The influence of hunger, shoal size and predator presence on foraging in bluntnose minnows

This study examines the compromise between predator avoidance and foraging in bluntnose minnows, Pimephales notatus, in shoals of different sizes and at three levels of hunger: 5, 24 or 72 h of deprivation. Trials were carried out in the laboratory with a predator present or absent. Foraging was affected significantly by shoal size, predator presence and hunger. Foraging latency decreased as shoal size and hunger level increased, but latency increased in the presence of a predator. Foraging rate was less when there was a predator present. In the absence of a predator, foraging rate increased as hunger level increased. At the 5 h hunger level, foraging rate increased as shoal size increased, in the absence but not the presence of a predator. At this low level of hunger, minnows in all shoal sizes fed at a low rate when the risk of predation was greater. At higher levels of hunger, fish in all shoal sizes fed at a high rate when no predator was present, so that foraging rate did not change across shoal size. When the predator was present at the higher hunger levels, only fish in larger, safer shoals fed at a rate greater than at the lowest hunger level.