Environmental forcing of stable age-structured populations with compensatory density dependent recruitment

Background/Question/Methods

There has been a tendency to explain behavior of marine populations such as cycles and increasing variability with fishing, in terms of populations that are deterministically unstable.  The potential explanatory power of populations that are actually stable, but constantly perturbed by environmental variability has been virtually ignored.

Results/Conclusions

The effects of environmental variability in growth and survival rates on age-structured populations with compensatory (hence stable) density-dependent recruitment can be characterized in terms of the frequency response of abundance, egg production, recruitment or catch time series to that variability.  The frequency response consists of two peaks, one at low frequencies, and one at the generational frequency of the species, a phenomenon referred to as “cohort resonance.”  Frequency selectivity becomes greater when adult survival is reduced, as in fishing or effects and environmental shift such as ocean acidification.  These effects are stronger in shorter-lived species, such as Pacific salmon.  An extreme form of this behavior provided an explanation of the cyclic dominant behavior of sockeye salmon (Oncorhynchus nerka) populations in the Fraser River, B.C. and Bristol Bay, Alaska.  Analysis of whether the addition of a constant number of hatchery fish to populations each year would reduce the coefficient of variation, thus reducing the detectability of cohort resonance showed that it would not.  The effect of increased adult mortality leading to stronger peaks at generational frequencies has been detected in Chinook salmon populations (O. tshawytscha).  Analysis of the effects of this strong frequency dependent sensitivity on probability of extinction has shown that populations are not necessarily more likely to go extinct when driven by environmental variability at generational frequencies.  These results demonstrate the possibility that many of the effects we may be attributing to unstable, nonlinear models may be due to stable models with environmental forcing.