Population structure and dynamics of walleye pollock, theragra chalcogramma

The population biology of walleye pollock, Theragra chalcogramma, is described including its life history, population dynamics, genetic structure and metapopiilation structure. Walleye pollock is an important species in the ecosystems of the subarctic Pacific Ocean, and is one of the world's largest fisheries. The population dynamics of pollock is driven by recruitment, which is associated with environmental variability. Management of pollock stocks is based on harvests from large geographic regions. However, lumping stocks within these regions may be adverse to conservation and management goals. Historical genetic studies of pollock have produced some conflicting results and comprehensive genetic studies are needed. A summary view of genetic structure in walleye pollock to date suggests a pattern of geographic stock structure, with varying levels of gene flow between major regions. Phenotypic differences between stocks, elemental composition of otoliths and parasite studies indicate restricted mixing of juveniles and adults. Genetic differences appear between broad regions, but resolution beftveen adjacent stocks, especially within the eastern Bering Sea, is currently lacking. Recent studies indicate genetic differentiation among pollock in the Gulf of Alaska and Bering Sea, possibly resulting from reduced gene flow owing to larval retention mechanisms or strong natal homing. The global population of pollock does not fit into a strict metapopiilation framework, but some neighbouring populations may be considered as metapopulations. Whether there is either density-driven migration of strong recruitment cohorts, or population sinks, is controversial and more information is needed. Stock mixing problems can be best addressed by means of high resolution genetic techniques in conjunction with tagging and the use of natural environmental markers.