Fishless-stream mayflies express behavioural flexibility in response to predatory fish

The fish avoidance behaviour of nymphs of the grazing mayfly Baetis rhodani (Ephemeroptera) was examined. The nymphs originated from a fishless stream. To study their responses to the addition of predator cues, nymphs were exposed to (1) chemical cues from a caged fish, or (2) a freely moving diurnal fish (European minnow, Phoxinus phoxinus). The nymphs immediately increased their refuge use when exposed to a live fish, whereas chemical cues alone did not cause any avoidance responses. In a second experiment, the effects of (I) fish chemicals and (2) a fish model plus fish chemicals on the diel refuge use patterns of Baetis nymphs were examined. In the presence of a fish model, more nymphs were outside the refuges during the first few hours of darkness, when fish predators are not active, than at any other time of day. A similar but non-significant trend was observed when only chemical cues of fish were present, whereas refuge use in the control treatment was aperiodic. Baetis nymphs were thus able to assess the fish predation risk and to respond by quickly adjustable, flexible avoidance behaviour. Spatial and temporal variation in predation risk, and adult dispersal between fishless and fish-inhabited streams, may be the key factors promoting the maintenance of flexible fish avoidance behaviour in stream-dwelling mayfly nymphs. (C) 1996 The Association for the Study of Animal Behaviour