Predator chemicals induce changes in mayfly life history traits: A whole-stream manipulation

In high-elevation streams of western Colorado, mayflies (Baetis bicaudatus) develop taster, but mature at a smaller size where trout are present compared to streams where fish are absent. These life history traits reduce the time of larval exposure to trout predation, but Cost reduced fecundity. We designed a field experiment involving manipulation of whole streams to determine whether these changes were caused by the presence of brook trout, and specifically, whether they could be triggered by trout chemical cues, fit 1999 and 2000. We introduced water from containers with brook trout (Salvelinus fontinalis) into five naturally fishless streams, and fishless stream water into live adjacent control streams to determine whether these cues alone could induce the mayfly life history traits we have observed in natural trout streams. As in previous small-scale experiments, the size at which mayflies matured declined significantly, in streams with added trout chemicals but did not change in streams with fishless water only. Thus, life history traits similar to those observed in the field were induced within the natural variability inherent ill streams. These results demonstrate the strength of this predator-prey interaction and indicate that brook trout are an important agent of natural selection oil mayfly life history traits.