Size-Dependent Diel Foraging Periodicity of a Mayfly Grazer in Streams with and without Fish

We tested the hypothesis that predation risk from diurnal fish influences the diel foraging periodicity of the mayfly grazer, Baetis tricaudatus Dodds, in a size-dependent manner by examining habitat use and foraging activity of mayflies in streams with and without these fish predators. In each stream diel habitat use was determined by counting the number of small (3 mm total length) and large (6 mm total length) Baetis larvae foraging on the upper surfaces of replicate, natural cobble during 4-8 sampling periods within a 24-h cycle over three seasons. Additionally, larvae were collected and preserved so that the diel periodicity in gut fullness could be determined. In fishless stream reaches small and large larvae exhibited seasonally consistent patterns of aperiodic, or weakly diurnal, periodicities in both gut fullness and the use of exposed, upper surfaces of stones. In contrast, the diel patterns of gut fullness and use of exposed substrate surfaces in stream reaches with abundant fish populations provided strong evidence that foraging by large Baetis was nocturnal and that small larvae were aperiodic foragers. Thus, when sympatric with diurnal fish predators, large Baetis appear to be constrained to feed at night when predation risk from day active fish is low. Conversely, small Baetis grazed from stones throughout the day and night and were unconstrained by these fish predators. This movement pattern of large larvae, from the underside of stones during the day to the sides and tops of stones at night in habitats where diurnal fish predators are present, is similar to the normal vertical migration observed for marine and freshwater zooplankton. The diel vertical migration of large Baetis larvae suggests that there is a tradeoff based inherently on an evaluation of predation risk and food reward. That this size-dependent impact of fish predators has not been quantified before could explain some of the contradictory patterns of Baetis diel periodicity evident within the literature and future studies need to consider this predator-mediated, size-dependent component of stream insect periodicities.