In Japan, large numbers of hatchery-reared masu salmon Oncorhynchus masou masou are released into rivers for stock enhancement. To evaluate the effects on life history characteristics of hybridization between wild and hatchery-reared fish, we conducted a crossbreeding experiment using wild-origin fish (1st generation in hatchery originating from wild, anadromous fish) and hatchery-reared fish (more than 15 generations in hatchery originating from stream-resident fish that have been crossbred

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... eatedly with highgrowth and early-maturity strains). Hybrid fish (wild-origin female×hatchery-reared male) smolted in March, which was an intermediate timing between that of hatchery-reared fish (December) and wild-origin fish (April). Also, 43.1% of hybrid females at age 1+exhibited sexual maturity, which was intermediate between hatchery-reared fish (75.7%) and wild-origin fish (6.8%). These results suggest that life history characteristics of local masu salmon populations may be altered by hybridization between wild and hatcheryreared fish.