Life History Diversity and Resilience of Spring Chinook Salmon in the Willamette River Basin (Oregon)

Chinook salmon often have been classified into two life history types based on the age at which juvenile fish migrate as smolts to the ocean.  A study in the Willamette River Basin over the last 10 years has shown that spring Chinook may migrate as subyearling (ocean-type) or yearling (stream-type) smolts.  Within each of these life histories, however, juvenile Chinook exhibit a complex diversity of migratory and rearing pathways they may follow from the time they first emerge from the gravel to their migration to the Pacific Ocean. 

Emigration from spawning areas begins shortly after Chinook emerge from the gravel as fry and continues to the second spring when fish migrate as yearling smolts.  Recent evidence indicates some Chinook also migrate from spawning rivers as 2-year smolts.  Large numbers of newly-emergent fry migrate downstream in late winter and early spring to rear in the lower reaches of spawning rivers and in the Willamette River; a dispersal that may cover over 100 km within a few weeks.  Small numbers of juvenile Chinook emigrate from spawning areas through summer, followed by a large peak of migrants in late fall and a smaller peak of yearling migrants in late winter through spring.  Juvenile Chinook may rear in the Willamette River for as little as one month up to sixteen months or longer before emigrating from the basin.  These multiple size and age classes of fish require a diverse and connected suite of rearing habitats. 

This diversity of life histories reflects adaptations to variable freshwater, estuarine, and marine environments, and provides resilience to the population.  For example, emigration of newly-emerged fry likely evolved because it afforded access to historically productive habitat in the Willamette Valley, in contrast to nutrient-poor and cold water habitats in the spawning tributaries.   This life history type has persisted despite anthropogenic changes to the river (including channel simplification and altered hydrologic regime because of dams), suggesting it is a highly productive component of Willamette spring Chinook.  The proportion of returning adults with a subyearling or yearling life history is variable between years and among rivers within the Willamette Basin, indicating that a variety of life history tactics can contribute to overall survival and resilience of the population.  Maintaining this diversity by protecting and restoring diverse habitats will help to insure the continued persistence of Willamette spring Chinook.