Feeding and food supply of salmons in the Okhotsk Sea in autumn of 2018-2019

In the autumn months of 2018-2019, food spectra of salmons were quite diverse in the Okhotsk Sea. Amphipods prevailed in the diet of juvenile pink and chum salmon that corresponded to these zooplankton species abundance in the southern part of the sea; other significant portions of their diet were presented by euphausiids, copepods, arrowworms, pteropods, and oikopleura. Decapods, fish and squids were in the diet of masu, coho and chinook salmon. Portion of zooplankton was about 100 % in the diet of pink salmon fingerlings, but only 98 % in the diet of chum salmon fingerlings with 2 % portion of squids. Grazing by salmons was insignificant: 0.2-0.3 % of the total zooplankton biomass within the periods of observation (18 days in 2018 and 13 days in 2019), or 0.04-2.0 % for certain taxonomic groups, except of pteropods and oikopleura with the consumed portions of 4-7 % and 7-12 %, respectively. However, the daily consumption by salmons relative to daily production of some groups was significant in October, 2018 and reached 97 % for amphipods and > 200 % for euphausiids in the northeastern Okhotsk Sea (biostatistical areas 6-8) and 48 and 27 %, respectively, in the entire surveyed area. This ratio was twice lower in October, 2019, as 24 % for amphipods and 12 % for euphausiids. Daily consumption of copepods and arrowworms never exceeded 2 % of their daily production. The most favorable conditions for nekton feeding in fall of both years were observed in the northwestern Okhotsk Sea, and the worst conditions — in its northeastern part, with moderate conditions without deficit of prey in the central part (biostatistical area 9) and southern part (biostatistical areas 12, 13), where the biomass of forage plankton exceeded the biomass of nekton in 30-97 times in 2018 and in 56-76 times in 2019.

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