Phenotype-dependent downstream dispersal under ordinary flow conditions in juvenile white-spotted char

Animal dispersal is often phenotype-dependent and can exert evolutionary pressures on populations in which it occurs. The evolutionary pressure arising from phenotype-dependent dispersal is called spatial sorting. We examined the evolutionary pressure arising from spatial sorting (sorting pressure) caused by downstream dispersal in juvenile white-spotted char Salvelinus leucomaenis under ordinary flow conditions. We conducted outdoor experiments using an artificial channel with 10 steps to investigate the relationship between phenotypic characteristics and the occurrence or distance of downstream dispersal during five daytime hours. Six experiments were conducted using young-of-the-year juveniles collected early in the morning of each experimental day. We focused on two phenotypes, fork length (body size) and station-holding (SH) behavior, where juveniles remain sedentary on the substrate. Juveniles were assigned to the “SH group” if they exhibited SH behavior for more than 10 s during a 540-s observation period, and to the “swimming group” if they exhibited SH behavior for less than 10 s. Juveniles in the swimming group had a higher occurrence of downstream dispersal than in the SH group. In addition, large juveniles in the SH group and small juveniles in the swimming group tended to show long dispersal distances. These results suggest an effective sorting pressure against juveniles with active swimming behavior. This sorting pressure may accumulate in isolated char populations located above a tall migration barrier and contribute to the creation and maintenance of the reported interpopulation variation in SH behavior.