Appendicularia in the Bering, Okhotsk, Chukchi Seas and North Pacific and their significance for feeding of nekton

Significance of larvaceans (class Appendicularia) for plankton community and feeding of nekton in the Far-Eastern Seas and North Pacific is underestimated, this group of species is poorly represented in scientific literature. The total biomass of larvaceans is below the stocks of dominant groups in the large-sized zooplankton, as copepods, euphausiids, arrowworms, amphipods, and coelenterates, but accounted together with their shells (called «houses») they form a comparable stock. In the studied area, the class Appendicularia is represented by four species: widely distributed Oikopleura vanhoeffeni, O. labradoriensis, and Fritillaria borealis and F. sp. (perhaps F. pacifica) in the southern periphery of this area. Larger and more numerous oikopleurids dominate by both abundance and biomass and are presented in all size fractions of zooplankton, whereas fritillarids are presented mostly in the small-sized fraction. Larvaceans distribute mainly in the upper epipelagic layer (55–97 %), i.e. in the layer of their prey concentration; their density is the highest in the coastal zone with the depth < 50 m and decreases in deeper areas. They are a significant portion in the diet of many nekton species (41 out of 151 species in the Trofology database of TINRO), including basic commercial fishes, as pollock, salmons, herring, polar cod, mackerels, sardine and some others. Their mucus houses glowing at night, with the animal inside, whose tail vibrates constantly providing movement and nutrition, are attractive for many plankton-eaters. Appendicularia have a high occurrence in the food of all size-classes of nekton, though it decreases for larger-sized fish of such mass fish species, as walleye pollock and pink and chum salmons.

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