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

TL;DR

This study models how environmental variability influences Pacific salmon populations, revealing that different life history stages respond uniquely to fluctuations, with implications for climate change and fishing impacts.

Contribution

It introduces an age-structured model analyzing frequency responses of salmon populations to environmental variability, highlighting the effects of survival and growth rate fluctuations.

Findings

Growth rate variability excites periodic resonance in age structure.

Survival variability causes low-frequency fluctuations in population size.

Decreased adult survival amplifies cohort resonance effects.

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.