The selective removal of large, old individuals through fishing fundamentally alters population structure and demographic diversity. Beyond direct impacts on reproduction and biomass, these individuals often serve as repositories of ecological knowledge, guiding younger fish to spawning grounds and feeding areas through social learning and cultural transmission. Loss of these "wise" individuals may disrupt migratory behaviour, causing populations to lose access to optimal spawning, foraging or overwintering habitats and reducing population resilience to harvest and environmental change.
Evidence from Australian fisheries reveals a striking pattern: migratory and spawning aggregation behaviours are significantly over-represented in depleted stocks. We found that ~75% of depleted or depleting stocks belong to species with seasonal aggregations or spawning migrations, a proportion roughly twice that observed in sustainably fished stocks where only 30-40% exhibit these behaviours.
Several non-mutually exclusive mechanisms could explain this finding. Seasonal aggregations may simply make fish more predictable and therefore easier to exploit, increasing the likelihood of overfishing. Alternatively, fishing on spawning aggregations may disproportionately remove the largest, oldest and most fecund individuals, reducing reproductive potential. A third possibility is that the selective loss of older individuals erodes socially transmitted knowledge of migration routes and aggregation locations, disrupting behaviours that underpin successful reproduction.
Our work provides an opportunity to re-evaluate the value of large, old fish beyond their well-recognised reproductive contributions. We argue that if cultural transmission contributes to the persistence of migratory and spawning behaviours, maintaining old age structure may also preserve the ecological memory embedded within populations, thereby supporting long-term population persistence in an increasingly variable world.