State of plankton community in the Kuril waters under recent renewal of mass feeding northward migrations of subtropical fish

Abstract

State of plankton community was monitored in the Pacific waters at Kuril Islands in July-August 2016, in conditions of mass feeding migrations of subtropical fish. Structure of the plankton community corresponded to its summer season: the large-sized fraction of zooplankton prevailed by biomass (74-86 %), the portions of medium- and small-sized fractions were 3-13 % and 11-15 %, respectively. The dominant groups in the large-sized fraction were copepods ( Neocalanus plumchrus, Metridia okhotensis, Metridia pacifica, Eucalanus bungii ), arrowworms ( Sagitta elegans ) and hyperiids ( Themisto pacifica ). Copepoda nauplii and copepodites of I-II stage, Euphausiacea furcilia and caliphyptophytes, and juvenile hyperiids were found in the small-sized and medium-sized fractions. Total biomass of the large-sized fraction was significantly higher in summer 2016 than in July-August 2015, obviously because of earlier phase of seasonal succession under colder thermal regime. In 2016, the biomasses of copepods and sagittas were at the mean long-term levels, and the biomass of euphausiids was above the norm. The biomasses of the small- and medium-sized fractions were above the norm, too. The total stock of plankton in the surveyed area in July-August 2016 was assessed in 92926^. 10³ t. High biomass of zooplankton in this area was favorable for feeding of subtropical fish.