The Okhotsk and Bering Seas and the subarctic zone of the North Pacific are surveyed well by the expeditions conducted by Pacific Fish. Res. Center (TINRO) in the last three decades (115 surveys in 1984-2013), with sampling of zooplankton for monitoring of feeding resources of commercial species. Mean total zooplankton biomass and biomasses of size fractions of zooplankton, its taxonomic groups and mass species are calculated, by biostatistical areas. General patterns of the biomass distribution between the biostatistical areas are determined and presented as the charts of the biomass averaged by the areas. The most abundant species of zooplankton with similar systematic and trophic status are spatially separated that allows them to reduce competition for food, for example the pairs of small-sized, medium-sized and large-sized Copepoda species ( Pseudocalanus minutus - Oithona similis, Metridia pacifica - M. okhotensis , Calanus glacialis - Neocalanus plumchrus ), the pair of Hyperiida species ( Themisto pacifica - T. libellula ) and four Euphausia species ( Thysanoessa raschii - Th. inermis - Th. longipes - Euphausia pacifica ) in the Okhotsk Sea. Detailed tables on biomass and stock of the main components of zooplankton are presented, by regions and biostatistical areas.
The most common regularities in spatial distribution of mass Euphausia species and their size classes and ontogenetic stages are determined for the Okhotsk Sea using the data averaging by biostatistical areas and applying the average results to the centers of the areas that allows to smooth small-scale patterns as patchiness. Thysanoessa raschii and Th. inermis occupy mostly shelf areas and Th. longipes and Euphausia pacifica - deep-water areas; among them Th. raschii and Th. longipes are more abundant and Th. inermis and E. pacifica are supposedly allochtonous species which penetrate to the Okhotsk Sea through the Kuril Straits, so they are distributed mainly in the southern part of the Sea. Mean biomass and abundance of all species in the upper (0-50 m) and deeper (50-200 m) layers are calculated for each stage and size class in the daytime and nighttime, separately: most of them, including both adults and furcilias, concentrate mainly in the deeper layer and their night biomass is higher in both layers though about a half of animals raise to the sea surface at night. Only eggs, nauplii and calyptopes of euphausiids don’t change their abundance considerably between day and night, and most of them occupy the upper layer round the clock. Obviously, the increasing of late furcilias and adults in the deeper layer at night is caused by their diurnal migration from the layers below 200 m. Seasons of the highest abundance for early stages come in natural order that is a sign of their short duration.
The BASIS expeditions in the Bering Sea were conducted by North Pacific Anadromous Fish Comission (NPAFC) in the 2003-2006 and 2007-2012, in relatively warm and relatively cold conditions, respectively. The ice cover of the Sea changed synchronously in its western and eastern parts and indicated the warm regime since 2001 to 2006 and the cold regime since 2006 till nowadays. The regime shift changed radically the structure of zooplankton community in the eastern Bering Sea: the dominance of small-sized and medium-sized zooplankton was replaced by the dominance of large-sized zooplankton, with abundant large species of euphausiids, hyperiids, copepods, and arrowworms. This restructuring was reflected in the diet of pacific salmon: the bulk of the pink, chum and sockeye salmons diet was presented by fingerlings of pollock, sand lance, and capelin, larvae of flounders and crabs, and fry of small demersal fish in 2003-2006, but by zooplankton as euphausiids, hyperiids, and pteropods since 2007. However, there weren’t so essential changes in the western Bering Sea, both in zooplankton structure and salmons diet; the year-to-year dynamics was significant here, too, but long-term tendencies were not observed.
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