The Role of Stoneflies in Enhancing Growth of Trout: A Test of the Importance of Predator-Predator Facilitation within a Stream Community

Abstract

Failure to understand subtle behavioral mechanisms can lead to mischaracterization of interactions within food webs. Laboratory studies indicating increased prey capture by fish when predatory invertebrates are present suggested a field experiment to evaluate the effect of stonefly removal on growth rate of fish. Individually marked juvenile cutthroat trout (Oncorynchus clarki) were introduced into replicate experimental stream channels beside a coastal Pacific rainforest stream. In stream channels with large perlid stoneflies (Doroneuria and Calineuria sp.) trout gained mass, whereas in channels without stoneflies trout lost body mass. There was no significant effect of stoneflies on trout mortality over the month-long experiments. Although total predator number and biomass were higher in channels with stoneflies (trout and stonefly biomass combined), trout apparently gained more from the way prey behavior was altered by the presence of stoneflies, than they lost from any effects of competition for common prey resources. The presence of significant facilitation amongst predators complicates our view of trophic structuring in stream communities, requiring that such non-intuitive interactions be more explicitly considered both by modelers and experimentalists seeking to evaluate the role of predators in these and in other communities.