Population Dynamic Consequences of Cohort Resonance

Understanding the impact of environmental forcing and fishing on population variability is a fundamental challenge in fisheries management. Analysis of stochastic age-structured models has identified cohort resonance as a dominant feature of the response of several marine species to environmental variability. Cohort resonance consists of greater sensitivity to specific time scales of environmental variability: 1) low frequencies and 2) generational time scales. We first show that the cohort resonance pattern of sensitivity and population variability increases with fishing. We then show that salmon are more sensitive to environmental variability than longer-lived species, but show that sensitivity changes more rapidly with fishing for longer-lived species. We then describe efforts to detect cohort resonance in existing salmon populations by asking if the addition of hatchery fish each year reduces variability in salmon. We have identified an extreme example of cohort resonance in cyclic populations of sockeye salmon and describe the conditions for cyclic behavior. Finally, we describe the interaction of cohort resonance and the spectrum of environmental variability on extinction risk. Cohort resonance appears to be an important population dynamic mechanism, by which inter-annual variability in the abundance of spawners has a significant effect on variability in recruitment.