Recovery process of masu salmon stock after the construction of fish ways

Abstract

　To evaluate the population recovery of masu salmon Oncorhynchus masou by fish ways, we investigated the number and distribution of spawning redds, population density and body size of juveniles for a total of 10 years before and after the construction of fish ways in the Shirai River, Hokkaido, Japan. Before the fish ways were constructed, most juveniles were distributed in the lowermost reach of the river. The population density of juveniles was high and their growth rate was low. After the fish ways were constructed, the distribution of spawning redds expanded into the upper reaches of the river, while in the lower reaches the population density of juveniles reduced and their growth rate increased. After construction, the number or size of potential smolts increased. The total number of spawning redds of the third generation after construction was 2.8 times as many as that of the first generation. We supposed that expansion of the spawning area and habitat of juveniles by constructing fish ways might lead to an increase of the number or size of potential smolts and an increase of the number of ascending adults and spawning redds.