Recovery of white-spotted charr Salvelinus leucomaenis following the removal of stocked red-spotted masu salmon Oncorhynchus masou ishikawae in a small headwater tributary of Lake Biwa, central Japan

The possible recovery of a white-spotted charr population in a small tributary of a river to Lake Biwa, following removal of previously stocked red-spotted masu salmon, was investigated by electrofishing. We captured 30 red-spotted masu salmon and two hybrids of these two species in 2014. Prior to the stocking of red-spotted masu salmon, the estimated number of white-spotted charr ≥ 100 mm standard length (SL) in the tributary was more than 200 individuals in 2005, which had reduced to fewer than 30 individuals by 2014 when 30 red-spotted masu salmon plus two charr/masu salmon hybrids were captured. However, no red-spotted masu salmon were captured from 2015 to 2017, indicating the success of a red-spotted masu salmon removal program. The estimated number of white-spotted charr ≥ 100 mm SL ranged from 25 to 91 individuals between 2015 and 2017, implying a great reduction of red-spotted masu. The estimated population size of the former increased further to 171–221 individuals in 2021, comparable to 2005. These results suggest that the white-spotted charr population declined due to stocking of red-spotted masu salmon, and recovered following removal of the latter.