Differences in response to environmental variability across Pacific salmon species
Pacific salmon populations are of concern because they supply substantial ecosystem services, yet are highly temporally variable, with some being listed as endangered, while others support sustainable fisheries. This high temporal variability influences the persistence of single populations and the stabilizing effects of the portfolio effect. Recent theoretical work suggests that cohort resonance may play a particularly important role in the response of Pacific salmon to their variable environment. The cohort resonance effect predicts that age-structured populations are more sensitive to variability in their environments on both short time scales (near the mean reproductive age) and very long time scales. Here we address how differences in life histories (spawning age distributions) across Pacific salmon species affect their dynamical responses through the cohort resonance effect. We explore these questions by comparative analyses of the Jacobian matrices resulting from linearization of density-dependent age-structured salmon models over a range of age structures, focusing on the two largest eigenvalues.
Results show that sensitivity to environmental variability on short time scales increases at older central spawning ages. Furthermore, sensitivity to high-frequency environmental variability increases with narrowness of the spawning age distribution. Age two central spawning is an exception, as is spawning primarily at age one with a smaller fraction of age two spawners (pink salmon). Thus, cohort resonant magnification of sensitivity to high-frequency environmental variability is expected across Pacific salmon, but is predicted to be greater for species with narrow spawning age distributions and with increased ages of first reproduction. The unique dynamics of pink salmon populations warrant further investigation to predict cohort resonance effects on this species and species with similar life histories.