FISHLESS-STREAM MAYFLIES EXPRESS BEHAVIORAL FLEXIBILITY IN RESPONSE TO PREDATORY FISH

The fish avoidance behaviour of nymphs of the grazing mayfly Baetis rh odani (Ephemeroptera) was examined. The nymphs originated from a fishl ess 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 n ymphs immediately increased their refuge use when exposed to a live fi sh, whereas chemical cues alone did not cause any avoidance responses. In a second experiment, the effects of (I) fish chemicals and (2) a f ish model plus fish chemicals on the diel refuge use patterns of Baeti s nymphs were examined. In the presence of a fish model, more nymphs w ere outside the refuges during the first few hours of darkness, when f ish predators are not active, than at any other time of day. A similar but non-significant trend was observed when only chemical cues of fis h were present, whereas refuge use in the control treatment was aperio dic. Baetis nymphs were thus able to assess the fish predation risk an d to respond by quickly adjustable, flexible avoidance behaviour. Spat ial and temporal variation in predation risk, and adult dispersal betw een fishless and fish-inhabited streams, may be the key factors promot ing the maintenance of flexible fish avoidance behaviour in stream-dwe lling mayfly nymphs. (C) 1996 The Association for the Study of Animal Behaviour