In this study, we identified the echo signals of anchovies on echograms using the dB-difference method; additionally, the daytime and nighttime distribution characteristics of anchovies were determined. Using the difference in frequency characteristics of 18, 38, 70, 120, and 200 kHz, the dB-difference intervals of −2.1 dB ≤ ΔMVBS_18–38 ≤ 4.4 dB, −9.7 dB ≤ ΔMVBS_70–38 ≤ 1.3 dB, −10.2 dB ≤ ΔMVBS_120–38 ≤ 2.3 dB, and −10.7 dB ≤ ΔMVBS_200–38 ≤ 0.3 dB were set to extract the echo signals of anchovies. The results revealed that anchovies showed a relatively higher distribution in the coast than in the open sea. In February, the average density of anchovies was observed to be 3.43 g/m², whereas in May, it was observed to be 6.02 g/m². Also, anchovies were found to be mainly distributed at 30-70 m water depths during the day, and at 30–50 m water depths during the night.

In this study, the distribution of giant N. jellyfish by the echo counting method was investigated in the East China Sea, where giant N. jellyfish are known to breed and migrate to the coastal waters of Korea mainly in summer. In addition, the distribution densities by the survey method were compared with the sighting and trawl surveys. In the case of the East China Sea area and the Gijang coastal area, a split beam type scientific echosounder (EK60, Simrad, Norway) and Acoustic data at 38 and 120 kHz were collected while moving at 6–7 kts. In the coastal waters of Korea, acoustics data at 38 and 120 kHz were collected with a split beam type scientific echosounder (EK80, Simrad, Norway) attached to the bottom of the R/V Tamgu No. 21. In the East China Sea, the average distribution densities of acoustic, sighting, and trawl surveys were 8355.7 (10⁻⁶ ind/m³), 162.2 (10⁻⁶ ind/m³), and 792.5 (10⁻⁶ ind/m³), respectively. The average densities in the coastal waters of Korea of acoustic, sighting, and trawl surveys were 2238.7 (10⁻⁶ ind/m³), 664.9 (10⁻⁶ ind/m³), and 432.9 (10⁻⁶ ind/m³), respectively. The average distribution density of the acoustic survey conducted on 21 July 2020 in the coastal waters of Gijang was 1024.5 (10⁻⁶ ind/m³), and the sighting survey showed 48.8 (10⁻⁶ ind/m³). The sighting surveys conducted on 22 July 2020 were 393.3 (10⁻⁶ ind/m³) and 19.6 (10⁻⁶ ind/m³). The average distribution density for the acoustic survey performed on 23 July 2020 was 99.0 (10⁻⁶ ind/m³), and for the sighting survey was 197.2 (10⁻⁶ ind/m³). When comparing the results of the acoustic survey with the results of the sighting and the trawl surveys, all surveys except for the survey conducted on 23 July 2020 showed that the acoustic survey was higher than other survey methods.

Fisheries and Aquatic Sciences

In this study, we examined the interannual distribution patterns of the giant jellyfish Nemopilema nomurai and their relationship with marine environmental factors using field survey data collected from the East China Sea from 2020 to 2024. Acoustic surveys and jellyfish-specific trawl sampling were conducted to analyze their vertical distribution characteristics in relation to water temperature, salinity, seawater density, and chlorophyll-a concentrations. The results revealed that N. nomurai were mainly distributed in the mid-to-lower water layers at depths of 40–60 m, particularly in areas with strong stratification between the surface and bottom waters. Temperatures and salinity at these depths were relatively stable, and jellyfish were concentrated within high-density layers (1024.6–1025.0 kg·m⁻³). Correlation analysis revealed significant positive relationships between jellyfish occurrence frequency and salinity and seawater density, whereas no significant relationship was observed with water temperature. The chlorophyll-a concentrations varied between the years but did not directly correspond to the primary habitat depths of jellyfish, although synchronous variations in jellyfish abundance occurred in some years. These results indicate that the vertical distribution of N. nomurai is primarily controlled by physical oceanographic factors rather than by chlorophyll-a concentrations, reflecting an ecological adaptation for efficient energy use.

In this study, acoustic, sighting, trawl, and marine environmental surveys were used to determine the vertical distribution and density of giant jellyfish that have been observed in Korean waters over the past 3 years. From 2020 to 2022, annual surveys were conducted in May and July in the East China Sea and waters adjacent to South Korea. The acoustic data were processed by identifying and eliminating all signals considered as noise while excluding those suspected to be jellyfish signals. Subsequently, a single target detection method was employed. Giant jellyfish are distributed mostly in the middle and low layers. In May 2021, the average population density of giant jellyfish was recorded as 11.6 (ind./ha), which was the highest density. In July 2022, this value decreased to 1.7 (ind./ha), marking the lowest density. The sighting survey, which allows for the identification of jellyfish distributed in the surface layer, exhibited a difference of approximately 0.13 times compared to the acoustic survey conducted in the middle and low layers in 2020. In 2021 and 2022, this difference was approximately 0.11 times and 0.24 times, respectively. The average of this difference was 0.16 times or greater.