1. The predominantly nocturnal constrained drift of stream invertebrates is commonly regarded as a behaviour that avoids encounters with visually foraging fish in the water column. The alternative explanation, that drift peaks are caused by bottom‐feeding, nocturnal predators, has rarely been tested. 
2. We examined these hypotheses by collecting invertebrate drift in five streams in northern Finland: one with brown trout (Salmotrutta, a drift‐feeding fish), one with alpine bullhead (Cottuspoecilopus, a benthic fish), one with both species, and two fishless streams. 
3. Drift by Baetis mayflies was aperiodic or slightly diurnal in both fishless streams on all sampling occasions. In contrast, drift was nocturnal in streams with trout and, to a lesser extent, in the stream with bullhead. Non‐dipteran prey drifted mainly nocturnally in all streams with fish, whereas Diptera larvae were less responsive to the presence of fish. 
4. In laboratory experiments, bullheads were night‐active, causing a much higher frequency of drift by touching Baetis at night than during the day. Thus, increased nocturnal drift may serve to avoid both visual predators (a pre‐contact response) and benthic fish (a post‐contact response). In streams with bottom‐feeding fish, nocturnal drift should be caused by increased drift by night rather than by reduced drift by day.

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Summary1. We examined direct and indirect behavioural responses of grazing may¯y nymphs (Baetis rhodani) to predation risk from a drift-feeding ®sh (European minnow; odour manipulated), and two types of invertebrate predators, Diura bicaudata (stone¯y) and Rhyacophila nubila (caddis larva). We also assessed the direct responses of the invertebrate predators to ®sh chemicals. 2. Based on diel gut content periodicity, D. bicaudata nymphs were strongly nocturnal foragers. R. nubila was also nocturnal, but only in a ®sh stream; in a stream lacking ®sh, their gut contents did not vary signi®cantly on a diel basis. In the laboratory, Diura was nocturnal even in the absence of ®sh and almost ceased moving when ®sh was present. Rhyacophila shifted from aperiodic to nocturnal foraging in the presence of ®sh. 3. The contrasting behaviours of the two predators may be explained by their respective ®eld distributions: D. bicaudata always co-occurs with ®sh, whereas R. nubila is also found in streams without ®sh. Therefore, a capacity for plastic anti®sh responses is bene®cial for R. nubila, but not for D. bicaudata. 4. Drift of large Baetis nymphs was aperiodic when Rhyacophila was present, but nocturnal in all other treatments. Drift rate was highest when both Diura and ®sh were present and lowest in treatments with Rhyacophila. Predatory stone¯ies, but not the caddis larva, induced a night-time peak in the drift of both Baetis sizeclasses. In the absence of predators, small Baetis drifted aperiodically. Interaction terms in three-way ANOVAs testing for the indirect eects of ®sh and invertebrate predators on may¯y drift periodicity and drift rate were all non-signi®cant, indicating that the response of Baetis to one type of predator was not modi®ed by the other predator. 5. Our results indicate that the drift periodicity of lotic may¯ies may be ®ne-tuned to variations in the multi-predator environment and that prey responses are sizespeci®c. We conclude that the eects of invertebrate predators on prey behaviour vary in relation to predator's foraging strategy and generalizations based on studies with only one type of predator should be avoided.

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 constrained fish, (3) water from a fish stream, (4) water from a fishless stream. LargeBaetis nymphs drifted mainly during night-time in treatments involving either the chemical or actual presence of fish, whereas no diel periodicity was observed when the water was not conditioned with fish odour. The response was strongest when the fish was uncaged, which suggests that visual or hydrodynamic cues are needed in addition to chemical ones for an accurate assessment of predation risk. Fish presence had no effect on the drift rates of small nymphs. Instead, they increased their refuge use in the presence of a live fish. Chemical cues alone did not have any effect on the refuge use of any of theBaetis size classes. Our results indicate active drift entry by mayfly nymphs. Because predation pressure is spatially and temporally variable, nymphs must sample the environment in order to locate predator-free areas or areas with low predation risk. Drifting should be the most energy-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 not a fixed prey response. Instead,Baetis nymphs are able to assess the prevailing predation pressure, and they adjust their foraging behaviour accordingly.