Analysis of frozen cores of gravel surrounding the egg pockets of chum salmon, Oncorhynchus kern, collected in the fall revealed that spawning activity by females purged about 75%, of the fine sediments from the stream bed. The egg pocket was one of four distinct vertical strata detected in the cores. There was an undisturbed layer below the egg pocket, and separate bridge and cover strata above the egg pocket, all defined by different particle size distributions. However, by spring most of the egg pockets had been infiltrated with fine sediment and the particle size distribution approached background levels. The most likely physical factors responsible for these results were: (1) intrusion of fine sediments through the cleaned surface gravel, (2) lateral subsurface migration of fine sediment into interstitial voids, (3) scour of the surface gravel and subsequent deposition of a sand rich bedload: and (4) superimposed spawning activity of other fish, causing disturbance of the cleaned surface gravel and exposing the egg pocket to intrusion of fine particles. We conclude that, while female salmon substantially affect the physical environment of their embryos, subsequent sediment transport processes and fine bedload flux tend to return this environment to pre-spawning conditions.
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