Genetic structure of leopard shark (Triakis semifasciata) populations along the Pacific coast of North America

• Significant population structure exists throughout the range of leopard sharks.
• There is a clear pattern of isolation by distance.
• Deep water is not a barrier to dispersal between island and mainland populations.
• Limited dispersal between California and less protected populations in Mexico

Poorly understood genetic structure throughout a species' range can impede the implementation of effective management strategies. Leopard sharks (Triakis semifasciata) are known to form seasonal aggregations along the California coastline, but little is known about broad-scale movement patterns once aggregations disperse. If aggregations represent isolated populations, increased protective measures of known aggregation sites may be required to reduce the risk of local extirpation. Five highly polymorphic microsatellite markers were used to analyze the genetic population structure of T. semifasciata. A total of 382 individuals were genotyped from six locations in California and one location in Mexico, spanning approximately 1800 km of coastline. In addition, the mitochondrial DNA control region was sequenced for five individuals in Mexico to compare to a previous analysis of T. semifasciata haplotypes found in California. Analyses of the genetic data show that T. semifasciata does not form one panmictic population and significant population structure is present. Pairwise FST tests and Bayesian clustering analyses show significant differentiation between northern and southern California populations (north and south of Point Conception, CA). Based on limited sampling, analyses of both microsatellites and mitochondrial DNA indicate that the Mexican population is genetically isolated from the California populations, making it more susceptible to local decline.