## 4 Discussion  
During the investigation period, we collected 22 species over the course of three mid-water trawl surveys, among which 19 species from 12,625 images were captured based on analyses of 118,064 Deep Vision stereo images. However, the remaining species, namely Japanese jack mackerel, Pacific rudderfish, and spearnose genadier, caught during the haul01 trawl were not identified using Deep Vision. This is essentially consistent with the findings of a previous study [17], in which 2 species (silvery put (_Gadiculus argenteus_) and Atlantic herring (_Clupea harengus_)) among 17 species caught in four mid-water trawls were not identified using Deep Vision. In this case, it was established that these two species were obscured from the camera's field of view by a school of blue whiting (_Micromesisius poutassou_) passing simultaneously through Deep Vision within the trawl. In the present study, we found than more than 50% of the populations of yellow croaker (87 individuals) and largehead hairtail (123 individuals) captured during haul01 were caught at a specific time, and that at other times, numbers of the three species (Japanese jack mackerel, Pacific rudderfish, and spearnose genadier) of less than five individuals were obscured from the camera's field of view. However, given that a larger number of individuals were captured from trawls (\(n=629\)) and Deep Vision (\(n=800\)) during haul02 and haul03, correspondingly higher proportions of individuals were identified from Deep Vision images. For haul01, the number of individuals from trawl catches was approximately 420% higher than that captured by Deep Vision. [PERSON] et al. [18] have reported that the number of polar cod (_Boreogadus saida_) captured using Deep Vision was lower than that obtained by trawling, as these fish passed through Deep Vision during the time in which the net was rising from  
Figure 6: Length comparison of yellow croaker (haul01) and Japanese jack mackerel (haul03) measured using two different methods. the trawled sea level to the survey vessel. Thus, we suspect that in the case of haul01 in the present study, some individuals moved to the codend whilst Deep Vision was being raised to the survey vessel. Contrastingly, the number of glowbelly recorded during haul02 was higher in Deep Vision (227 individuals) than in the trawl (168 individuals), which we assume to be indicative of the fact that some individuals escaped from the net between the body of the Deep Vision apparatus and the codend, given that the glowbelly is a small species with a total length of 4 cm or less. This would be consistent with the findings of [PERSON] and [PERSON] [17], who have reported that the trawl catches of some fish species were underestimated by 47%, owing to a loss of the individuals of species with body lengths of less than 10 cm during their passage to the codend. With respect to haul03, we found that large numbers of Japanese jack mackerel and largehead hairtail were captured in Deep Vision images compared with those captured by trawling. We believe these differences to be attributable to an error made in calculating the number of Japanese jack mackerel caught by the trawl, due to dividing the entire catch and not obtaining total catch measurements, whereas in the case of the largehead hairtail, numbers were underestimated, as some individuals caught in the net were not sampled while opening the codend and collecting catches. Differences in the total populations captured and photographed based on fish species analyzed in the three trawl surveys with installed Deep Vision ranged from 0% to 2000% (mean 321%). In contrast to many of the other fish species, club mackerel showed a significant difference (2000%), with one individual being recorded by Deep Vision and 21 from trawls, followed by skinnycheek lanternfish with 33 based on Deep Vision images and 610 from trawls (1748%). [PERSON] and [PERSON] [17] have also reported wide variations of between 0% to 1055% (mean 106.9%) on the basis of four trawl surveys.