Size-selective mortality evolutionarily alters collective behaviour in response to predation risk in a zebrafish (Danio rerio) harvest-induced selection model.

Intensive multigenerational size-selective mortality has been found to alter collective properties like shoaling via evolutionary adaptations of individual-level behavioural traits. For example, experimental populations of zebrafish (Danio rerio) experiencing intensive large size-selective mortality over multiple generations developed less cohesive shoaling in the laboratory, presumably because these fish were more attentive to environmental rather than social cues. By contrast, zebrafish exposed to small size-selective mortality evolved increased group cohesion. However, these studies were conducted in the absence of explicit predation risk by a live predator. In this study, we examined if size-selective mortality led to divergent responses in shoaling behaviour in the same zebrafish selection lines in the presence and absence of a live predator. The large line generated via size-selective harvest of smaller fish over five generations formed significantly more cohesive shoals than the control line, independent of whether a live predator was present or not. By contrast, the small line generated by size-selective mortality of large fish over five generations did not differ from controls in their shoaling behaviour in the presence or absence of a live predator. All zebrafish groups generally formed more cohesive shoals in the presence of a predator and became more dispersive over a 2-week experimental period. We conclude that the systematic removal of smallest individuals, thereby saving the largest fish from harvest, evolutionarily fosters increased group cohesiveness. These evolutionary adaptations might affect natural predation mortality and catchability by fishing gears.