Northwest Fisheries Science Center

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Document Type: Journal Article
Center: NWFSC
Document ID: 8372
Title: Seasonal variation exceeds effects of salmon carcass additions on benthic foodwebs in the Elwha River
Author: S. A. Morley, Holly J. Coe, L. S. Dunphy, J. J. Duda, M. L. McHenry, Brian R. Beckman, Mel Elofson, Sonny Sampson, L. Ward
Publication Year: 2016
Journal: Ecosphere
Volume: 7
Issue: 8
Pages: e01422
Keywords: benthic invertebrates, Elwha River, hydroelectric dams, Pacific salmon, salmon subsidies, nutrient limitation, periphyton, stable isotopes, Washington,

Dam removal and other fish barrier removal projects in western North America are assumed to boost freshwater productivity via the transport of marine-derived nutrients from recolonizing Pacific salmon (Oncorhynchus spp.).  In anticipation of the removal of two hydroelectric dams on the Elwha River in Washington State, we tested this hypothesis with a salmon carcass addition experiment.  Our study was designed to examine how background nutrient dynamics and benthic food webs vary seasonally, and how these features respond to salmon subsidies.  We conducted our experiment in six side channels of the Elwha River, each with a spatially paired reference and treatment reach.  Each reach was sampled on multiple occasions from October 2007 to August 2008, before and after carcass placement.  We evaluated nutrient limitation status; measured water chemistry, periphyton, benthic invertebrates, and juvenile rainbow trout (O. mykiss) response; and traced salmon-derived nutrient uptake using stable isotopes.  Outside of winter, algal accrual was limited by both nitrogen and phosphorous and remained so even in the presence of salmon carcasses.  One month after salmon addition, dissolved inorganic nitrogen levels doubled in treatment reaches.  Two months after addition, benthic algal accrual was significantly elevated.  We detected no changes in invertebrate or fish metrics, with the exception of 15N enrichment.  Natural seasonal variability was greater than salmon effects for the majority of our response metrics.  Yet seasonality and synchronicity of nutrient supply and demand are often overlooked in nutrient enhancement studies.  Timing and magnitude of salmon-derived nitrogen utilization suggest that uptake of dissolved nutrients was favored over direct consumption of carcasses.  The highest proportion of salmon-derived nitrogen was incorporated by herbivores (18–30%) and peaked 1–2 months after carcass addition.  Peak nitrogen enrichment in predators (11–16%) occurred 2–3 months after addition.  All taxa returned to background δ15N levels by 7 months.  Since this study was conducted, both dams on the Elwha River were removed over 2011–2014 to open over 90% of the basin to anadromous fishes.  We anticipate that as the full portfolio of salmon species expands through the basin, nutrient supply and demand will come into better balance.


 Write-up of salmon carcass placement conducted on Elwha River upstream and downstream of former Elwha Dam

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Theme: Habitats to Support Sustainable Fisheries and Recovered Populations
Foci: Develop effective and efficient habitat restoration and conservation techniques.