Influence of autumn body size on following spring migration probability and timing in juvenile brown trout (Salmo trutta)

Body size is frequently invoked to explain within-population variation in migratory behaviour because of its far-reaching implications for physiological and ecological processes. Here, we investigated how autumn body size influenced spring river-to-sea migration probability and timing in juvenile brown trout (Salmo trutta) in the river Nybroån, Sweden. To achieve this, we used a combination of individual-based tracking and trapping during two consecutive years. We report a non-linear relationship between autumn body size and probability of spring migration, with migration likelihood increasing steeply up to a body length of ~110 mm before declining with further increasing body size. Among migrants, larger individuals migrated earlier than smaller conspecifics. Our results highlight that autumn body size may be used as a reliable predictor of migratory behaviours in juvenile brown trout and underscore the utility of using recruitment surveys to forecast life-history transitions and suggest that probabilistic reaction norms based on body size can support monitoring and conservation strategies in migratory fish populations.