Life history diversity and contribution to population stability in adult Chinook salmon of the Willamette and Sandy rivers, Oregon

Life history diversity can buffer populations against environmental variability by spreading risks spatially and temporally. We surveyed five populations of wild spring Chinook salmon ( Oncorhynchus tshawytscha) in the Willamette and Sandy watersheds (Oregon) over 18–22 run years and identified 10–17 life histories contributing to adult returns. Variability in adult returns of individual populations was reduced by 23%–57% through asynchronous contributions of life histories because of differences in rearing environments and migratory behavior. Life histories remained largely unchanged from 1946 to 1951 in populations with access to historic habitats, but the age of smolts and returning adults was significantly younger in populations where dams blocked access. Portfolio effects increased when abundance was reduced in populations with relatively high initial abundance but not in populations where abundance was already low. In simulations, the loss of life history diversity with a subsequent reduction in adult returns diminished portfolio effects regardless of the initial size of the population. Long-term monitoring of populations is important for conservation and recovery, and should include measures of both diversity and abundance.