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.

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.

Fisheries and Aquatic Sciences

Fisheries and Aquatic Sciences

Assessing fishery resources is crucial for sustainable marine ecosystem management and the operation of fisheries. This study integrates trawl surveys, environmental DNA (eDNA) analysis, and scientific echo sounder techniques to analyze the fishery resource distribution of and seasonal variations in the East China Sea. Surveys were conducted in April, July, August, and November 2022, utilizing bottom trawl sampling, eDNA metabarcoding, and acoustic data collection. The results revealed temporal differences in species composition, with crustaceans dominating in terms of abundance and fish species in biomass. The integration of eDNA analysis provided broader species detection, including cryptic and pelagic species, while acoustic techniques enabled large-scale resource assessment. However, discrepancies between methods highlighted the need for methodological refinement. Dominant species exhibited seasonal variation, with Portunus trituberculatus prevailing in spring (April), Trachurus japonicus and Scomber japonicus in summer (July–August), and Pampus argenteus in late autumn (November). A comparative analysis revealed that eDNA is sensitive to pelagic and cryptic species, trawl surveys effectively detect demersal fish, and acoustics allow for broad-scale biomass estimation, highlighting the complementary value of method integration.

Nautical area scattering coefficient (NASC) data from small coastal acoustic surveys are strongly zero-inflated and spatially autocorrelated. We analysed 3324 integration intervals from 19 surveys in Jinhae Bay, Korea: May–September 2023 and March–September 2024–2025. Signals meeting a 38–120 kHz backscattering-strength-difference mask were treated as small pelagic fish and integrated at 38 kHz. Concurrent set-net and eDNA sampling showed a multi-species assemblage. Four predictor sets combining hydrographic, seasonal, spatial and topographic variables were compared using a two-part random forest under nested cross-validation with 1500 m spatial blocks. Zeros comprised 67.39% of observations. Hydrographic models achieved detection AUCs of 0.874–0.882 and positive-intensity log-scale R² values of 0.325–0.330, versus 0.701 and 0.062 for topography plus month. Drop-column analysis showed that surface salinity carried the unique hydrographic signal and that surface temperature was redundant with salinity and month; topographic variables added nothing once coordinates were included. Survey identity alone reproduced the full-model AUC (0.882 versus 0.885): spatial-block performance largely reflects discrimination between surveys, not local interpolation. Leave-one-survey-out and leave-one-year-out validation fell to 0.503 ± 0.122 and 0.541 ± 0.123. Detection was more predictable than intensity, but the apparent skill is largely between-survey; the model should not predict unobserved occasions or estimate abundance.