An age‐stage structured population model for the coexistence of alternative migratory tactics in salmonid fishes

Abstract Alternative tactics refer to the discontinuous patterns of phenotypic variation, often expressed as an association of several phenotypes. These tactics are widely observed in several taxa, with individuals in a population choosing a phenotype from multiple options depending on their status. In salmonids, alternative migratory tactics‐where some individuals undergo feeding migration to the ocean and others remain in their natal streams are associated with exaggerated phenotypic variations. Such discrete alternative phenotypes can have complex impacts on ecological and evolutionary processes (e.g., reproduction and the trait changes due to natural selection) owing to the large differences in demographic rates between tactics. In this study, to explore the eco‐evolutionary consequences of alternative migratory tactics in salmonid fishes, we constructed an age‐stage structured population model focusing on females. As individuals choose a tactic based on their own status, we incorporated the size structure of juveniles into the population dynamics and explored the evolution of the threshold trait. At the evolutionary endpoint, the threshold trait maximized the population size. Furthermore, the coexistence of resident and migrant tactics within a population occurred when the river environment was relatively poor and/or the ocean environment was rich; otherwise, all individuals adopted the resident tactic. This model provides several insights into the eco‐evolutionary response of the population to environmental change. For example, the increase in survivorship during migration increased the population size and the fraction of migrants as an ecological and evolutionary response, respectively.