Role of maturation and mortality in portfolio effects and climate resilience

The portfolio effect plays a critical role in population productivity and stability. Age structure of spawning salmon represents an example of portfolio effects such that the risks of experiencing unfavorable conditions are spread across time. However, the distribution of maturation ages for Pacific salmon (Oncorhynchus spp.) is increasingly concentrated into fewer and younger ages, which may impact population resilience to climate change. We explored the population dynamics of Sacramento River fall-run Chinook salmon (O. tshawytscha) under different age structure scenarios using a life-cycle model and compared two mechanisms that can underlie these changes--mortality and maturation. In addition, we tested whether age structure promotes resilience to drought. We found that high age structure diversity increased the stability of population size and harvest compared with low diversity. However, mean population size responded differently depending on the underlying mechanism. Reduced mortality of adult fish ages 4-5 increased escapement, whereas delayed maturation decreased escapement. Overall, high age structure diversity was able to buffer against the adverse effects of droughts by reducing the variability of population size and harvest compared with low diversity. Our results suggest that age structure promotes stability of salmon in an increasingly variable climate.