Size-selective harvesting evolutionarily alters collective behaviour, vulnerability to fishing and natural predation in zebrafish

Intensive size-selective harvesting alters group behavioural traits like shoaling by changing individual vigilance. These changes could affect vulnerability to fishing gears and natural predators. We tested this experimentally using zebrafish (Danio rerio) as a model in a multi-generation harvest selection experiment. We used 3 lines of zebrafish generated through selective harvesting for large, small &; random body-size for 5 generations followed by no selection for 10 generations to test evolutionarily fixed outcomes. We predicted that positive size-selection would result in a reduction in shoal cohesion, reduced fishing but increased natural mortality. We exposed zebrafish groups to an enclosed predator to test for collective behaviour, to a trap and trawl to test for fishing vulnerability and to a live cichlid predator to test for vulnerability to natural mortality. The large-harvested line demonstrated a significant reduction on shoal cohesion. Significantly fewer individuals of the large-harvested line were caught by the trap and trawl compared to controls indicating that adaptation to large size-selection selected for behavioural traits that reduced vulnerability to fishing. With the predator, the small-harvested line suffered significantly higher mortality compared to controls. Thus, evolutionary adaptations of fish to large size-selection may help stocks survive against fishing and not substantially increase natural mortality.