Shifting-balance stock structure in North Pacific walleye pollock (Gadus chalcogrammus)

High levels of gene flow are expected to produce genetic homogeneity among open-ocean populations of walleye pollock (Gadus chalcogrammus) because of few restrictions on migration. Although most genetic studies confirm this prediction, many surveys of morphology have detected differences among populations. Here, sequences of mitochondrial DNA cytochrome oxidase subunit I in nine samples (n ¼ 433) from Japan to Puget Sound were used to evaluate genetic population structure. Two haplotypes varied clinally across the North Pacific. These clines are likely the result of the isolation of populations in ice-age refugia, secondary post-glacial contact, and restricted long-distance dispersal. Overall, FST ¼ 0.030 (p , 0.001), but the greatest partition was attributable to differences between Asian and North American populations (FCT ¼ 0.058, p ¼ 0.036). Isolation by distance was detected across the North Pacific, but differentiation among populations within regions was minimal (FSC ¼ 0.007, p , 0.092). Climate variability on decadal – centennial scales produces shifts in local abundance, which prevent the appearance of genetically discrete stocks. These cycles of boom and bust, combined with high levels of gene flow, lead to different stock structures, as viewed with morphological, life history, and genetic markers.