Thomas P. Quinn
January 19, 2021
Migrations affect the population dynamics, life history, evolution, and connections of animals to natural ecosystems and humans. Many species and populations display partial migration (some individuals migrate and some do not), and differential migration (migration distance varies). Partial migration is widely distributed in fishes but the term differential migration is much less commonly applied, despite the occurrence of this phenomenon. This paper briefly reviews the extent of differential migration in Pacific salmon and trout (genus Oncorhynchus ), a very extensively studied group. Three hypotheses are presented to explain the patterns among species: 1) phylogenetic relationships, 2) the prevalence of partial migration (i.e., variation in anadromy), and 3) life history patterns (iteroparous or semelparous, and duration spent feeding at sea prior to maturation). Each hypothesis has some support but none is consistent with all patterns. The prevalence of differential migration, ranging from essentially non-existent to common within a species, reflects phylogeny and life history, interacting with the geographic features of the region where juvenile salmon enter the ocean. Notwithstanding the uncertain evolution of this behavior, it has very clear implications for salmon conservation, as it strongly affects exposure to predators, patterns of fishery exploitation and also uptake of toxic contaminants.