A riverscape perspective of Pacific salmonids and aquatic habitats prior to large‐scale dam removal in the Elwha River, Washington, USA

Abstract

Dam removal has been increasingly proposed as a river restoration technique. In 2011, two large hydroelectric dams will be removed from Washington State's 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 This article is a U.S. Government work and is in the public domain in the USA. 36 Fisheries Management and EcologyThis article is a U.S. government work, and is not subject to copyright in the United States.(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.

Polymorphic mountain whitefish (Prosopium williamsoni) in a coastal riverscape: size class assemblages, distribution, and habitat associations

“…abundance data were standardized as relative density (D r ) at both the unit and reach scales to account for variation in unit and bin length, and to reduce variance in the datasets (Brenkman et al 2012). At the unit scale, morphotype relative density was calculated for quantifying differences between habitat types:…”

“…At the reach scale, relative density was calculated in a similar fashion where f i = number fish per 1-km custom bin, l i =length (km) of custom bin, f t = total number of fish, and l t = total length (km) sampled. Positive and negative values of relative density indicated densities of mountain whitefish that were above and below the average density for the entire length of stream sampled, respectively (Brenkman et al 2012). Relative density was highly skewed (leptokurtic) at both the unit and reach scales, and therefore was transformed using the natural logarithm (x i + 1) for subsequent analysis (Zar 1984).…”

Evolving conceptions of the role of large dams in social-ecological resilience

“…Olympic National Park is a World Heritage Site and Biosphere Reserve (see Fig. 2) which brought more attention to how construction of the two dams had negatively impacted populations of salmon within the park (Brenkman et al 2012). Salmon, including chinook (Oncorhynchus tshawytscha), coho (O. kisutch), pink (O. gorbuscha), chum (O. keta), and sockeye (O. nerka), and steelhead (O. mykiss) had all but disappeared because they lost access to approximately 95% of the watershed due to the construction of the dams.…”

Identification of multiple genetically distinct populations of Chinook salmon (Oncorhynchus tshawytscha) in a small coastal watershed

“…It has become evident that genetic data can inform many decisions relating to management strategies, especially those that are aimed at maintaining abundance and genetic diversity in natural salmon populations (Olsen et al 2000;Eldridge and Killebrew 2007;Grandjean et al 2009;Brenkman et al 2012;Matala et al 2012). Without such information there is great risk of losing important life history variation that enables resilience of anadromous species to changing environments.…”

“…Contributing to evolution of these migratory patterns is their homing fidelity to natal spawning rivers that allow for reproductive isolation and development of unique evolutionary trajectories (Waples 2001;Quinn 2004). Anthropogenic activities including harvest, waterway development, hatchery production, and land use practices have altered salmon populations and their associated freshwater ecosystems; the result is often reduced life-history and genetic variability (Yoshiyama et al 1998;Brenkman et al 2012).…”