Identifying Life History Variation to Inform Recovery Planning for Upper Willamette River Chinook Salmon

Life history variability contributes to population production, stability and resilience and arises from differences in developmental pathways.  We are investigating life history variation in Upper Willamette River (UWR) spring Chinook Salmon to inform recovery planning for this ESA-listed stock.  Specifically, we have been analyzing adult and juvenile scale morphology, microchemistry and microstructure of otoliths, and screw trap data from outmigrating juveniles.  Multiple life history pathways have been identified, including a life history that rears and grows at exceptional rates in reservoirs. To what degree this life history and others are adaptive remains unknown, but the reservoir pathway results in part from blocked downstream passage at high-head dams.  More broadly, a continuum of outmigration timing and rearing habitat use can be found across UWR Chinook Salmon, consistent with variation observed range-wide, and phenotypic plasticity likely contributes to variation within and among populations. We are developing a modeling framework to estimate relative fitness of life history variants that will include predictions about how life history compostion responds to various recovery actions.  We propose that the pathway concept is useful for identifying life history types and continua, and for generating hypotheses about specific selective pressures that generate and maintain life history variation.