Estimating Coho Salmon Rearing Habitat and Smolt Production Losses in a Large River Basin, and Implications for Habitat Restoration

Abstract

To develop a habitat restoration strategy for the 8,270‐km2 Skagit River basin, we estimated changes in smolt production of coho salmon Oncorhynchus kisutch since European settlement began in the basin, based on changes in summer and winter rearing habitat areas. We assessed changes in coho salmon smolt production by habitat type and by cause of habitat alteration. We estimated that the coho salmon smolt production capacity of summer habitats in the Skagit River basin has been reduced from 1.28 million smolts to 0.98 million smolts (–24%) and that the production capacity of winter habitats has been reduced from 1.77 million to 1.17 million smolts (–34%). The largest proportion of summer non‐main‐stem habitat losses has occurred in side‐channel sloughs (41%), followed by losses in small tributaries (31%) and distributary sloughs (29%). The largest loss of winter habitats has occurred in side‐channel sloughs (52%), followed by losses in distributary sloughs (37%) and small tributaries (11%). By type of impact, hydromodification (diking, ditching, dredging) associated with agricultural and urban lands accounts for 73% of summer habitat losses and 91% of winter habitat losses. Blocking culverts on small tributaries account for 13% of the decrease in summer habitat and 6% of the decrease in winter habitat. Forestry activities account for 9% of summer habitat losses and 3% of winter habitat losses. Limitations of the analysis and implications for developing a habitat restoration strategy are discussed.

A new measure of longitudinal connectivity for stream networks

“…In contrast, 460 potadromous species can make use of habitats above and below the same barriers (assuming that sufficient habitat exists on each side of the barrier to sustain a separate population). The increased sensitivity of diadromous species is exemplified by widespread declines of diadromous species (Pacific Rivers Council 1993;Beechie et al 1994;Moyle 1995;Berkamp et al 2000), for which fragmentation loss of connectivity has been at least in part to blame. Nonetheless, the 465 impacts on potadromous fish (e.g.e.g., Moyle 1995;Muhar 1996;Dunham et al 1997;Fagan et al 2002;Morita and Yamamoto 2002) and aquatic invertebrates (Master 1990) are also widespread.…”

“…An advantage of the approach presented here is that it allows the evaluation of impacts on individual projects at thea riverscape scale of the entire river network that includes cumulative impacts of many small developments (e.g.e.g.,, Beechie et al 1994;Pringle 2003). As 535 relationships with structural connectivity and biological communities develop through further research in fluvial landscape ecology, a standardized method such as we propose here will allow comparison of multiple watersheds and will aid in identifying extirpation hotspots or areas in need of enhanced protective measures.…”

“…Methods to measure effects of multiple barriers at the extent of the entire landscape are in their infancy considered ; (Beechie et al 1994;Wheeler et al 2005). …”

“…Other structures, such as road culverts, impede the movement of aquatic species due to high current velocities, shallow depths, and vertical drops at outflows (Warren and Pardew 1998;95 Wheeler et al 2005). Though impacts of the latter typically occur at smaller extents, they cannot be marginalized given their pervasive nature (Beechie et al 1994). Furthermore, cumulative effects of multiple barriers are rarely considered (Beechie et al 1994;Wheeler et al 2005) and are poorly understood because methods are not available to measure their effects at the extent of the entire landscape are in their infancy.…”

Grays River Watershed and Biological Assessment, 2006 Final Report.

“…Kasahara and Wondzell (2003) found that in a model of a wide alluvial valley, removal of secondary channels reduced hyporheic discharge by 25%. Complex off-channel habitats also are important juvenile rearing habitats and have been associated with increased productivity (Beechie et al 1994). Bedform diversity associated with inchannel LWD also has been associated with salmon spawning habitat .…”