The Fish Community of the Upper Epipelagic Layer of Russian Pacific Waters during the “Renaissance” of the Japanese Sardine in the Early 21st Century

The interannual dynamics of integral parameters of the species structure of the upper epipelagic fish community in the Pacific waters off the Kuril Islands in 2019–2023 are discussed in the context of a new increase in Japanese sardine (Sardinops melanostictus) abundance. The separate position of trawl surveys conducted in 2004–2013 (before the period of mass expansion of subtropical migrants here: Japanese sardine and chub mackerel, Scomber japonicus) is shown by the multidimensional scaling method. During this period, the biomass was dominated by pink salmon (Oncorhynchus gorbuscha) and mesopelagic fish species of the families Myctophidae and Microstomatidae. After 2014 and up to the present, Japanese sardine and chub mackerel held the dominant positions in the fish community, with a decreasing role of the latter species. During the last 5 years of monitoring (2019–2023), the species structure of the pelagic fish community in the Russian Pacific waters did not change dramatically, but there was an increasing trend of uneven redistribution of total ichthyomass among all species of the community in favor of the Japanese sardine. The polydominance index decreased significantly from 4.85 ± 0.44 in 2004–2013 to 2.37 ± 0.21 in 2019–2022 (P < 0.05, t-test: 4.33), as well as the index of species evenness from 0.41 ± 0.04 to 0.15 ± 0.02 (P < 0.05, t-test: 5.25). The growth of Russian and foreign catches of Japanese sardine over the last 8 years indicates its “renaissance” in the early 21st century.