Because we don’t tend to to think of fish as being very smart, we don’t think they could have memories that matter. In that, we are mistaken, according to a recent article by science writer Katarina Zimmer at Nautilus:

For as long as scientists have been watching, the world’s largest herring population has migrated up to 800 miles every year from wintering areas in northern Norway to spawning grounds along the country’s southwest coast. The herring larvae grow faster in the warmer southern waters, feeding off the rich zooplankton that thrives there.

But a few years ago, scientists noticed something odd. When the spring rolled around in 2021, instead of heading all the way south, the adult fish migrated a few hundred miles to Lofoten in northern Norway and then stopped, and they’ve largely stuck around there ever since, says Aril Slotte, a fisheries biologist at Norway’s Institute for Marine Research. The scientists track the fish’s movements with radiofrequency identification tags and other methods. “This is the first time something dramatic like this has happened,” says Slotte.

The shift is worrying. Fewer baby herring might survive in the colder and less plankton-rich conditions in the north, something that could cause population declines in the long term. The collapse of migration patterns may also have ripple effects on the fish, birds, and mammals along the coast that prey on the herring larvae as they drift north from spawning grounds to the Barents Sea in the north during the first year of their lives. Slotte is especially concerned about the many bird colonies that rely on these drifting herring babies to feed their young. Without herring larvae, “it could be quite dramatic for those populations,” he says.

Too many old, wise fish were being harvested, causing the population as a whole to lose its way. “How These Fish Lost Their Memory,” May 29, 2025

From an open-access study at Nature by Slotte and colleagues:

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.

Slotte, A., Salthaug, A., Vatnehol, S. et al. Herring spawned poleward following fishery-induced collective memory loss. Nature (2025). https://doi.org/10.1038/s41586-025-08983-3

That makes sense if we recall that fish can’t relay a history or write things down so they were depending on older fish to know the way.