The Importance of Beaver Ponds to Coho Salmon Production in the Stillaguamish River Basin, Washington, USA

—The use of beaver Castor canadensis ponds by juvenile coho salmon Oncorhynchus kisutch and other fishes has been well established. However, the population-level effects on coho salmon resulting from the widespread removal of millions of beaver and their dams from Pacific Coast watersheds have not been examined. We assessed the current and historic distributions of beaver ponds and other coho salmon rearing habitat in the Stillaguamish River, a 1,771-km2 drainage basin in Washington and found that the greatest reduction in coho salmon smolt production capacity originated from the extensive loss of beaver ponds. We estimated the current summer smolt production potential (SPP) to be 965,000 smolts, compared with a historic summer SPP of 2.5 million smolts. Overall, current summer habitat capacity was reduced by 61% compared with historic levels, most of the reduction resulting from the loss of beaver ponds. Current summer SPP from beaver ponds and sloughs was reduced by 89% and 68%, respectively, compared with historic SPP. A more dramatic reduction in winter habitat capacity was found; the current winter SPP was estimated at 971,000 smolts, compared with a historic winter SPP of 7.1 million smolts. In terms of winter habitat capacity, we estimated a 94% reduction in beaver pond SPP, a 68% loss in SPP of sloughs, a 9% loss in SPP of tributary habitat, and an overall SPP reduction of 86%. Most of the overall reduction resulted from the loss of beaver ponds. Our analysis suggests that summer habitat historically limited smolt production capacity, whereas both summer and winter habitats currently exert equal limits on production. Watershed-scale restoration activities designed to increase coho salmon production should emphasize the creation of ponds and other slow-water environments; increasing beaver populations may be a simple and effective means of creating slow-water habitat. North American river networks contain numerous reaches dammed by beaver Castor canadensis, and the spatial distribution of beaver dams controls fundamental geomorphological and ecological processes (Rudemann and Schoonmaker 1938; McDowell and Naiman 1986; Johnston and Naiman 1990; Pollock et al. 2003). Of particular interest on the Pacific Coast of North America is the rearing habitat that beaver ponds provide for juvenile salmonids, most notably coho salmon Oncorhynchus kisutch. Coho salmon populations in parts of California, Oregon, and Washington are listed as threatened under the U.S. Endangered Species Act. When beaver impound streams by building dams, they substantially alter stream hydraulics in ways that benefit many fish species (Murphy et al. 1989; Snodgrass and Meffe 1998). However, early * Corresponding author: [email protected] Received August 13, 2003; accepted September 30, 2003 research suggested that beaver dams might be detrimental to fish, such as hindering fish passage for anadromous salmonids; therefore, until recently, it was common for fish managers to remove beaver dams (Salyer 1935; Reid 1952). It has also been demonstrated that beaver dams seasonally restrict movement of fishes (Rupp 1954; Gard 1961; Murphy et al. 1989; Schlosser 1995). However, more than 80 North American fishes have been documented in beaver ponds, including 48 species that commonly use these habitats, and the beaver ponds’ overall benefit to numerous fishes has been well documented (Pollock et al. 2003). Beaver ponds usually have slow current velocities and large edge-to-surface-area ratios, and therefore contain extensive cover and a highly productive environment for both vegetation and aquatic invertebrates; these conditions provide fish with foraging opportunities not found in unimpounded stream habitat (Hanson and Campbell 1963; Keast and Fox 1990). The slow water also means that energy expenditures for foraging are less than