Genetic isolation by distance and localized fjord population structure in Pacific cod (Gadus macrocephalus): limited effective dispersal in the northeastern Pacific Ocean

Genetic population structure of Pacific cod, Gadus macrocephalus, was examined across much of its northeastern Pacific range by screening variation at 11 microsatellite DNA loci. Estimates of F-ST (0.005 +/- 0.002) and R-ST (0.010 +/- 0.003) over all samples suggested that effective dispersal is limited among populations. Genetic divergence was highly correlated with geographic distance in an isolation-by-distance (IBD) pattern along the entire coastal continuum in the northeastern Pacific Ocean (similar to 4000 km; r(2) = 0.83), extending from Washington State to the Aleutian Islands, and over smaller geographic distances for three locations in Alaska (similar to 1700 km; r(2) = 0.56). Slopes of IBD regressions suggested average dispersal distance between birth and reproduction of less than 30 km. Exceptions to this pattern were found in samples taken from fjord environments in the Georgia Basin (the Strait of Georgia (Canada) and Puget Sound (USA)), where populations were differentiated from coastal cod. Our results showed population structure at spatial scales relevant to fisheries management, both caused by limited dispersal along the coast and by sharp barriers to migration isolating smaller stocks in coastal fjord environments.