Spatio-Temporal Distribution of Giant Jellyfish (Nemopilema nomurai)

Abstract

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.