+1 for following Eschmeyer's and now synonymizing under O. masou based on recent mtDNA analyses.

Yamamoto, S, Morita, K, Kikko, T, Kawamura, K, Sato, S, Gwo, J-C. Phylogeography of a salmonid fish, masu salmon Oncorhynchus masou subspecies-complex, with disjunct distributions across the temperate northern Pacific. Freshwater Biology. 2020; 65: 698– 715. https://doi.org/10.1111/fwb.13460

No marked regional or subspecies-specific mtDNA haplotype associations were recognised, except for O. masou subsp., a lacustrine form endemic to Lake Biwa, an ancient lake in central Honshu. The landlocked subspecies O. masou formosanus, with a disjunct distribution on Taiwan Island, exhibited no diagnostic population features differing from the other subspecies, in either mtDNA or msDNA markers. Mismatch distribution and Bayesian skyline plot analyses indicated relatively recent range expansion and rapid population growth for masu salmon during the last glacial period (c. 0.1–0.15 Ma).

Most characteristically, O. m. formosanus on Taiwan Island showed no prominent population feature to distinguish it from the other three subspecies, in both mtDNA and nuclear DNA markers. This result is surprising because O. m. formosanus population is separated from other masu salmon populations by a relatively long distance. The lack of clear genetic differentiation of O. m. formosanus in both types of DNA markers could be caused by recent large-scale dispersal and colonisation events.

Gong, L., Liu, LQ., Guo, BY. et al. The complete mitochondrial genome of Oncorhynchus masou formosanus (Salmoniformes: Salmonidae) and phylogenetic studies of Salmoninae. Conservation Genet Resour 9, 281–284 (2017). https://doi.org/10.1007/s12686-016-0673-1