Rainbow trout (Oncorhynchus mykiss) invasion in Hokkaido streams, northern Japan, in relation to flow variability and biotic interactions

Factors controlling invasion success of rainbow trout ( Oncorhynchus mykiss ) and interactions with native masu salmon ( Oncorhynchus masou ) were examined by field investigations. The results suggested that flow regime has a major role in controlling rainbow trout invasion success. Although competitive relationships were found between the two species, masu salmon were not likely to prevent trout invasion. Analyses on the distributions and abundances of the two species indicated that the occurrence of rainbow trout was limited to streams with stable flows, and the density of each species was negatively related with that of the other. Habitat use and diet analyses revealed a pattern of their resource partitioning; rainbow trout occupy positions near the bottom or cover, foraging benthic prey, while masu salmon feed drifting prey at the middle layers of the water column. This resource partitioning was suggested to reflect a diet shift by rainbow trout. Our results also have a management implication that dams, which control flow regime, can be a key factor in managing rainbow trout invasion and its effects.