Seasonal patterns of downstream movement in partially migratory stream-dwelling Dolly Varden

Migration is a common behaviour among salmonids, but not all individuals within a population migrate, instead becoming residents and remaining in their natal streams. This phenomenon, known as "partial migration," is well studied among sea-run and lake-run populations; however, the lower migratory benefits and costs for mainstem-run individuals question whether the same mechanisms can be applied for stream-dwelling salmonids. In this study, we investigated main stem-run timing and body condition of partially migratory stream-dwelling Dolly Varden charr (Salvelinus malma) in the Shiisorapuchi River in central Hokkaido, Japan. Based on commonly observed patterns in sea-run and lake-run salmonids, we predicted that migration would occur during spring and consist predominantly of age 1 + females with some small subordinate males. Traps were placed at the mouths of two small tributaries along the Shiisorapuchi River periodically from May to December 2015. Adipose fin samples were taken for DNA to sex the individuals. Consistent to initial predictions, downstream movement occurs only once in the spring, dominated by age-1 + females. Trapped fishes (presumed migrants) were slightly longer but slimmer compared to the fish caught in the tributaries (presumed resident), which may be equivalent to smolts in anadromous populations. Regardless of migratory habitat, mechanisms driving partial migration in salmonids may be the same as long as production between natal stream and feeding habitats is significantly different.