mm FL were found to feed maily on trichopteran larvae, terrestrial animals and the benthic fishes such as Noemacheilus toni.4. In the Lake Shikotsu, where two alien salmonid species, S.trutta and Oncorhynchus mykiss, coexist, the former fed on benthic animals whereas the latter fed on swimming-type aminals in an inlet stream, the Bifue River, suggesting their food resource partitioning. In the lake, S. trutta predated the fishes such as threespine stickleback, while-spotted charr and kokanee salom.5. The niche width of prey animals in S. turtta was found to be lower than that in O. mykiss.6. This stydy strongly suggests that S. trutta provides a large impact on the native fish community mainly due to its piscivorous feeding.