Frequency responses of age-structured populations: Pacific salmon as an example

Abstract

Increasing evidence of the effects of changing climate on physical ocean conditions and long-term changes in fish populations adds to the need to understand the effects of stochastic forcing on marine populations. Cohort resonance is of particular interest because it involves selective sensitivity to specific time scales of environmental variability, including that of mean age of reproduction, and, more importantly, very low frequencies (i.e., trends). We present an age-structured model for two Pacific salmon species with environmental variability in survival rate and in individual growth rate, hence spawning age distribution. We use computed frequency response curves and analysis of the linearized dynamics to obtain two main results. First, the frequency response of the population is affected by the life history stage at which variability affects the population; varying growth rate tends to excite periodic resonance in age structure, while varying survival tends to excite low frequency fluctuation with more effect on total population size. Second, decreasing adult survival strengthens the cohort resonance effect at all frequencies, a finding that addresses the question of how fishing and climate change will interact.

Changing central Pacific El Niños reduce stability of North American salmon survival rates

“…We used an age-structured, density-dependent salmon population model (45) to investigate how the observed increased correlation of coho and Chinook salmon survival rates could increase aggregate variability of simulated coho and Chinook salmon spawning abundance. Specifically, we investigated how aggregate variability of the spawning abundance of returning coho and Chinook salmon changes as survival becomes increasingly homogenous.…”

Synergistic effects of fishing-induced demographic changes and climate variation on fish population dynamics

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Demographic Diversity and Sustainable Fisheries

“…Understanding transient dynamics is important in the management of natural resources [18], [19], [20] because a large part of what we actually observe are transient dynamics. In this study, transient population dynamics are measured in two ways.…”