Effects of Environmental Forcing on Age-Structured Populations: Northern California Dungeness Crab (Cancer magister) as an Example

Evaluation of potential environmental effects on recruitment requires knowing how environmental forcing can affect recruitment and subsequent catch, and how these effects depend on life history. General relationships between a time series of an environmental forcing variable, the resulting time series of recruitment, and the subsequent time series of catch or abundance are described here for a fishery in which environmental forcing affects density-dependent recruitment and the population is fished at a constant rate. For a stable population with overcompensatory, density-dependent recruitment, the recruitment time series will resemble the environmental time series except that frequencies near 1/(2 × mean age in the population) will be emphasized. The amount of selective emphasis will depend on the degree of overcompensation, harvest rate, and width of the cohort size distribution (in a size-selective fishery). For a population harvested at constant rate, the catch time series will resemble the recruitment time series except that lower frequencies will be emphasized. The degree of selective emphasis will depend on harvest rate and width of the cohort size distribution. Application of these results to Dungeness crab (Cancer magister) research has allowed analysis of combined effects of density dependence and environmental forcing. They show (1) that criteria formerly used to evaluate density-dependent recruitment mechanisms were too strict, (2) that female harvest could reduce harvest variability due to environmental forcing, and (3) how a nonlinear influence of spring wind stress in combination with density-dependent recruitment could cause the observed catch record. Implications for other crustacean populations are discussed.