Differential response of marine populations to climate forcing

In searching for causes of fluctuations in marine populations, investigators often assume that populations respond on the same time scale as the environmental forcing period, but this may not hold true. Here we show how the response of populations to variable recruitment changes with the degree of overcompensation using models of two species with similar age structure but different density-dependent recruitment, chinook salmon (Oncorhynchus tshawytscha) and Dungeness crab (Cancer magister). For compensatory density dependence, as in chinook salmon, variability in recruitment tends to follow the period in environmental variability over all time scales. For overcompensatory density dependence, as in Dungeness crab, variability in recruitment follows the environmental variability only for periods much greater than the maximum age of the population. For periods in environmental variability less than the maximum age, the dominant period of the population response is slightly larger than the length of the age structure. Here, strong overcompensatory recruitment acts to filter out potentially good recruitment years, resulting in dominant periodicities slightly larger than the length of the age structure. These mechanisms appear to explain the differences between observed spectra of Dungeness crab and chinook salmon.