The effects of severe flooding on native masu salmon and nonnative rainbow trout in the Atsuta River, Hokkaido, Japan

摘要

Debris flow has potentially pronounced effects on streams and fish populations. In August 2010, a severe flood, which was coupled with wide-ranging landslides and massive debris flows, occurred in the Atsuta River. In this study, we evaluated the effects of floods on salmonid populations by examining the abundance and density of masu salmon Oncorhynchus masou and rainbow trout O. mykiss in the Atsuta River and comparing these values in the post-flood period to those in the pre-flood period. The estimated abundance and density of masu salmon did not change between the pre- and post-flood periods. In contrast, the density of adult (age-1and older) rainbow trout decreased dramatically. Although the density of age-0 rainbow trout decreased after the floods, there was no significant difference between the pre- and post-flood values. These results suggest that the effects of severe floods differ among species and these differences may be attributed to differences between masu salmon and rainbow trout in the timing of life-history events, such as fry emergence and spawning.