Genetic differentiation, effective population size and gene fl ow in marine fi shes: implications for stock management

Abstract

Many commercially exploited marine fi sh and mol- lusc species exhibit no or a low degree of genetic differentiation in neutral marker genes. This lack of genetic differentiation, typically attributed to high degree of gene flow in marine environments, has sometimes supported the thinking that genetically indistinguishable stocks can be managed as being one panmictic population. Recent comparative studies of neutral marker gene and quantitative trait differentia- tion in a wide variety of taxa - including several ma- rine organisms - show that a high degree of genetic differentiation (as measured by Q ST ) in ecologically and economically important traits is a common place occurrence, even when the degree of differentiation in neutral marker genes (as measured by F ST ) is low or absent. In fact, among the empirical studies made so far, the outcome Q ST > F ST is pervasive. This ac- cords with the increasing evidence that natal homing and self-replenishment of local populations may be more common in marine habitats than previously an- ticipated. If so, the low degree of genetic differentia- tion in neutral genetic markers could be a simple con- sequence of the large effective population size (N e ) of many marine populations, effectively buffering them against differentiation due to genetic drift. However, genetic markers linked to parts of the genome under directional selection should readily diverge in al- lele frequencies especially when N e is high. In fact, several recent studies have discovered that such loci provide a way to differentiate among stocks undif- ferentiated in neutral marker genes. Hence, the study of adaptive rather than neutral genetic differentiation among fish and shellfish populations might provide practical tools for stock identification and thereby contribute to improved fi sheries policies.