## 4 Discussion  
The results of previous studies on the distribution of fish species by water depth showed that _Engraulis japonicus_ shoal was mainly distributed at a depth range of 12-50 m, and the volume scattering strength distribution of _Engraulis japonicus_ shoal was around \(-50\) dB. In addition, in another previous study, the volume scattering strength of the large _Engraulis japonicus_ shoal with a high level of aggregation was in the range of \(-44.0\) to \(-28.0\) dB, and the large _Engraulis japonicus_ shoal was distributed at a depth range of 10-75 m.  
Figure 10: Vertical distribution of herring. Figure 9: Vertical distribution of common squid. Furthermore, the previous study on the distribution of Atlantic herring (_Clupea harengus_) by water depth showed the distribution of the species in the range of 10-125 m. A previous study on the distribution of _Todarodes pacificus_ in the East Sea of South Korea showed the distribution of the species mainly at 60 m depth. Therefore, when comparing the results of this study with those of previous studies, it can be concluded that the distribution of species by depth was similar [14; 15; 16; 17; 18; 19]. The spatiotemporal distribution of each fish species was represented by extracting the Nautical Area-Scattering Coefficient (NASC m\({}^{2}\)/nm\({}^{2}\)) and is illustrated in the figure below, and the size of NASC was represented by the height of the bar graph. The results of the spatiotemporal distribution of _Gadus macrocephalus_ showed a higher level of distribution in the south of the area, and a higher level of distribution was identified in coastal waters than in the open ocean. In addition, _Gadus macrocephalus_ was distributed in the open ocean waters of Ulsan (Figure 11). When comparing the results of this study with the results acquired by using the pop-up electronic tag for the movement route of _Gadus macrocephalus_, the location of recovering the _Gadus macrocephalus_ pop-up tag in 2011 was 34.53\({}^{\circ}\) N, 128.49\({}^{\circ}\) E. Additionally, in the analysis result of the hydroacoustic survey, the identified location of distribution in the coastal waters was 35.5\({}^{\circ}\) N, 129.5\({}^{\circ}\) E. Furthermore, the location of recovering the pop-up tag in 2014 was 34.93\({}^{\circ}\) N, 128.84\({}^{\circ}\) E. In the analysis result of the hydroacoustic survey, the identified location of distribution in the open ocean was 35.5\({}^{\circ}\) N, 129.85\({}^{\circ}\) E. It was concluded that the movement route of _Gadus macrocephalus_ shown in the result of previous studies and the location of the distribution of _Gadus macrocephalus_ obtained using hydroacoustic survey were very similar. However, there is little research on _Gadus macrocephalus_ movement, and, thus, it is thought that further studies are needed to obtain data on various routes of _Gadus macrocephalus_ by year (Figure 12). Figure 11: Spatiotemporal distribution of pacific cod. The results of the spatiotemporal distribution of _Engraulis japonicus_ showed high aggregation of the species in the coastal waters and open ocean of Ulsan in the survey area and low aggregation in the adjacent waters of Pohang. In a previous study that identified the distribution of _Engraulis japonicus_ in the southern part of the East Sea, lower aggregation of the species was shown in coastal waters of Pohang, and high aggregation was observed in coastal waters and open ocean of Ulsan. Comparing the two sets of results, it is judged that the two results are considerably similar in many aspects [16]. However, the _Engraulis japonicus_ shoal is a migratory species and shows aggregation at various depths, and, thus, further research is thought to be necessary (Figures 13 and 14). Figure 12: The migration route of pacific cod [20]. Figure 13: Spatiotemporal distribution of anchovy. The spatiotemporal distribution of _Todarodes pacificus_ showed a high aggregation in the coastal waters and open ocean in Ulsan and a relatively low aggregation in the coastal waters of Pohang (Figure 15).