During the deep-ocean water pumping in Rausu Town, Hokkaido, Japan, zooplankton were ltered using a strainer with 420 μm. We conducted a study based on the samples collected in 20072008, 2008‒2009, and 2022‒2023. The study aimed to evaluate two topics: the annual changes in the zooplankton community structure and the life cycle of the dominant copepod Metridia okhotsnsis. Our ndings revealed that the zooplankton abundance and biomass were lower in 20222023 compared to 2007‒2009. Additionally, the second abundant taxa in zooplankton and copepod communities varied between 2022‒2023 and 2007‒2009. However, the population structure, sex ratio, and gonad maturation of adult females of the dominant copepod M. okhotensis showed common seasonal patterns throughout the examined periods. These seasonal phenomena are considered to be a re ection of the life cycles of this species.

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