Timber harvest intensifies spawning-salmon disturbance of macroinvertebrates in southeastern Alaskan streams

Abstract

We addressed how timber harvest can interact with salmon (Oncorhynchus spp.) spawning activities to influence benthic macroinvertebrate communities in streams on Prince of Wales Island, Alaska. We predicted that spawning salmon would cause greater disturbance to macroinvertebrates in streams in watersheds with high than with low timber-harvest intensity because finer sediments that accumulate in streams after timber harvesting would be readily dislodged by spawning salmon and lower volumes of large wood in the streams would decrease availability of macroinvertebrate refugia from salmon activity. We used a modified Hess sampler to collect benthic macroinvertebrates from 6 riffles in each of 7 streams before and during the annual salmon run. Diptera biomass was lower and Plecoptera biomass was higher during the salmon run than before it. During the run, insect biomass, scraper biomass, and Ephemeroptera biomass were higher in streams with low than with high timber-harvest intensity, possibly because the finer sediments in the high timber-harvest intensity streams were more readily dislodged by salmon. Macroinvertebrate community structure (density and biomass) differed between before and during the run. Epeorus longimanus, Baetis, Seratella tibialis, Suwallia, Chironomidae, and Simuliidae were significant indicators of before-salmon benthic communities, whereas Sweltsa and Zapada cinctipes typified during-salmon communities. Some taxa probably are more tolerant of salmon disturbance than others, and their life histories might be adapted to the autumn salmon run. Overall, our results indicate that strong interactive effects can occur between anthropogenic activities and natural disturbance and that timber-harvest activity can intensify the effects of spawning-salmon disturbance on macroinvertebrates.