The Sea Journal of the Korean Society of Oceanography ISSN:1226-2978(Print) 2671-8820(Online) 한국해양학회지 바다

Using acoustic data collected in the western Indian Ocean, tropical tuna catch data from the Indian Ocean Tuna Commission, and Copernicus-based oceanographic data, this study investigated the relationships among the catch distributions of bigeye tuna, skipjack tuna, and yellowfin tuna, the distribution of prey organisms, and oceanographic conditions. The Nautical Area Scattering Coefficient (NASC) was calculated using 38 kHz acoustic data obtained from the R/V Isabu of the Korea Institute of Ocean Science and Technology in December 2021, and was compared with tropical tuna catches and temperature, salinity, and dissolved oxygen data. The results showed that NASC was relatively high in the 9°-12°S and 3°-6°S areas, whereas tropical tuna catches tended to be concentrated in the northwestern and equatorial regions. Although all three tropical tuna species showed high catches in the northern and equatorial regions, their spatial distributions differed among species. Skipjack tuna and yellowfin tuna were mainly concentrated north of 3°S, whereas bigeye tuna was also partially caught south of 9°S. The Spearman correlation coefficient between NASC and tropical tuna catches was very low and not statistically significant (ρ = 0.029, p = 0.172). This was considered to be attributable to the spatiotemporal mismatch between the acoustic and catch data, as well as the limitations of the sample-based catch data. The oceanographic analysis showed that the northern and equatorial regions, where tuna catches were concentrated, partially overlapped with areas characterized by relatively high temperature and dissolved oxygen concentrations at a depth of 34 m. These conditions showed some similarity to the preferred environmental ranges of tropical tunas reported in previous studies. In addition, some significant correlations were observed between NASC and oceanographic variables; however, the absolute values of the correlation coefficients were generally low, indicating weak relationships. This study provides a preliminary integrated comparison of acoustic data, tropical tuna catch data, and oceanographic data in the western Indian Ocean, and its findings are expected to serve as baseline information for future investigations of ecological linkages among tuna resources, prey organisms, and oceanographic conditions.