Migration may confer benefits, such as foraging opportunity and predator avoidance, but partial migration, i.e. that not the whole population migrates, can be expected if benefits differ between individual phenotypes. We here show that seasonal migration from lakes to connected streams in the freshwater fish roach is strongly affected by risk of cormorant predation. We show that the duration away from the lake, as well as roach individual body size, influence the probability of being predated by cormorants. We also present novel field data showing that cormorant predation acts as a selective pressure against individuals with bold personality, providing a mechanism behind the evolution and maintenance of behavioural phenotypic diversity along the bold-shy continuum. Accordingly, we argue that bold individuals have a higher migratory propensity to avoid cormorants, and that cormorant predation on bold individuals may shift animal-personality composition of roach populations, with consequences for selective processes and lake system composition and function.