Optimal reproduction in salmon spawning substrates linked to grain size and fish length

Abstract

Millions of dollars are spent annually on revitalizing salmon spawning in riverbeds where redd building by female salmon is inhibited by sediment that is too big for fish to move. Yet the conditions necessary for productive spawning remain unclear. There is no gauge for quantifying how grain size influences the reproductive potential of coarse-bedded rivers. Hence, managers lack a quantitative basis for optimizing spawning habitat restoration for reproductive value. To overcome this limitation, we studied spawning by Chinook, sockeye, and pink salmon (Oncorhynchus tshawytscha, O. nerka, and O. gorbuscha) in creeks and rivers of California and the Pacific Northwest. Our analysis shows that coarse substrates have been substantially undervalued as spawning habitat in previous work. We present a field-calibrated approach for estimating the number of redds and eggs a substrate can accommodate from measurements of grain size and fish length. Bigger fish can move larger sediment and thus use more riverbed area for spawning. They also tend to have higher fecundity, and so can deposit more eggs per redd. However, because redd area increases with fish length, the number of eggs a substrate can accommodate is maximized for moderate-sized fish. This previously unrecognized tradeoff raises the possibility that differences in grain size help regulate river-to-river differences in salmon size. Thus, population diversity and species resilience may be linked to lithologic, geomorphic, and climatic factors that determine grain size in rivers. Our approach provides a tool for managing grain-size distributions in support of optimal reproductive potential and species resilience.

Climate and topography control the size and flux of sediment produced on steep mountain slopes

“…Thus, future applications of the approach will contribute to new process-based understanding of hillslope weathering and erosion in steep landscapes. This, in turn, will permit more mechanistic understanding of grain size variations in channel networks (9,44) and thus reveal how geology, climate, and topography influence riverine habitats (45). Moreover, as shown here, our approach can improve understanding of the role of sediment supply in the feedbacks between climate, erosion, and tectonics that drive landscape evolution across sites where the origins of sediment can be traced.…”

Large-scale dam removal on the Elwha River, Washington, USA: River channel and floodplain geomorphic change

“…By summer 2013, grain size on the main channel bed was dominantly granule and pebble material (Fig. 8), which, although still evolving, represented an increased proportion of anadromous fish spawning habitat compared to the too coarse cobble bed before dam removal or the too fine mud and sand bed of 2011-2012 (Kondolf and Wolman, 1993; see also Riebe et al, 2014).…”

Multiscale hydrogeomorphic influences on bull trout (Salvelinus confluentus) spawning habitat

“…Adfluvial bull trout in the Flathead Lake and River system are among the largest in their native range (Fraley and Shepard 1989), however. This suggests that the link between body length and spawning gravel size selection that has been established for anadromous salmonids (Kondolf and Wolman 1993;Riebe et al 2014) can be ambiguous (see also Roni and Quinn 1995).…”

“…Geomorphic factors constraining the abundance of potential salmonid spawning habitat, from patch to basin scales, include streambed grain size, flow and sediment regimes, stream gradient, hydraulic roughness, and the mobility of spawning substrates (e.g., Kondolf and Wolman 1993;Buffington et al 2004;Moir et al 2009;Riebe et al 2014). Valley confinement, as a result of its effects on channel migration, high-flow hydraulics, riparian vegetation, and groundwater dynamics, produces distinct geomorphic process domains (Montgomery 1999) that affect salmonids (Bellmore and Baxter 2014).…”