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Managing for Resilience:

Life history diversity in Skagit River Chinook salmon and its consequences for population ecology and management

Eric Beamer, Skagit River System Cooperative

Correigh Greene, Northwest Fisheries Science Center

Case Study: Skagit River

We discuss resilience in the context of life history diversity in Chinook salmon of the Skagit River. The Skagit River supports the largest population of wild Chinook salmon in the Puget Sound ESU, despite historic overharvest and habitat loss that has threatened the watershed’s six spawning populations. Nevertheless, these populations have retained much of their historical variation. This variation exists in adults (five age classes) as well as in juveniles (four classes: fry migrants, delta fry, parr migrants, and yearlings). In our presentation, we examine the possible causes of variation in juvenile life histories, and present evidence that life history variation is a product of genetics, environmental variation, and density-dependent behavior. Using our knowledge of Skagit River Chinook salmon, we examine how life history variation influences population dynamics via a stochastic life cycle model in which the degree of both juvenile and adult life history variation is manipulated. Our results indicate that life history variation presents a flip side to the concept of resilience, allowing populations to grow at faster rates despite significant environmental insults, but also resulting in much greater variation in growth rate. This variation could cause a population to swing below critical thresholds during extended periods of high mortality. Our life cycle approach allows us to examine how management actions can improve life history diversity within the Skagit watershed. However, limited local understanding and interest in life history diversity may present challenges for implementing management actions that best facilitate resilience in Chinook salmon.

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