## 4 Discussion
In the present study, we suspect that the large difference between the numbers of skinnycheek lanternfish recorded using Deep Vision and from trawl catches of haul01 can be attributed to an error in estimating the number of individuals in the total catch by measuring only a few individuals, owing to the large catch size. Consequently, we believe that it would be necessary to increase the number of individuals measured in order to accurately compare the two survey methods. When comparing fish body lengths measured based on trawl catch and Deep Vision approaches, we found that largehead hairtail and glowbelly captured in haul01 and Japanese jack mackerel captured haul03 showed statistically significant differences (Table 3). In the case of largehead hairtail caught in haul01, it was difficult to precisely compare differences, as the number of samples measured using Deep Vision images (seven individuals) was notably small. In contrast, in the case of Japanese jack mackerel, the number of samples measured (Deep Vision 244 and mid-water trawl 100) was comparatively large, which is considered to be similar to numbers reported by [PERSON] [15]. These authors found that, based on a comparison of lengths, the two species Atlantic horse mackerel (_Trachurus trachurus_) and Atlantic mackerel showed a statistically significant difference in Deep Vision image and trawl catch analyses, which was attributable to the particularly small differences in length, owing to the large number of samples and the small length distribution range. During the present survey, 10 species were collected from haul01 and 6 species from each of hauls 02 and 03. In previous bottom trawl surveys conducted in the seas off Jeju Island [19, 20, 21], a variety of fish species were identified differing to those we recorded, which is believed to reflect differences in the additional species of benthic fish caught using bottom trawls. Among these species, Indian perch (_Jaydia lineata_), yellow croaker, yearnose grenadier, and glowbelly were also captured in the present study. In terms of the number of individuals per unit volume for different hauls, we found that skinnycheek lanternfish in haul01 at 4239 ind./km\({}^{3}\) (41.4%) and glowbelly in haul02 at 310 ind./km\({}^{3}\) (68.7%) were the predominant species. Skinnycheek lanternfish are mainly found at depths of around 100 m or more, although are known to occur at depths of between 80 and 130 m during the daytime and from 30 to 70 m at night [22]. In Tosa Bay, Japan, [PERSON] et al. [23] have analyzed glowbelly caught by commercial fishing boats at depths of between 100 to 300 m. Accordingly, given that the present survey was conducted by trawling at a depth of approximately 100 m during the daytime, we were, in all probability, sampling from a zone in which skinnycheek lanternfish and glowbelly are more likely to be captured. In haul03, Japanese jack mackerel was the predominantly recorded species with an estimated population of 1149 ind./km\({}^{3}\) (94.5%). In this regard, we believe that the collected mackerel had gathered for spawning, as Japanese jack mackerel inhabit depths of between 10 and 100 m [24] and the main spawning season of these mackerel in the seas around Jeju extends from March to July [25]. However, given that 50% of Japanese jack mackerel females are known to have a mature body length of 26.6 cm [25], it is considered that not all Japanese jack mackerel collected at this time were engaged in spawning. When analyzing the data, we established that there was a time difference between the acoustic signal of the scientific echosounder and the image captured by the Deep Vision installed on the mid-water trawl, depending on the installation distance, and that there is also likely to have been variations depending on the swimming ability of fish schools. When using only the depth data recorded by Deep Vision, errors may occur in determining the actual distribution depth of fish schools [26].