Herring spawned poleward following fishery-induced collective memory loss.

Entrainment is a process in schooling migratory fish whereby routes to suitable habitats are transferred from repeat spawners to recruits over generations through social learning. Selective fisheries targeting older fish may therefore result in collective memory loss and disrupted migration culture. The world's largest herring (Clupea harengus) population has traditionally migrated up to 1,300 km southward from wintering areas in northern Norwegian waters to spawn at the west coast. This conservative strategy is proposed to be a trade-off between high energetic swimming costs and enhanced larval survival under improved growth conditions. Here an analysis of extensive data from fisheries, scientific surveys and tagging experiments demonstrates an abrupt approximately 800-km poleward shift in main spawning. The new migration was established by a large cohort recruiting when the abundance of older fish was critically low due to age-selective fisheries. The threshold of memory required for cultural transfer was probably not met-a situation that was further exacerbated by reduced spatiotemporal overlap between older fish and recruits driven by migration constraints and climate change. Finally, a minority of survivors from older generations adopted the migration culture from the recruits instead of the historically opposite. This may have profound consequences for production and coastal ecology, challenging the management of migratory schooling fish.