Japanese char Salvelinus leucomaenis, red-spotted masu salmon Oncorhynchus masou ishikawae, hybrid individuals were collected from a tributary of the Ibigawa River, Gifu Prefecture, Japan. The char and hybrids were mainly distributed in the headwater area where eyed eggs of red-spotted masu salmon are released in each year. Principal component analysis (PCA) of the morphological features of parental and hybrid individuals indicated that hybrid body shape was similar to that of char. Partial nucleotide sequences of mitochondrial DNA indicated the female parental species to be red-spotted masu salmon, without exception. Genotyping of three loci of short interspersed repetitive elements (SINE) of inserted alleles showed all hybrid individuals to be F1, with no evidence of backcrossing. These results indicated that the introduced red-spotted masu salmon hybridized with male char, with a high survival rate of sterile F1 offspring.
