Habitat niche separation of the nonnative rainbow trout and native masu salmon in the Atsuta River, Hokkaido, Japan

Abstract

The mechanisms by which nonnative species establish populations can be classified into two broad categories: they usurp the niches of native species through interspecific competition, or they avoid this intense interspecific competition by making use of minimal niche overlap with the native species. In this study, we considered how a nonnative salmonid species, the rainbow trout Oncorhynchus mykiss, established a population in the presence of the native salmonid species, the masu salmon O. masou, in Hokkaido, Japan. Circumstantial field evidence shows that the masu salmon exceeds the rainbow trout in abundance and suggests that these species use different types of cover habitat (rainbow trout abundance increases with increasing abundance of large woody debris aggregates, whereas masu salmon abundance increases with increasing abundance of undercut banks). These results imply that the rainbow trout established a population due to minimal niche overlap with the masu salmon, and not by competitive exclusion of the native species.

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

“…In order to examine the effects of floods on masu salmon and rainbow trout, 10 reaches were established along the main stem of the Detomata Stream (reaches 1À10, 50 m/reach; Figure 1). The locations of the reaches and the methods that were used to measure the environmental factors followed those described by Hasegawa et al (2010). Within each reach, the reach area, pool/riffle ratio, size of the undercut bank, and the size of LWD were measured according to the methods in Hasegawa et al (2010).…”

“…The locations of the reaches and the methods that were used to measure the environmental factors followed those described by Hasegawa et al (2010). Within each reach, the reach area, pool/riffle ratio, size of the undercut bank, and the size of LWD were measured according to the methods in Hasegawa et al (2010). If LWD and undercut bank were not available within a reach, we reestablished the reach nearby the existing reach in order to include at least one LWD and undercut bank within each reach.…”

“…If LWD and undercut bank were not available within a reach, we reestablished the reach nearby the existing reach in order to include at least one LWD and undercut bank within each reach. Although there is a possibility that this process would have an influence on the result of the field surveys, especially on the amount of an undercut bank and LDW, this was done because our study focused on the abundance of salmonid fish, which is affected by the amount of LWD and undercut bank (Hasegawa et al 2010). The data in Hasegawa et al (2010) were used as the pre-flood environmental factors.…”

“…Although there is a possibility that this process would have an influence on the result of the field surveys, especially on the amount of an undercut bank and LDW, this was done because our study focused on the abundance of salmonid fish, which is affected by the amount of LWD and undercut bank (Hasegawa et al 2010). The data in Hasegawa et al (2010) were used as the pre-flood environmental factors. The values of the pre-and postflood parameters were compared using Kruskal-Wallis tests in SPSS ver.…”

“…In the Atsuta River, native masu salmon Oncorhynchus masou and nonnative rainbow trout O. mykiss coexist and it was expected that these populations were affected by the floods. Hasegawa et al (2010) investigated the abundance of masu salmon and rainbow trout as well as the river's environmental factors in 2006 and 2007 and these preflood data provide a rare opportunity to investigate the effects of severe floods on salmonid fish. In this study, we examined the abundance and density of masu salmon and rainbow trout and compared these values in the post-flood period with those in the preflood period in order to evaluate the influence of floods on these salmonid populations.…”