Individual and Temporal Variation in Prey Switching by Brown Trout, Salmo trutta

Foraging tactics of brown trout (Salmo trutta) from a wild population were observed in a laboratory stream. Floating tent caterpillars (Malacosoma americanum) and mealworm larvae (Tenebrio molitor) were introduced to the stream by conveyor belt and the ratio of prey species was changed over an 11 d period from 0:1-5:1. Two of five individuals appeared to switch to the most abundant prey species. Preference values based on a model of switching (Murdoch, 1969) differed among individuals and varied as much as five-fold among 5 min intervals. In some individuals diet preferences reversed during 30 min bouts; short-term changes in feeding preference were not generally predictive of longer-term trends. Individual differences were detected in the expression of negative bias toward tent caterpillars. Some effects of exposure to relatively unpalatable prey appeared to extend over at least a 24 d period. Studies of foraging variability should contribute to an understanding of the role of switching in stabilizing prey communities and to the successful application of optimal foraging theory to stream fishes.