Social interactions shape the timing of spawning migrations in an anadromous fish

Mass migrations are found throughout the animal kingdom and are often undertaken by coordinated social groups. However, surprisingly little is known about how social interactions influence migratory timing. Anadromous fishes such as salmon make extensive breeding migrations between marine and freshwater ecosystems. Returning adult salmon tend to move in discrete temporal pulses, which are typically thought to be triggered by abiotic environmental stimuli (e.g. changes in river flow or temperature). However, most studies reveal only weak correlations between abiotic factors and the timing of spawning runs. Here, we demonstrate that social interactions provide a plausible alternative or additional explanation for such patterns. We first provide an example of the phenomenon using 20 years of data on sockeye salmon, Oncorhynchus nerka, ascending a stream in pulses in the absence of any obvious environmental triggers. Next, we present a model that reproduces the pulses observed in the data, simply by including social interactions among individuals. Deviations between the empirical data and the social model results suggest that salmon may alter their individual behaviour in response to annual fluctuations in density. We hope our results, demonstrating the role that social influence can play on migration timing, will motivate further studies exploring how social interactions may shape the movements of other migratory taxa. Understanding how individuals integrate social information with internal and exogenous drivers of migratory behaviour is vital, particularly in the face of a changing climate, which is changing both the cues used for, and the optimal timing of, migrations.