Population genetic structure in Atlantic Cod (Gadus morhua) from the North Atlantic and Barents Sea: contrasting or concordant patterns in mtDNA sequence and microsatellite data?

We summarize the results of studies of mitochondrial DNA sequence variation in Atlantic cod (Gadus morhua) from the North Atlantic and adjacent areas. Population genetic structures in the Northwest and Northeast Atlantic and Barents Sea differ markedly. Whereas all populations from the Northwest Atlantic are dominated by a single common genotype and show low haplotype and nucleotide diversity, samples from Norway and the Barents Sea support multiple genotypes at intermediate frequencies and show much higher genetic diversity. Replicate samples taken in the same year at a single location off Newfoundland, or in different years from the Gulf of Maine and Georges Bank, are as dissimilar as samples taken from opposite sides of the Laurentian Channel. Essentially none of the genetic variance in the Northwest Atlantic is attributable to subdivision among samples, whereas a substantial proportion is attributable to trans-Atlantic differentiation. Re-evaluation of comparable microsatellite data supports the conclusion of extremely limited genetic differentiation among populations in the Northwest Atlantic. The mtDNA data support the hypothesis that cod throughout the Northwestern Atlantic have experienced a bottleneck in population size relative to their source population in the Old World, and show that mtDNA haplotypes are sensitive markers of population structure.