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The Pacific herring (Clupea pallasii) is one of the most important species in the commercial fisheries distributed in the North Pacific Ocean and the northeastern European seas. This teleost has marine and lake ecological forms a long its distribution in the Holarctic. However, the level of genetic differentiation between these two forms is not well known. In the present study, we used ddRAD-sequencing to genotype 54 specimens from twelve wild Pacific herring populations from the Kara Sea and the Russian part of the northwestern Pacific Ocean for unveiling the genetic structure of Pacific herring. We found that the Kara Sea population is significantly distinct from Pacific Ocean populations. It was demonstrated that lake populations of Pacific herring differ from one another as well as from marine specimens. Our results show that fresh and brackish water Pacific herring, which inhabit lakes, can be distinguished as a separate lake ecological form. Moreover, we demonstrate that each observed lake Pacific herring population has its own and unique genetic legacy.

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Pacific herring (Clupea pallasii) is an essential target of commercial fishing in the North Pacific Ocean. Previous studies have suggested the existence of marine and lake ecological forms of this species within its range. The lake ecological form of herring has a shortened life cycle, spending the winter and spawning in brackish waters near the shoreline without long migrations for feeding; it also has a relatively smaller body size than the marine form. Genetic-based studies have shown that brackish water Pacific herring not only can be distinguished as a separate lake ecological form but possibly has its genetic legacy. Here, as part of an ongoing study, using ddRAD-sequencing data for marine and lake ecological forms from a total of 54 individuals and methods of comparative bioinformatics, we describe genomic signatures of freshwater adaptivity in Pacific herring. In total, 253 genes containing discriminating SNPs were found, and part of those genes was organized into genome clusters, also known as “genomic islands of divergence”. Moreover, the Tajima’s D test showed that these loci are under directional selection in the lake populations of the Pacific herring. Yet, most discriminating loci between the lake and marine ecological forms of Pacific herring do not intersect (by gene name) with those in other known marine fish species with known freshwater/brackish populations. However, some are associated with the same physiological trait—osmoregulation.