Ichthyoplankton of the shelf waters of southeastern Sakhalin and Aniva Bay

Fish eggs and larvae are a crucial component of the meroplankton community and, during periods of mass development, significantly supplement the food base of planktivorous organisms. In coastal waters, they serve as an important food source for juvenile salmon that have migrated to the sea. The Sakhalin branch of the Russian Federal Research Institute of Fisheries and Oceanography has developed and is implementing a program to study the food supply of Pacific salmon. This program has included ichthyoplankton collections in the confluence areas of salmon rivers over several years. This report is based on data collected from 2018 to 2022 at selected test sites off southeastern Sakhalin and in the eastern part of Aniva Bay (See Fig. 1). Ichthyoplankton sampling was conducted from May to October following a standardized station scheme. At the water’s edge, sampling involved filtering 100 L of water through a plankton net. At isobaths of 5, 10, and 20 m, the net was towed vertically from the bottom to the surface using a conical net with a 50 cm diameter opening, 0.35 mm mesh size, and a 3 m filter cone length. A total of 519 ichthyoplankton samples were collected and processed. To understand the general distribution of eggs and larvae of walleye pollock Gadus chalcogrammus (Pallas, 1814) and Bering flounder Hippoglossoides robustus (Schmidt, 1904) observed in the coastal area, results from marine ichthyoplankton studies conducted in 2012, 2014, and 2020 were also utilized. During the research, eggs and larvae of 32 species and supraspecific taxa from 11 families were recorded (See Table 1). Along the southeastern coast of Sakhalin, the number of fish species decreased from south to north. The total contribution of cold-water high-boreal and Arctic-boreal species increased in the northern direction (See Fig. 2a). Demersal fishes - mainly littoral and sublittoral species (See Fig. 2b) - primarily formed the biotopic composition. Minimal species similarity was noted for the site in Aniva Bay compared to other sites along southeastern Sakhalin (See Table 2). The quantitative similarity of ichthyoplankton, based on averaged data, was low, indicating differences in the quality of the study sites. At the same time, seasonal values of the Chekanovsky-Sorensen index in some areas were very high (See Table 3). The highest number of ichthyoplankton species was generally recorded from May to mid-June (See Fig. 3a). During this period, increased values of abundance, biomass, and caloric content of fish eggs and larvae were observed in most areas (See Figs. 3b, 3c, 3d). Thus, the maxima of biomass and caloric content at all sites coincided with the mass migration of young Pacific salmon from freshwater. The data obtained at the study sites show significant differences in the structure of ichthyoplankton communities. Bottom sediments play an important role in their formation. To analyze the results of ship surveys based on the configuration of the coastline, shelf width, and hydrological features, the surveyed water area was divided into four sections: northern (53°-54°50'N), middle (51°-53°N), southern (south of 51°N latitude to Cape Terpeniya), and southeastern Sakhalin (from Cape Terpeniya to Cape Svobodnyy). In 2020, notable differences between these sections were observed in the distribution of pollock eggs, predominantly at stages I and IV. The similarity in the ratio of egg development stages in the two southern areas suggests the possibility of a single pollock spawning area there (See Fig. 6). Bering flounder is not as active a migrant as walleye pollock and is largely confined to specific areas where it forms local groups. In June 2020, only eggs at stage IV showed a clear distinction between the southern section of northeastern Sakhalin and southeastern Sakhalin, with 2.5% in the former area and 38% in the latter (See Fig. 8). Significantly higher total egg densities indicated the isolation of Bering flounder reproduction in southeastern Sakhalin. According to data collected from 2018 to 2022, water temperatures at coastal sites in May and June were several degrees higher (See Fig. 9). This difference in warming influences both the onset of fish spawning and the duration of embryogenesis. This was confirmed by the timing of the mass development of eggs and larvae at coastal sites in southeastern Sakhalin and at depths greater than 20 m, as well as by the ratio of egg development stages. The number of walleye pollock eggs and larvae in the coastal area decreased markedly from north to south, corresponding to the distribution of spawning grounds along the southeastern coast of Sakhalin. No such pattern was observed in Bering flounder. This distribution may result from independent spawning by separate flounder groups near the marine boundaries of the study sites. The article contains 9 Figures, 5 Tables, 38 References. the authors are grateful to colleagues from the Laboratory of Hydrobiology of the Sakhalin branch of VNIRO, who devoted a lot of effort to long-term sampling, often in extreme conditions, in the coastal water of southeastern Sakhalin. The Authors declare no conflict of interest.

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