Disturbance of small-stream habitat by spawning sockeye salmon in Alaska

We examined how the spatially and temporally predictable disturbance caused by a spawning run of native sockeye salmon Oncorhynchus nerka temporarily affected benthic and invertebrate drift communities of two small streams in southwestern Alaska. Surface gravel size temporarily increased as a conseq uence of this mechanical disturbance, and periphyton biomass decreased in o ne stream. The postspawning rise in periphyton biomass in Bear Creek (large r sockeye salmon run) likely reflects nutrient release by decomposing socke ye salmon. Total benthic invertebrate and larval chironomid density decreas ed during spawning in one stream, but most invertebrate taxa in both stream s did not exhibit clear spawning-related impacts. In contrast, invertebrate drift almost always increased during daylight hours when sockeye salmon we re present, but it was difficult to separate mechanical effects of sockeye salmon acting via redd construction versus swimming. Total daytime drift de nsity during spawning was four times greater in Bear Creek (4 sockeye salmo n/m stream) and 2.5 times greater in Whitefish Creek (0.5 sockeye salmon/m stream), compared with pre- and postspawning values. In experimental enclos ures, total drift and drift by Ephemeroptera (mayflies), Hydracarina (water mites), and Plecoptera (stoneflies) were also significantly higher in the presence of swimming sockeye salmon. Juvenile coho salmon Oncorhynchus kisu tch consumed significantly more chironomid larvae during spawning in Whitef ish Creek, apparently capitalizing on the increased chironomid drift caused by the spawning-run disturbance. Results are discussed in terms of the imp ortance of salmonids to the stream ecosystems in which they spawn, with ref erence to benthic community structure and increased access to invertebrates in the drift by resident fishes.