Seven specimens of natural hybrids of salmonid fishes were collected during 1971 to 1976 in the Nippara River, an upper tribute in the Tama River, Tokyo.
In morphometric, meristic, and numerical characters as well as coloration, the natural hybrids showed intermediate character between the two native salmonids, Oncorhynchus masou and Salvelinus pluvius, which are only representative of salmonids in the drainage.Four specimens of artificially crossed hybrids between the same combination (O.masou×S.pluvius) were studied.
Published records of natural salmonid hybrids as the cross between O.masou and S.leucomaenis, from Hokkaido, Sado Island, and unpublished record from Aomori Prefecture seem to support results of the cross of the same combination of species reported here from the Nippara River.