Effects of trout on the diel periodicity of drifting in baetid mayflies.

Some benthic invertebrates in streams make frequent, short journeys downstream in the water column (=drifting). In most streams there are larger numbers of invertebrates in the drift at night than during the day. We tested the hypothesis that nocturnal drifting is a response to avoid predation from fish that feed in the water column during the day. We surveyed diel patterns of drifting by nymphs of the mayfly Baetis coelestis in several streams containing (n=5) and lacking (n=7) populations of rainbow trout, Oncorhynchus mykiss. Drifting was more nocturnal in the presence of trout (85% of daily drift occurred at night) than in their absence (50% of daily drift occurred at night). This shift in periodicity is due to reduced daytime drifting in streams with trout, because at a given nighttime drift density, the daytime drift density of B. coelestis was lower in streams occupied by trout than in troutless streams. Large size classes of B. coelestis were underrepresented in the daytime drift in trout streams compared to nighttime drift in trout streams, and to both day and night drift in troutless streams. Differences in daytime drift density between streams with and without trout were the result of differences in mayfly drift behaviour among streams because predation rates by trout were too low to significantly reduce densities of drifting B. coelestis. We tested for rapid (over 3 days) phenotypic responses to trout presence by adding trout in cages to three of the troutless streams. Nighttime drifting was unaffected by the addition of trout, but daytime drift densities were reduced by 28% below cages containing trout relative to control cages (lacking trout) placed upstream. Drift responses were measured 15 m downstream of the cages suggesting that mayflies detected trout using chemical cues. Overall, these data support the hypothesis that infrequent daytime drifting is an avoidance response to fish that feed in the water column during the day. Avoidance is more pronounced in large individuals and is, at least partially, a phenotypic response mediated by chemical cues.