Quantifying the role of woody debris in providing bioenergetically favorable habitat for juvenile salmon

The habitat complexity of a riverine ecosystem influences the bioenergetics of drift feeding fish. 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 river that lacked large wood. 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 for the configurations we simulated. In the scenario we analyzed for illustration, LWD additions could quadruple the potential growth area available before that limit was reached for the configurations selected for demonstration. Wood depletion in the world's rivers has been documented extensively, leading to widespread attempts by river managers to reverse this trend by adding wood to simplified aquatic habitats. 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.