Declining survival increases sensitivity of salmon populations to synchronizing environmental forcing through cohort resonance

Background/Question/Methods

A strong decline of Pacific salmon populations has been observed over the last two decades, even leading to the closure of fisheries for several consecutive years. Understanding what are the factors that have caused this decline is of particular interest if one wants to prevent these populations to go extinct. Because of the complexity of salmon life cycle, involving freshwater and marine components, the study of salmon population dynamics is a challenging issue. We develop here a a stochastic age-structured model to investigate the combined effects of environmental and anthropogenic forcing on the dynamics and persistence of Pacific salmon populations. We are particularly interested on the effects of cohort resonance phenomenon, first described by Bjornstad et al (1999, 2004), on the sensitivity of salmon populations dynamics to perturbations and the synchronization of the dynamics among several salmon stocks in a region. As a case of study we focus on the dynamics of two stream-type Chinook salmon (Oncorhynchus tschawytscha) runs: the Snake River spring/summer run and the Upper Columbia River spring run.

Results/Conclusions

The populations of study have declined dramatically since the 1970s and has been linked to the development of the Federal Columbia River Power System, which has involved the implementation of 8 dams all along the Columbia River. We show that during this decline these populations exhibit a cyclic dynamics of period 5-6 years which coincides with a Pacific Decadal Oscillation outbreak at the same period. We found that the sensitivity of the two salmon runs to stochastic forcing (either white noise or cyclic forcing) increases with a decrease in their survival rate, with greater sensitivity on time scales that are very long (i.e., slowly varying) and time scales near the generation time of the population. This pattern of variability corresponds to the cohort resonance phenomenon. We show that this phenomenon can be of crucial importance for the conservation of salmon populations since it leads to an increase in stocks dynamics synchronization and therefore an increase in sensitivity to further potential perturbations through the portfolio effect.