Estimating the size of the spawning population and evaluating environmental controls on migration for a critically endangered Asian salmonid, Sakhalin taimen

Sakhalin taimen Parahucho perryi, an east Asian fish noted to be one of the largest salmonids in the world, is threatened throughout its range in northern Japan and neighboring Russian Federation. We report here on the first effort to enumerate and characterize the spawning run of a river population. We applied sonar and video methods in a tributary of the Sarufutsu River in Hokkaido, Japan, and evaluated environmental controls on migration. Over two years we estimated the tributary population to range from 335 to 425. We found passage rate by our site to increase with temperature and decrease with river discharge, and migratory cues were reinforced by strong diel fluctuations in environmental conditions. Finally, we report evidence of males arriving early to the spawning grounds in this species. Given our results and data on the recreational fishery, we conclude that a substantial number of individuals in the population are affected by angling, underscoring the need to establish fishing regulations. Further, our study indicates passage success can vary over the migration period, and efforts at modifying or removing impediments, and devoting more research to factors controlling passage, could ultimately improve the status of this species.