EFFECTS OF PREDATION RISK ON FORAGING BEHAVIOR OF JUVENILE RAINBOW-TROUT (ONCORHYNCHUS-MYKISS)

The effects of a simulated avian predator attack on the food intake and foraging behavior of wild juvenile rainbow trout, Oncorhynchus mykiss, were examined in laboratory streams. Exposure to a simulated predator attack reduced the amount of food eaten for several minutes after an attack. The percent reduction in foraging rate was not related to the base-line foraging rate of the fry, a measure of feeding motivation, indicating that fry were not balancing risk and hunger. Fry at risk responded differently from safe fry to individually presented food items. Following exposure to a simulated attack, fry initially ate fewer items than did nonattacked fry, but over the course of the 10-min experiment, fry at risk ate as much as safe fry, which suggests a degree of compensation for lost foraging opportunities. I hypothesized that fry at risk from overhead avian predators should avoid risk by temporarily suspending foraging, since the risk is acute, short-term suspension of foraging has low energetic costs, and compensation is often possible. Fry at risk from piscivorous fish should be sensitive to predator behavior and should balance risk and energy intake, since risk is more likely to be sustained, and prolonged suspension of foraging has a high energetic cost.