Effects of chum salmon redd excavation on benthic communities in a stream in the Pacific Northwest

We studied effects of redd excavation by wild chum salmon Oncorhynchus keta on insect communities in a stream in the Pacific Northwest. During the salmon redd excavation, the total mean insect densities in the spawning reach decreased to 10.8-14.7% of their predisturbance values. Mean densities of the major taxa, mayflies Baetis spp. and Cinygmula spp., midges Orthocladiinae, stoneflies Paraperla spp., and black flies Simuliidae, in the spawning reach were 0.4-55.1% of the predisturbance values during redd excavation. Densities of these taxa in the spawning reach were 1 : 1-46.6% of densities in the nonspawning reach during salmon redd excavation. Twenty-nine days after redd excavation, the densities of Baetis spp., Orthocladimae, and Simuliidae in the spawning reach exceeded the values of the same taxa in the nonspawning reach. Statistical analyses revealed that the presence or absence of spawning salmon was significantly associated with the distances (or dissimilarities) among stream insect communities (reach X sampling date combination) and the densities of insect taxa (except for Simuliidae); salmon redd excavation reduced insect densities and altered insect community structure.