Large river habitat complexity and productivity of Puget Sound Chinook salmon

Abstract

While numerous studies have shown that floodplain habitat complexity can be important to fish ecology, few quantify how watershed-scale complexity influences productivity. This scale mismatch complicates population conservation and recovery strategies that evaluate recovery at regional or multi-basin scales. We used outputs from a habitat status and trends monitoring program for ten of Puget Sound’s large river systems to examine whether juvenile Chinook salmon productivity relates to watershed-scale habitat complexity. We derived habitat complexity metrics that quantified wood jam densities, side and braid to main channel ratios, and node densities from a remote sensing census of Puget Sound’s large river systems. Principal component analysis revealed that 91% of variance in these metrics could be explained by two principal components. These metrics revealed gradients in habitat complexity across Puget Sound which were sensitive to changes in complexity as a result of restoration actions in one watershed. Mixed effects models revealed that the second principle component term (PC2) describing habitat complexity was positively related to log transformed subyearling Chinook per spawner productivity rates from 6–18 cohorts per watershed. Total subyearling productivity (subyearlings per spawner) and fry productivity (subyearling fry per spawner) rates were best described by models that included a positive effect of habitat complexity (PC2) and negative relationships with log transformed peak flow recurrence interval, suggestive of reduced survival due to egg destruction during floods. Total subyearling productivity (subyearlings per spawner) and parr productivity (subyearling parr per spawner) rates were best described by models that included a positive effect of habitat complexity (PC2) and negative relationships with log transformed spawner density, suggestive of density dependent limits on juvenile rearing habitat. We also found that coefficient of variation for log transformed subyearling productivity and subyearling fry productivity rates declined with increasing habitat complexity, supporting the idea that habitat complexity buffers populations from annual variation in environmental conditions. Therefore, we conclude that our watershed-scale census-based approach provided habitat complexity metrics that explained some of the variability in productivity of subyearling juveniles among Chinook salmon populations. Furthermore, this approach may provide a useful means to track and evaluate aggregate effects of habitat changes on the productivity of Endangered Species Act (ESA) listed Chinook salmon populations over time.

Applying cumulative effects to strategically advance large‐scale ecosystem restoration

“…For species such as smelt and salmon, survival across a mix of habitats in multiple parts of the estuary, tidal river, and watershed must be considered in conjunction with flow management. For instance, habitat restoration in a Puget Sound watershed measurably increased habitat complexity, which in turn was positively associated with Chinook salmon productivity (Hall et al 2018), and reconnection of estuarine habitat on the Oregon coast resulted in greater life-history diversity for a population of coho salmon (Oncorhynchus kisutch; Jones et al 2014). Relationships between life histories and habitat connectivity add complex dimensions to restoration planning, monitoring, and evaluation (WebTable 1; Herbold et al 2018).…”

Influences of valley form and land use on large river and floodplain habitats in Puget Sound

“…Similarly, wood can be removed from channels for flood conveyance without substantially altering side‐channel or braid length. Because the combination of these metrics is related to salmon productivity in several watersheds (Hall et al, ), quantifying all five metrics offers a more complete habitat assessment.…”

“…The main channel was the flow path carrying the most water, braids were secondary wetted flow paths separated from other flow paths by gravel bars, and side channels were secondary wetted flow paths separated by vegetated islands (Beechie et al, ). Both braids and side channels contain important salmon and steelhead habitats, and strongly influence production of juvenile salmonids from rivers (Beechie et al, ; Hall et al, ; Whited et al, ). The number of side‐channel or braid channel nodes was the number of channels multiplied by two (one node where the side channel or braid separates from another flow path and a second where it rejoins).…”

“…Since the late 1800s, urban and agricultural land uses have removed significant areas of floodplain forest and reduced the extent of floodplain habitats for fishes (e.g., Beechie et al, 2001;Konrad, 2015). Habitat complexity and wood cover in river and floodplain habitats are important spawning and rearing habitats for both Chinook salmon and steelhead (Beechie, Liermann, Beamer, & Henderson, 2005;Hall et al, 2018;Pess, Liermann, McHenry, Peters, & Bennett, 2012;Whited, Kimball, Lorang, & Stanford, 2013).…”

Potential for ecological nonlinearities and thresholds to inform Pacific salmon management

“…Currently, available data, including previously published studies, can be used to explore salmon outcomes in relation to ecological conditions (Table 2). For example, emergent approaches such as satellite photo analyses allow us to quantify habitat conditions at vast scales commensurate with spatial extents of managed stocks, which we can compare to stock-level metrics of salmon performance (Hall et al 2018). In addition, oceanographic models of the North Pacific are proving skillful in explaining and predicting variation in survival and recruitment of marine fishes (Tolimieri et al 2018), and could be similarly aligned to different (Large et al 2013, Samhouri et al 2017.…”