DIEL PATTERNS OF DRIFT BY 5 SPECIES OF MAYFLY AT DIFFERENT LEVELS OF FISH PREDATION

Stream insects that drift downstream in the water column are at risk f rom fishes that feed visually in midwater, and the restriction of drif ting to nighttime is hypothesized to have evolved as an avoidance resp onse to this type of predator. I tested the hypothesis that low daytim e drifting is a proximate response to the presence of drift-feeding fi shes by manipulating fish density (brook trout, Salvelinus fontinalis) in two field experiments. Daytime drift activity of five prey taxa (a ll mayflies: Ephemeroptera) was low in both experiments and did not ch ange following fish removal. By night, however, nighttime drift activi ty of two mayflies, Baetis and Paraleptophlebia, was increased in area s containing fish. The other three mayflies, Ephemerella, Stenonema, a nd Eurylophella, which showed less drift activity overall, displayed n o consistent responses to fish presence. The original hypothesis predi cts that risk from drift-feeding fishes should result in reduced dayti me drifting. The increases in nighttime drifting away from patches den se with trout were not predicted by the original hypothesis, but may b e an additional avoidance response to reduce longterm predation risk i n streams where trout are distributed patchily in space.