Abstract
Coastal Atlantic cod (Gadus morhua) in the Northeast Atlantic has seen a continuous decline since the industrialization of the coastal fishery, and management needs to address the spatial and temporal complexities of coexisting cod stocks. Toward that end, genetic analyses and oceanographic modelling of coastal and oceanic cod larval drift patterns were combined to elucidate the mechanisms responsible for an observed genetic cline over a >1500 km stretch along the coast of Norway. The results indicate that the north–south cline in coastal cod represents an extended contact zone between genetically divergent North Sea and Northeast Arctic cod and is maintained by two-way gene flow: by northward drift of pelagic eggs and larvae and by southward spawning migrations of Northeast Arctic cod. Computer simulations verify that the genetic cline can be established rapidly if gene flow into coastal populations is substantial. The shape of the cline, on the other hand, was found to be largely insensitive to the total amount of gene flow and therefore carries little information on extent of gene flow into and among coastal populations.
Highlights
A major role for genetic analyses of commercially important species is to uncover population structuring and aid management by providing information on the appropriate geographic partitioning of the resource for assessment and harvesting or protection (Ryman and Utter 1987; Waples et al 2008)
Most of the single-nucleotide polymorphisms (SNPs) that were genotyped are located within the four known chromosome inversions in the Atlantic cod genome
SNPs located within inversion tended to have correlated genotypes and be in linkage disequilibrium
Summary
A major role for genetic analyses of commercially important species is to uncover population structuring and aid management by providing information on the appropriate geographic partitioning of the resource for assessment and harvesting or protection (Ryman and Utter 1987; Waples et al 2008). This goal is challenging when, as is often the case for many marine organisms, for example, genetic patterns take the form of gradual trends or clines rather than distinct geographic patches (Spies et al 2015). A way forward in such situations is to try to clarify the mechanism(s) behind the genetic cline and thereby gain insights into patterns of population connectivity and isolation and use that insight to inform management
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