Disturbance of Small-Stream Habitat by Spawning Sockeye Salmon in Alaska

Abstract

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 consequence of this mechanical disturbance, and periphyton biomass decreased in one stream. The postspawning rise in periphyton biomass in Bear Creek (larger sockeye salmon run) likely reflects nutrient release by decomposing sockeye salmon. Total benthic invertebrate and larval chironomid density decreased during spawning in one stream, but most invertebrate taxa in both streams did not exhibit clear spawning-related impacts. In contrast, invertebrate drift almost always increased during daylight hours when sockeye salmon were present, but it was difficult to separate mechanical effects of sockeye salmon acting via redd construction versus swimming. Total daytime drift density during spawning was four times greater in Bear Creek (4 sockeye salmon/m stream) and 2.5 times greater in Whitefish Creek (0.5 sockeye salmon/m stream), compared with pre- and postspawning values. In experimental enclosures, 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 kisutch consumed significantly more chironomid larvae during spawning in Whitefish Creek, apparently capitalizing on the increased chironomid drift caused by the spawning-run disturbance. Results are discussed in terms of the importance of salmonids to the stream ecosystems in which they spawn, with reference to benthic community structure and increased access to invertebrates in the drift by resident fishes.