Evolutionary bistability of life history decision in male masu salmon

Within the salmonid species, some male juveniles after spawning in fresh water stream migrate to the ocean and return to their natal streams after maturation, while others stay and mature in the fresh water stream only. Migration is relative to the size of the individuals. This is an evolutionary outcome according to the status-dependent strategy model, which assumes that the juveniles exhibit the optimal tactic based on their status. In this paper, we consider the case in which the density of adult residents suppresses juvenile growth, and explore the dynamics of alternative tactics and the evolution of threshold size. We show that a fraction of the migratory tactic that might converge into a stable state or continue to fluctuate wildly, and that the evolutionary outcome might be evolutionarily bistable, resulting in a clearly different threshold size. In the case of evolutionary bistability, two threshold sizes differ in ecological dynamics either by stable fraction of migratory tactic or showing two-year periodic fluctuation.