Tidal gradients, fine-scale homing, and a potential cryptic ecotype of wild spawning pink salmon (Oncorhynchus gorbuscha)

Abstract

The homing behavior of salmon is a remarkable natural phenomenon, critical for shaping the ecology and evolution of populations, yet the spatial scale at which it occurs is poorly understood. This study investigated the spatial scale and mechanisms driving homing and spawning site-choice behavior in pink salmon in Prince William Sound, Alaska. Molecular pedigree analyses of nearly 15,000 adult spawners in five streams revealed that pink salmon can exhibit fine-scale site fidelity within a stream, returning to the same few meters of streams as their parents. Homing behaviors were driven in part by a salinity gradient between intertidal and freshwater environments, with individuals incubated in freshwater environments more than twice as likely to spawn upstream of the high tide line than those incubated in the intertidal. Our findings challenge the traditional view still held by some that pink salmon populations are genetically and phenotypically homogenous due to their short freshwater residency as juveniles and high rates of dispersal as returning adults (i.e., straying). This study has important implications for rates of inbreeding, local adaptation, and gene flow within populations, and is particularly relevant to the management of salmon hatcheries, given the high incidence of hatchery-origin pink salmon, reared in freshwater hatchery environments, that stray into wild populations in Prince William Sound.