Genetic identification of stocks of marine fish and shellfish

Marine stock-enhancement programs should use information on population structure to optimize enhancement strategies and to design and implement operations so as to protect the genetic character and diversity of existing wild stocks, thereby maintaining their productivity and evolutionary potential. Genetic methods of stock identification allow direct tests of reproductive isolation and gene flow among populations, and most use naturally occurring "marks" (= genotypes) and so avoid the cost and difficulty of applying physical tags. Protein electrophoresis (used to generate extensive population data for numerous species for over 20 yrs) requires fresh or frozen tissue samples, is relatively inexpensive, and resolves well-characterized Mendelian gene markers. Newer nuclear and mitochondrial DNA-based methods often reveal more genetic variation and have considerably less stringent tissue requirements. Their disadvantages, which include higher costs per locus, less mature technology, and a paucity of established databases, are diminishing as techniques develop. Restriction-enzyme analysis of mtDNA and length polymorphism analysis of nuclear mini- and microsatellites are particularly promising for stock identification. Genetic analyses of protein and/or DNA variation in marine species have revealed the existence of multiple species in many cases where only one was previously recognized, as well as the presence of numerous, reproductively isolated stocks in many fish and shellfish species. The review of selected allozyme and DNA studies of various species in the Australia-New Zealand and the southeastern U.S.-Gulf of Mexico regions and of Atlantic cod and chum salmon provides examples of the power and limitations of these genetic approaches and illustrates the varied levels and patterns of population subdivision exhibited by marine organisms.