Small-Scale Recovery of a Native White-Spotted Charr Population by Suppression of Non-Native Brown Trout

Abstract

If eradication is not feasible, suppressing a population size of non-native species by removal is an alternative option for the conservation of native species that are impacted by non-native species invasions. This study reports on the suppression of non-native brown trout for the conservation of native white-spotted charr in an upstream of Monbetsu Stream, Hokkaido, northern Japan. Brown trout removals were conducted from 2014 to 2023 by electrofishing. The population size of brown trout was dramatically reduced during the first three years of removals, and the ratio of white-spotted charr to brown trout abundance gradually increased. However, the population size of white-spotted charr did not fully recover, which can be explained by the openness of the system and likely also by the influence of interfering factors observed during the study, such as changes in stream morphology due to flooding and suspected illegal fish stocking. The abundance of young-of-the-year brown trout also tended to be reduced when numerous mature females had been removed in previous years. This study suggests that suppressing non-native fish species, even on a limited scale, can contribute to the conservation of native species. However, when preventing migration from downstream into the target stream reach is unfeasible, ongoing suppression of non-native species becomes essential to ensure the conservation of native species in the area.