Northwest Fisheries Science Center

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Document Type: Journal Article
Center: NWFSC
Document ID: 1804
Title: A riverscape perspective of Pacific salmonids and aquatic habitats prior to large-scale dam removal in the Elwha River, Washington, USA
Author: S. J. Brenkman, J. J. Duda, C. E. Torgersen, E. Welty, G. R. Pess, R. Peters, M. McHenry
Publication Year: 2012
Journal: Fisheries Management and Ecology
Volume: 19
Issue: 1
Pages: 36-53
Keywords: dam removal, recolonization, riverscape, salmonid, snorkel survey, spatially continuous,

Dam removal has been increasingly proposed as a river restoration technique.  In 2011, two large hydroelectric dams will be removed from Washington States Elwha River.  Ten anadromous fish populations are expected to recolonise historical habitats after dam removal.  A key to understanding watershed recolonisation is the collection of spatially continuous information on fish and aquatic habitats.  A riverscape approach with an emphasis on biological data has rarely been applied in mid–sized, wilderness rivers, particularly in consecutive years prior to dam removal.  Concurrent snorkel and habitat surveys were conducted from the headwaters to the mouth (rkm 65–0) of the Elwha River in 2007 and 2008.  This riverscape approach characterised the spatial extent, assemblage structure and patterns of relative density of Pacific salmonids.  The presence of dams influenced the longitudinal patterns of fish assemblages, and species richness was the highest downstream of the dams, where anadromous salmonids still have access.  The percent composition of salmonids was similar in both years for rainbow trout, Oncorhynchus mykiss (Walbaum), coastal cutthroat trout, Oncorhynchus clarkii clarkii (Richardson) (89%; 88%), Chinook salmon, Oncorhynchus tshawytscha (Walbaum) (8%; 9%), and bull trout, Salvelinus confluentus (Suckley) (3% in both years).  Spatial patterns of abundance for rainbow and cutthroat trout (r = 0.76) and bull trout (r = 0.70) were also consistent between years.  Multivariate and univariate methods detected differences in habitat structure along the river profile caused by natural and anthropogenic factors.  The riverscape view highlighted species–specific biological hotspots and revealed that 60–69% of federally threatened bull trout occurred near or below the dams.  Spatially continuous surveys will be vital in evaluating the effectiveness of upcoming dam removal projects at restoring anadromous salmonids.

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Theme: Habitats to Support Sustainable Fisheries and Recovered Populations
Foci: Develop effective and efficient habitat restoration and conservation techniques.
Characterize habitat effects on ecosystem processes, ecological interactions and the health of organisms.