Resource partitioning between non-native white-spotted charr and native red-spotted masu salmon in Shikoku, southwestern Japan: population- and individual-level analyses

White-spotted charr (Salvelinus leucomaenis subspp.) and masu salmon (Oncorhynchus masou subspp.) are common stream salmonids in the Japanese Archipelago. In Shikoku Island, although white-spotted charr is not originally distributed, its introduced populations have been established in several streams and caused population decline of red-spotted masu salmon (Oncorhynchus masou ishikawae) in some streams. In this study, we examined effects of non-native white-spotted charr on prey use by native red-spotted masu salmon and described food resource partitioning between the two species at the population and individual levels using their allopatric and sympatric streams. Comparisons of the diet between allopatric (without charr) and sympatric red-spotted masu salmon (with charr) indicated no significant difference at both population and individual levels, suggesting that white-spotted charr had no effect on prey use by red-spotted masu salmon. Comparisons of the diet between red-spotted masu salmon and white-spotted charr in the sympatric stream indicated significant differences at both population and individual levels. The population level analyses showed a clear pattern of partitioning that the diets of red-spotted masu salmon and white-spotted charr were dominated by terrestrial and aquatic prey, respectively. Furthermore, individual-level analyses showed that among-individual variation in prey use was larger in white-spotted charr than in red-spotted masu salmon. The high individual variation in prey use by white-spotted charr may facilitate the coexistence of the two species at the local habitat scale.