The effect of hunger, shoal size and the presence of a predator on shoal cohesiveness in bluntnose minnows, Pimephales notatus Rafinesque

Shoals of fish vary in their degree of cohesiveness as certain conditions such as hunger, predation risk and shoal size vary: this study relates this variation to the benefits of shoaling. Shoals of bluntnose minnows, Pimephales notatus Rafinesque, consisting of different numbers of individuals were starved for 5, 24 or 72 h and allowed to forage in the presence or absence of a predator. Cohesiveness was measured as the dispersion offish within the shoal (the radius of the longest axis of the shoal corrected for the number of fish in the shoal), straggle frequency (the number of movements by individuals of at least 5 body lengths away from the shoal) and aggressive interactions between shoal members. Cohesiveness increased as shoal size and the benefits per individual increased. Cohesiveness also increased in the presence of a predator, as did the importance of shoaling as a means of reducing predation risk. Cohesivenes decreased as hunger level increased, possibly as a result of increased competition for food among shoal members.