Effect of Harvest and Effective Population Size on Genetic Diversity in a Striped Bass Population

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A major factor that contributes to loss of genetic variation in natural populations is a small effective population size. In species with a complex life history that involves overlapping generations and delayed maturity, the impact of infrequent annual reproductive bottlenecks is likely to be small because effective population size is defined by the number of individuals contributing to a generation and not to a single year-class. The striped bass Morone saxatilis is a long-lived species with overlapping generations and age structure, whose recreational and commercial importance has made it a target of intense harvest. We analyzed allele frequency fluctuation among juvenile year-classes of the Santee–Cooper, South Carolina, population from 1990 through 1994 with three independently segregating polymorphic nuclear DNA loci to examine genetic drift and estimate the number of breeders each year. Significant fluctuations in allele frequencies among juvenile year-classes were observed, and most of the variation was attributed to a small number of parents in 1992. The potential impact of this year-class is likely to be low because 1992 was a poor recruitment year, and striped bass have multiple opportunities to breed. However, high adult mortality due to fishing may increase the impact of the 1992 year-class by decreasing the number of adult age-classes in this population. Thus, high exploitation in species with overlapping generations can reduce the long-term effective population size by abrogating the possibility of multiple breeding opportunities.