THE IMPACT OF BROOK TROUT (SALVELINUS-FONTINALIS) ON AN EXPERIMENTAL STREAM BENTHIC COMMUNITY - THE ROLE OF SPATIAL AND SIZE REFUGIA

1. The importance of space and size refugia in allowing stream benthic insects to avoid predation by fish was tested by measuring the impact of brook trout on drifting, epibenthic and infaunal insects of different taxa and size-classes. Experiments were carried out in a replicated series of pebble-bottom, outdoor channels fed by a boreal forest stream in Quebec, Canada. 2. Significant effects of fish on the size-structure of the insect populations were observed on at least one of the two sampling dates for epibenthic and drifting animals, but never for those in the infauna. 3. The presence of fish significantly reduced the density of many epibenthic and drifting insects (maximum of 36% and 24% of the taxa, respectively), but this impact varied with time. In contrast, negative impacts on infaunal insects were few (< 11% of the taxa). Some small to medium-sized benthic Chironomidae and Ephemeroptera actually increased in the presence of fish. 4. In general, insects smaller than 2 mm were not significantly reduced by fish predation in either the drift or the epibenthos and can thus be considered to have occupied a size-refuge. The effects of fish on larger insects varied according to the taxon and the sampling date. 5. Overall, these results suggest that predation by brook trout can significantly reduce the density of large to medium-sized epibenthic and drifting insects, and that rock interstices can be effective refugia from brook trout predation. Since the infauna constitutes the largest proportion of the benthos, these results may explain the lack of fish predatory impact frequently reported in streams.