Intergeneric Hybridization between Sympatric Kirikuchi Char and Red‐Spotted Masu Salmon in a Small Japanese Mountain Stream

Hybridization occurs widely across salmonid species. Intergeneric hybridization of salmonid species in nature, however, has been rarely reported. Using microsatellite and mitochondrial DNA (mtDNA) markers, we obtained evidence of intergeneric hybridization between threatened, naturally sympatric Kirikuchi char Salvelinus leucomaenis japonicus and red‐spotted masu salmon Oncorhynchus masou ishikawae in an upper drainage of the Totsu River system, which is located on Kii Peninsula, central Honshu, Japan. Fifteen suspected hybrid juveniles were captured in August 2005 in a tributary within a surveyed drainage, where an intergeneric pair consisting of a female Kirikuchi char and a male red‐spotted masu salmon was observed during the preceding spawning season. The suspected hybrids all appeared to be F 1 on the basis of microsatellite genotypes at five loci, which is consistent with the previously reported infertility of such hybrids. The maternal parent species was presumed to be the Kirikuchi char, since all hybrids had this species' mtDNA haplotype. The estimated number of hybrids was equivalent to that of age‐0 Kirikuchi char, suggesting that at least half of the annual reproduction of Kirikuchi char in the tributary was disturbed by the present hybridization. Six of nine hybrid recaptures were collected from the same tributary in 2006, indicating that hybrids could survive and stay in their nursery tributary at least until age 1. This study suggests that intergeneric hybridization negatively affects long‐term persistence of both parental species, especially Kirikuchi char, through reproductive disturbances and competition for resources.