The aim of this study was to assess seasonal variation in the food-web structure of fish assemblages in the East (two sites) and the South (one site) Seas of Korea, and to compare the isotopic niche areas between the regions. To do this, we analyzed the community structures and the δ¹³C and δ¹⁵N values for fish assemblages, and their potential food sources collected during May and October 2020. There were spatial differences in the diversity and dominant species of fish assemblages between the two seas. The fish assemblages in the South Sea had relatively wide ranges of δ¹³C and δ¹⁵N (−22.4‰ to −15.3‰ and 7.4‰ to 13.8‰, respectively) compared to those (−22.1‰ to −18.0‰ and 9.8‰ to 13.6‰, respectively) in the East Sea. The δ¹³C and δ¹⁵N values of suspended particulate organic matter, zooplankton, and fish assemblages differed significantly among sites and between seasons (PERMANOVA, p < 0.05, in all cases). Moreover, isotopic niche indices were relatively higher in the South Sea compared to those in the East Sea. Such differences in food-web characteristics among sites are likely due to the specific environmental effects (especially, major currents) on the differences in the species compositions and, therefore, their trophic relationships. Overall, these results allow for a deeper understanding of the changing trophic diversity and community structure of fish assemblages resulting from climate variability.

The spatial and temporal distribution of euphausiid krill Euphausia pacifica (Crustacea: Malacostraca) and of fish schools were observed along acoustic transects at the southwestern East Sea coastline of Korea. Two-frequency (38- and 120-kHz) acoustic backscatter data were examined from April to July 2010. A decibel identification window (S_{V 120–38}) and school detection algorithm identified Euphausia pacifica and fish schools in the acoustic backscatter, respectively. The E. pacifica was regularly observed in middle of southern waters, where phytoplankton was abundant during spring, and irregularly during summer, when phytoplankton was homogeneously distributed. Using the distorted-wave Born approximation model, the acoustic density we calculated of E. pacifica was higher in spring (April: 75.9 mg m⁻², May: 85.3 mg m⁻²) than in summer (June: 71.4 mg m⁻², July: 54.1 mg m⁻²). The fish schools observed by the acoustic data tended to significantly increase from spring to summer. Although major fish species, such as anchovies and herring, fed on copepods and euphausiids in the survey area, the temporal and spatial distribution of E. pacifica was weakly correlated with the distribution of the fish schools. These findings aid in our understanding of the temporal and spatial distribution dynamics of euphausiids and fish schools in the food web of the coastal southwestern East Sea.

Pacific herring (Clupea pallasii Valenciennes, 1847) is a commercially important species that inhabits the coastal waters of the North Pacific from Korea to California, USA. This study analyzed the target strength (TS; dB re 1 m²) of Pacific herring individuals (n = 14, total length (L_T) = 21.3–32.3 cm) at 38 and 120 kHz using ex-situ measurements and the Kirchhoff-ray mode (KRM) model. The least-squares regressions of the TS–L_T relationship for the ex-situ measurements were TS_38kHz = 20 log₁₀(L_T) − 70.10 (r = 0.17) and TS_120kHz = 20 log₁₀(L_T) − 70.59 (r = 0.10). The least-squares regressions for the KRM model were TS_38kHz = 20 log₁₀(L_T) − 68.39 (r = 0.40) and TS_120kHz = 20 log₁₀(L_T) − 69.74 (r = 0.49). The b₂₀ value of the KRM model was 1.71 dB higher than that of the ex-situ measurement at 38 kHz but similar at 120 kHz. These results provide basic data to evaluate the distribution and abundance of Pacific herring using fisheries’ acoustic technology.

Acoustic surveys were used to locate coastal Pacific herring (Clupea pallasii) spawning grounds, and the spatial and temporal patterns of their spawning in Jinhae Bay, Korea were examined. We deployed mooring with a newly designed autonomous echosounder for ~70 days during the Pacific herring spawning season in Jinhae Bay, from January to April 2018. At the same time, ship-based acoustic surveys were conducted to identify the spatial distribution twice, at 38 and 120 kHz, onboard the fishing vessel in January and April 2018 in the bay. Fish school signals, including those from adult Pacific herring, are often detected through ship-based acoustic surveys in January, from outside the bay. In the spring, weak scattering signals from fish larvae and zooplankton were continuously detected inside the bay. Backscatter at the mooring in the center of Jinhae Bay was low from mid-January to early March, gradually increasing to higher levels until the end of March. The backscatter observed from the mooring correlated well with ship-based acoustic surveys in the center of the bay. This study proposes that the mooring type acoustic echosounder is a valuable tool for temporal abundance information and other aspects of fish behavior.