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

Abstract

a b s t r a c tThe 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.

River reaches with impaired riparian tree cover and channel morphology have reduced thermal resilience

“…Large wood is a significant catalyst for morphological change and has the potential to act as a self-restoration tool for degraded river channels (Osei, Harvey, & Gurnell, 2015). LWD recruitment improves hydrology and morphology by promoting processes that create natural features including pools, channel bars, and sediment storage (Elosegi, Daez, Flores, & Molinero, 2017;Pilotto, Harvey, Wharton, & Pusch, 2016;Wohl, 2017) and also has significant benefits for salmonids (Gustafsson, Greenberg, & Bergman, 2014;Hafs, Harrison, Utz, & Dunne, 2014). An approach that focuses on restoring natural riparian and physical processes will provide the most sustainable approach to climate change proofing of impaired temperate rivers.…”

Recent advances quantifying the large wood dynamics in river basins: New methods and remaining challenges

“…The understanding of large wood entrainment and motion allowed the first attempts to use models to simulate large wood transport. Without explicitly taking account of the influence of large wood on the hydraulics, computational fluid dynamics (CFD; i.e., one‐ or two‐dimensional (1‐D or 2‐D) hydraulic modeling) have been used, first computing the hydraulics and then using the results to calculate large wood mobilization and deposition [ He et al ., ; Merten et al ., ; Comiti et al ., ; Hafs et al ., ]. As an example, Merten et al [] applied the 1‐D HEC‐Ras model to estimate unit stream power, stage, mean water velocity, energy grade slope, and the hydrodynamic drag acting on wood pieces lying on the river bed.…”

Does variable channel morphology lead to dynamic salmon habitat?

“…Freshwater habitat is important to Pacific salmonid species as some spend one or more years rearing in river systems as juveniles (Meehan and Bjornn, 1991). Structural cover elements in stream channels, such as in-stream large wood (LW), also serve an important role in sheltering juveniles from predation and in reducing water velocities (Bjornn and Reiser, 1991;Hafs et al, 2014) such that LW quantities are often positively associated with juvenile salmonid abundance (Cederholm et al, 1997;Benke and Wallace, 2003;Pess et al, 2012). Metabolic efficiency and growth rate depends on stream temperature and availability of food sources (Becker and Genoway, 1979;Murphy and Meehan, 1991), in addition to dissolved oxygen and turbidity levels (Bjornn and Reiser, 1991).…”

“…Previous research highlights the connection between channel morphology and topography and larger-scale sediment supply dynamics, such as those imposed by channel-hillslope coupling (Hoffman and Gabet, 2007;Hassan et al, 2019). Due to a limited number of long-term datasets, habitat modeling studies typically focus on short timescales or on spatial aspects of habitat (Harrison et al, 2011;Cienciala and Hassan, 2013;Hafs et al, 2014;Carnie et al, 2016), or examine conditions over longer timescales but assume static morphology (Fabris et al, 2017). Several flume-based and dam removal studies have noted that variations in sediment supply lead to changes in channel bed topography, such as bar building and erosion (Lisle et al, 1993;Dietrich et al, 2005;Venditti et al, 2012;Major et al, 2017) and pool filling (Hoffman and Gabet, 2007;Major et al, 2017).…”