Occurrence of the white-spotted charr Salvelinus leucomaenis in Lake Biwa, central Japan

Near the southern limit of its Japanese distribution range, the white-spotted charr Salvelinus leucomaenis inhabits headwater tributaries of inlet rivers to Lake Biwa, Shiga Prefecture, although failing to migrate to the lake from those tributaries. Even though lower depths (>30 m) of the north basin of the lake are cold throughout the year, the only salmonid inhabiting the lake is the Biwa salmon Oncorhynchus sp. However, between 2018 and 2021, five white-spotted charr were caught in Lake Biwa, their phenotype being characterized by large whitish spots, reminiscent of the anadromous form off northern Japan. Mitochondrial DNA Cyt b haplotype analysis data supported their origin as hatchery-reared specimens from Samegai Trout Farm (Shiga Prefecture), the latter having sourced white-spotted charr from several Japanese populations, including northern Japan (descending anadromous exists). Such white-spotted charr, upon release from the hatchery, may have migrated to the lake during winter, due to a retained migration instinct.