The influence of light level on the functional response of a zooplanktonivorous fish.

Light and vision are clearly of significance in foraging behaviour by underyearling common bream [Abramis brama (L.)]. These fish are effective predators at 1.25 Lux but they were also shown to be capable of taking prey, at a reduced rate, at a much lower light intensity (less than 5x10 Lux). In the latter case they may have been using sensory modes other than vision, perhaps involving tactile and/or olfactory stimuli.We investigated the influence of light level on the functional response of bream to Daphnia magna prey. At 1.25 Lux the predator showed a typical type II response. However, the relatively unfavourable conditions in the lower light intensity appear to have been responsible for generating a sigmoid type III functional response. Observations, using infra-red sensitive equipment, suggested a behavioural basis for this result. Thus, the predator's attack rate was not constant, but increased with prey density. The significance of the type III functional response is discussed, both in terms of predator energetics and predator-prey population stability.