A mechanism for interference between stream predators: responses of the stonefly Agnetina capitata to the presence of sculpins.

Mottled sculpins (Cottus bairdi) have a strong negative effect on the ability of the stonefly Agnetina capitata to capture some types of mayfly prey. To determine the mechanism for this interference effect, behavior of Agnetina in the presence and absence of sculpins was observed over 24 h periods (12 h light, 12 h dark), using an infra-red sensitive camera and a time-lapse video recorder. Agnetina larvae reacted to the presence of sculpins by significantly reducing the time they spent off the bottom of the substrate, and by significantly decreasing the amount of time spent moving on the substrate. These experiments suggest that in the presence of fish, stonefly diets may contain a smaller proportion of prey that tend to frequent tops and sides of stones. This behavioral flexibility may be important in streams in that it allows stoneflies to advantageously shift their diets when fish population densities are low.