Effects of Watershed Characteristics, Stream Temperature, Condition, and Disease on Prespawn Survival in Upper Willamette River Chinook Salmon

Spring- and summer-run adult salmon migrate upstream and hold in tributaries for months prior to spawning. Consequently, there is concern that climate warming will increase the costs of holding and rates of prespawn mortality, and may hinder conservation efforts. Upper Willamette Valley Spring Chinook salmon are ESA-listed and the ESU is composed of populations returning to several tributaries. Tributary populations include natural production, hatchery production, and production from adults collected and transported above high-head dams ("outplanting"). Notably, annual prespawn mortality rates vary widely within and among tributaries (5-94%). In an effort to identify factors contributing to prespawn mortality and identify potential mitigation strategies, we monitored prespawn mortality in relation to initial adult condition, behavior, disease expression, and environmental conditions during holding in two outplant populations in 2008-2010. Within and among tributaries, prespawn mortality rates were associated with high stream temperatures, poor initial salmon condition, and expression of disease. The relative importance and interaction of these factors are not yet well understood, but we hypothesize that all potential causal pathways are strongly influenced by temperature. We are currently investigating the association between watershed-scale factors, thermal regime and prespawn mortality using existing habitat and prespawn mortality data, geologic and topographic maps, temperature data, and GIS analyses for 12 Willamette River spawning locations. Within sub-basin, we are using longitudinal temperature mapping to understand associations between watershed characteristics (underlying geology, slope, aspect, landcover, etc.), thermal regime, the distribution of potential thermal refugia within streams, and observed prespawn mortality rates. We are also comparing longitudinal profiles to adult movement behaviors in a subset of tributaries to test for evidence of behavioral thermoregulation in streams with temperatures that approach lethal limits. Collectively, these data will help determine which scale(s) have the largest control on stream thermal regime, the relative importance of temperature regime vs. other factors in the expression of prespawn mortality, and will aid in the identification of management actions to minimize prespawn mortality and mitigate anticipated effects of climate change.