Influence of Riparian Vegetation on Posteruption Survival of Coho Salmon Fingerlings on the West-Side Streams of Mount St. Helens, Washington

Abstract

The 1980 eruption of Mount St. Helens impacted salmon streams on the west side of the mountain primarily by debris and mud deposits; depositions of ash and large wood were relatively minor disturbances. We examined factors related to juvenile coho salmon (Oncorhynchus kisutch) disappearance during the summer and winter months of 1981 and 1982. Correlations exist between the survival of anadromous fish, instream vegetative debris cover, and water temperature. Summer mortality was related to high stream temperatures and winter mortality to the lack of large organic debris. Recovery of riparian vegetation would reduce stream temperatures and cause debris to be retained. Tree growth data suggest trees will be tall enough to effectively shade the third-and fourth-order streams in 5-20 years, and that it will be 50-75 years before the trees are large enough to create organic debris structures when they fall into a stream. These results imply that management activities that promote large organic debris will enhance fish survival.