Temporal change in the distribution and composition of native, introduced, and hybrid charrs in northern Japan

Abstract

Introductions of non-native species have caused various negative impacts on native species and their ecosystems. Hybridization is particularly prevalent among closely related species, and can result in displacement, hybrid swarms, or the disruption of a locally adapted gene complex. Although hybridization between native and non-native species is widespread, long-term monitoring is generally lacking. In this study, we compared the distribution and composition of native white-spotted charr (Salvelinus leucomaenis), introduced brook trout (Salvelinus fontinalis), and their hybrids in the upper Sorachi River, Hokkaido, Japan in 2003 and 2013, especially focusing on (1) if genetic introgression or hybrid swarm has occurred and (2) if white-spotted charr have declined, since a previous study indicated a potentially harmful asymmetric hybridization with the mothers of hybrids being all white-spotted charr. We found no evidence of decline in native white-spotted charr; rather, the distribution and abundance of introduced brook trout had decreased. Of 142 charr (i.e., genus Salvelinus) collected, 18 individuals (13%) were hybrids but no unidirectional hybridization was observed. However, most of the hybrids were post-F1 individuals with biased mating with white-spotted charr. The effects of long-term introgression on native white-spotted charr should be further examined.