Biochemical Population Genetics of Pacific Herring (Clupea pallasi)

Studies of stock structure in Pacific herring (Clupea pallasi) using geographic variation in morphology and growth, and tagging methods, have shown that Pacific herring are subdivided into numerous more or less distinct populations or stocks having limited migration between stocks. Homing of adults to previous spawning areas and larval retention mechanisms may enhance reproductive isolation between spawning areas and have been postulated to produce genetic differences between stocks. In this study the geographic distributions of inherited biochemical markers were used to measure the genetic component of stock structure in Pacific herring. The gene products of 40 protein-coding loci were examined by starch–gel electrophoresis in 21 samples collected from locations extending over most of the range of Pacific herring in the North Pacific Ocean and Bering Sea. These results show that there is a very small genetic component to the stock structure described by nongenetic methods. No significant allele-frequency differences were detected among three samples of Asian herring. In the eastern Bering Sea, the northern populations were genetically distinct from southern populations on the Seward Peninsula and in Bristol Bay, but no genetic structure was detected within these areas. In the eastern North Pacific Ocean, the Gulf of Alaska populations were genetically distinct from the remaining southern populations. Some genetic structure was detected in the Gulf of Alaska but not among the remaining southern populations. Using the stepping-stone model of migration and the observed amount of genetic divergence among populations, we show that small-scale genetic differences would not be expected, given the amount of migration observed between spawning areas. Another important result of this study is the discovery of two genetic races, Asian–Bering Sea herring and eastern North Pacific herring, having an average Nei genetic distance (D) between samples of the two races of 0.039; D between populations within each race averaged 0.0009. We postulate that the two North Pacific races of herring arose as a result of repeated Pleistocene glaciation on the southern coast of Alaska, which created a barrier to gene flow and permitted the two oceanic groups to diverge genetically.