Feasibility of Using Otolith and Scale Techniques to Characterize Life History Variation of Spring Chinook Salmon in the Willamette Valley

Pacific salmon populations have suffered declines and local extirpations in the Pacific Northwest over the last century as a result of physical and functional changes to their freshwater environments, yet persist in highly altered habitats.  Life history variation in Pacific salmonids  has been suggested as one factor that contributes to population resistance, persistence and resilience by reducing the demographic effects of anthropogenic perturbations to habitats. Recent analyses of screw trap data suggest that juvenile Chinook salmon life history strategies are variable within and among Willamette Valley Basin (WVB) populations originating from adults transported above dams for spawning, including traits that resemble both an ocean-type life history with subyearling emigration in summer or fall as well as a stream-type life history with yearling emigration the following spring.  Currently it is unknown whether juvenile life history types represent adaptive responses to high growth potential in reservoir habitats or result from seasonally constrained downstream passage of dams.  Additionally, relative survival to adulthood for different life history types is unknown, but is of management importance as action agencies implement the ESA Willamette Biological Opinion. We are using a multidimensional approach to resolve life history types and test the ability of otolith microstructure and microchemistry combined with scale morphological characteristics to accurately distinguish life history types of juvenile salmon with three primary goals:  1) to determine the degree of life history variability expressed in Willamette River subbasins (McKenzie and Middle Fork Willamette basins); 2) to quantify the size at freshwater emigration and the relative importance of main-stem Willamette River and Columbia River estuary habitat use; and 3) to evaluate the ability of scale and otolith analyses to resolve the natal stream, juvenile rearing habitat, and ocean entry timing for adult salmon of unknown origin.  Preliminary otolith microchemical analyses suggest individual variability in mainstem Willamette habitat use and size at freshwater emigration from throughout the Willamette sub-basins.  In the Middle Fork Willamette and McKenzie River sub-basins, water samples show distinguishable differences in alkaline earth elements between reservoir and natal rearing outplant streams that are conserved as otolith geochemical signatures. Differences in survival of individuals is likely a function of life history strategies expressed such as size or timing of out-migration.  Consequently, determining the juvenile rearing habitat and emigration ecology of these populations will enhance the understanding of the interaction between life history variation and anthropogenic disturbance and assist in developing appropriate management strategies.