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Salmonids are well known for their natal homing behaviour, meaning they return to breed in the same area where they originated. However, not all individuals return to their natal breeding grounds—a behavioural trait known as straying. The prevalence of straying is difficult to explore and therefore quantitative estimates for straying are seldom reported. In this study, otolith microchemistry and genetics were combined to investigate patterns of straying over ecological and evolutionary time, respectively, between neighbouring rivers flowing into Mariager fjord, Denmark. Otolith microchemistry was used to determine the river of origin for sea trout (Salmo trutta) upon their return to freshwater and 288 SNP markers were used to determine genetic structure among the rivers in the fjord. In this system, where the distance between rivers is short, otolith microchemistry achieved 80% accuracy in assigning juvenile brown trout to their natal river, thus allowing us to determine that approximately 43% of the adult sea trout had returned to non‐natal rivers to spawn, with a similar proportion of strayers and natal homers in all of the rivers. Genetic analysis further supported that there was substantial gene flow among individuals originating from different rivers, indicating that sea trout in Mariager fjord make up one population. The findings obtained from otolith microchemistry and genetics complement each other and provide further evidence that sea trout in this system migrate to non‐natal rivers and spawn there, which consequently affects the genetic structure of the population. [ABSTRACT FROM AUTHOR]

Anadromous brown trout display a continuum of life-history strategies with high intraspecific variability across their distribution range. While many of these strategies have been widely studied, some have received considerably less attention. In this study, we investigated intraspecific differences in two overlooked life-history strategies—outmigration phenology (spring vs. autumn migrants) and the prevalence of straying—in brown trout originating from four rivers flowing into the same fjord system. Juvenile brown trout (Salmo trutta) were tagged with passive integrated transponder (PIT) tags in their natal river, and were subsequently followed during their outmigration and subsequent return to freshwater. We found that patterns of outmigration (spring vs. autumn migrant, day of season) and return to freshwater (return likelihood, strayers vs. nonstrayers) differed between rivers, as did the factors (length, condition) affecting these outcomes. We also found evidence that outmigration phenology was linked to the likelihood of returning in a river-specific way. Our findings suggest that the costs and benefits of autumn migration and straying differ depending on an individual's home river. [ABSTRACT FROM AUTHOR]