The population structure of North Pacific pollock is complex and scientific understanding is evolving. In Russia, West Bering Sea, Navarinsky, Sea of Okhotsk, and Sea of Japan pollock stocks are considered reproductively isolated from one another, albeit with some inter-migration between them, and are addressed separately from one another by the Russian authorities for purposes of stock assessment (Kotenev and Glubokov 2007); (Federal Fisheries Agency, Kamchatka Branch 2020). Russian authorities also recognize that there are fish that originate in the Japanese EEZ harvested in Russian waters (i.e., a separate Japanese Pacific stock) (Ovsyannikova, S.L. 2012).

Within these groupings, the degree of differentiation among various populations is nuanced. Complications in elucidating pollock population structure include life histories that entail different habitats for spawning, overwintering, and feeding; larval drift from one region to another; and different population dynamics at low and high abundance (Kotenev and Glubokov 2007); (Grant, W.S. et al. 2010).

This profile is structured around the Russian fisheries South Kurils assessment and management unit, which is split into Sea of Okhotsk and Pacific Ocean subzones, in an area of stock mixing. Sea of Okhotsk pollock are thought to be a main contributor to the South Kurils fishery—particularly fish from its southern subpopulation, which experienced considerable growth in catch in 2009-2012 in comparison with the preceding two decades (1990s and early 2000s) (Kim, Sen Tok 2012)(TINRO 2020). Fish from the Japanese Pacific stock are also harvested here (Ovsyannikova, S.L. 2012). Besides the migrating stocks, there are also localized spawning areas in the South Kurils, but they are not temporally stable (TINRO 2020). A separate assessment unit is considered in the Petropavlovsk-Commander zone of East Kamchatka and the North Kurils.