Predation on mayfly nymph, <i>Baetis rhodani</i>, by native and introduced <i>Gammarus</i>: direct effects and the facilitation of predation by salmonids.

1. In a series of laboratory experiments, we assessed the predatory nature of the native Irish amphipod, <i>Gammarus duebeni celticus</i>, and the introduced <i>G. pulex</i>, towards the mayfly nymph <i>Baetis rhodani</i>. We also investigated alterations in microhabitat use and drift behaviour of <i>B. rhodani</i> in the presence of <i>Gammarus</i>, and indirect predatory interactions with juvenile Atlantic salmon, <i>Salmo salar</i>. 2. In trials with single predators and prey, B. rhodani survival was significantly lower when Gammarus were free to interact with nymphs as than when Gammarus were isolated from them. The invader G. pulex reduced the survival of B. rhodani more rapidly than did the native G. d. celticus. Both Gammarus spp. were active predators. 3. In `patch' experiments, B. rhodani survival was significantly lower both when G. pulex and G. d. celticus were present, although the effect of the two Gammarus species did not differ. Again, active predation of nymphs by Gammarus was observed. Significantly more nymphs occurred on the top and sides of a tile, and per capita drifts were significantly higher, when Gammarus were present. Baetis rhodani per capita drift was also significantly higher in the presence of the introduced G. pulex than with the native G. d. celticus. 4. Gammarus facilitated predation by salmon parr of B. rhodani by significantly increasing fish–nymph encounters on exposed gravel and in the drift. There were no differential effects of the two Gammarus spp. on fish –B. rhodani encounters or consumption. 5. We conclude that Gammarus as a predator can have lethal, nonlethal, direct and indirect effects in freshwaters. We stress the need for recognition of this predatory role when assigning Gammarus spp. to a `Functional Feeding Group'.