Spatial expansion and increased population density of masu salmon parr independent of river restoration

Spatial expansion and increased population abundance of masu salmon Oncorhynchus masou have been recently reported following habitat restoration in some rivers, but no studies have examined these types of changes under natural conditions. We found spatial expansion and increased population density of masu salmon parr in rivers of eastern Hokkaido where no habitat restoration had occurred during the period examined. Changes in the distribution and population densities of parr are often regarded as effects of river habitat destruction and restoration, yet such changes should be evaluated while taking into account the effects of other ocean environmental conditions as well.