PREDATOR DETECTION AND AVOIDANCE BY LOTIC MAYFLY NYMPHS OF DIFFERENT SIZE

We studied antipredatory responses of lotic mayfly (Baetis) nymphs in a factorial experiment with four levels of fish presence: (1) a freely foraging fish (the European minnow, Phoxinus phoxinus), (2) a constra ined fish, (3) water from a fish stream, (4) water from a fishless str eam. Large Baetis nymphs drifted mainly during night-time in treatment s involving either the chemical, or actual presence of fish, whereas n o diel periodicity was observed when the water was not conditioned wit h fish odour. The response was strongest when the fish was uncaged, wh ich suggests that visual or hydrodynamic cues are needed in addition t o chemical ones for an accurate assessment of predation risk. Fish pre sence had no effect on the drift rates of small nymphs. Instead, they increased their refuge use in the presence of a live fish. Chemical cu es alone did not have any effect on the refuge use of any of the Baeti s size classes. Our results in dicate active drift entry by mayfly nym phs. Because predation pressure is spatially and temporally variable, nymphs must sample the environment in order to locate predator-free ar eas or areas with low predation risk. Drifting should be the most ener gy-saving way to do this. To avoid the risk from Visually feeding fish , large individuals can sample safely (i.e. enter drift) only at night -time, while the small ones can also do this safely during the day. We suggest that, contrary to some earlier assumptions, mayfly drift is n ot a fixed prey response. Instead, Baetis nymphs are able to assess th e prevailing predation pressure, and they adjust their foraging behavi our accordingly.