Environmental and Genetic Influences on Stock Identification Characters

Characters used to identify fish stocks can be divided into three groups: those that are purely genetic, those that are purely environmental, and those that may reflect both genetic and environmental variation. Early studies characterized stocks on the basis of phenotypic variation in life-history, meristic, morphometric, and life history traits. These characters are quantitative genetic traits, typically controlled by many genes and affected by the environment in which those genes are expressed. They also are generally related to fitness and thus, molded by natural and sexual selection, they reflect local adaptation. Several molecular genetic techniques have been developed to directly examine genetic variation within and between groups (e.g., protein allozymes, mitochondrial DNA, and microsatellite DNA). These genetic markers usually are assumed to be neutral or nearly neutral to selection, although this is not always the case. Finally, environmental markers, such as the elemental composition of otoliths, have also been used to delineate stock structure in recent decades. These markers, signatures of the habitats or areas occupied at each life history stage, are usually thought to reflect purely environmental differences between groups of fish, although it has been suggested that genetic effects also may contribute to differences between individuals, stocks, or species. This chapter focuses on a comparison between the "traditional" approaches to stock identification using phenotypic characters, in particular meristic, morphometric, and life history traits, and the newer approaches using molecular genetic markers.