Amazing stuff!
"Herring had a spawning ‘culture.’ Overfishing obliterated it
For more than a century, the world’s biggest stock of the oily, nutritious herring has migrated annually to spawn in the same waters off southern Norway. But in 2021 only a few fish arrived, leaving boat crews with empty nets and pressing questions. ...
Excessive harvests in prior years eliminated “cultural” knowledge passed down from older generations of herring, so that younger fish didn’t find their way to the traditional spawning grounds.
Adult herring spend the winter in Norway’s uppermost fjords, then swim as far as 1300 kilometers down the coast to Møre to reproduce. The researchers noticed that after 2020 most young herring were no longer spawning near Møre. Instead, they had stopped short on their southbound journey, near Lofoten, which is 800 kilometers away. ...
Without older fish as guides, the younger ones stopped halfway down the coast to spawn in Lofoten. And now, with these fish returning there year after year, the new pattern seems established. ..."
From the abstract:
"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."
Fig. 1: Migration strategies of herring in a dynamic environment.
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