Quantifying the Role of Woody Debris in Providing Bioenergetically Favorable Habitat for Juvenile Salmon

Andrew W. Hafs , Biology, Bemidji State University, Bemidji, MN
Lee R. Harrison , National Marine Fisheries Service, Long Beach, CA
Ryan M. Utz , National Ecological Observatory Network, Boulder, CO
Thomas Dunne , Bren School of Environmental Science and Management, University of California, Santa Barbra
We coupled hydrodynamic and bioenergetic models to assess the influence of habitat complexity generated by large woody debris (LWD) on the growth potential of juvenile Chinook salmon (Oncorhynchus tshawytscha) in a restored section of the Merced River. Simulations indicated how LWD diversified the flow field, creating pronounced velocity gradients, which enhanced fish feeding and resting activities at the sub-meter scale. Fluid drag created by individual wood structures increased under higher wood loading amounts, leading to a 5-19% reduction in the reach-averaged velocity. The reach-scale growth potential was asymptotically related to wood loading, suggesting that the river became saturated with LWD and additional loading would produce minimal benefit. In the scenario analyzed, LWD additions quadrupled the potential growth area available before that limit was reached. Wood depletion in the world’s rivers has been documented extensively, leading to widespread attempts by river managers to reverse this trend. However, systematic prediction of the effects of wood on fish growth has not been previously accomplished. We offer a quantitative approach for assessing the influence of wood on habitat potential for fish growth at the microhabitat and reach-scales. Additionally, results from a field validation of the drift feeding bioenergetics model will be presented.