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 from fishes that feed visually in midwater, and the restriction of drifting to nighttime is hypothesized to have evolved as an avoidance response to this type of predator. I tested the hypothesis that low daytime drifting is a proximate response to the presence of drift-feeding fishes by manipulating fish density (brook trout, Salvelinus fontinalis) in two field experiments. Daytime drift activity of five prey taxa (all mayflies: Ephemeroptera) was low in both experiments and did not change following fish removal. By night, however, nighttime drift activity of two mayflies, Baetis and Paraleptophlebia, was increased in areas containing fish. The other three mayflies, Ephemerella, Stenonema, and Eurylophella, which showed less drift activity overall, displayed no consistent responses to fish presence. The original hypothesis predicts that risk from drift-feeding fishes should result in reduced daytime drifting. The increases in nighttime drifting away from patches dense with trout were not predicted by the original hypothesis, but may be an additional avoidance response to reduce longterm predation risk in streams where trout are distributed patchily in space.