## 2 Materials and Methods  
### Measurement Apparatus  
In this study, we conducted a mid-water trawl survey with Deep Vision installed as part of a fishery acoustic survey using a scientific echosounder for major pelagic fish species in the coastal waters off Jeju Island, Korea, in October 2020 and July 2021, onboard R/V Tampu23, the fisheries scientific research vessel of the National Institute of Fisheries Science (Figure 1). The acoustic surveys of pelagic fish were carried out along a set acoustic transects in the surveyed area, and a fishery survey using mid-water trawls was conducted in parallel, to identify the schools of pelagic fish. The acoustic data for volume backscattering strength (SV, dB re 1 m\({}^{-1}\)) were collected using a Simrad EK80 scientific echosounder using 18, 38, 70, 120, 200, and 333-kHz split-beam transducers, which were mounted on the bottom of the ship, the hull depth of which was approximately 5.4 m. Prior to conducting acoustic surveys, the acoustic system was calibrated using a calibration sphere. A pulse length of 1024 ms and ping rate of 2 s were employed at all frequencies, with a constant ship speed of 9 knots between trawl surveys. For the purposes of the fishery survey, we conducted three 30 min mid-water trawls at a ship speed of approximately 4.7 knots using trawl gear consisting of a hand rope (97 m), net pendant (97 m), and codend mesh size (30 mm) (Table 1). During towing, fish school signals detected by the scientific echosounder were examined in real time and the towing depth of the mid-water trawl was altered to catch the fish according to the distribution depth of the fish schools (Figure 2). The fish thus captured were identified at species level, and the population, wet weight (to within 0.1 kg), and total length (to within 0.1 cm) were measured. In the case of a large catch numbers, the total catch was divided, and we obtained measurements from up to 100 individuals. After the fishery survey, a marine environment survey was conducted using CTD (conductivity-temperature-depth; Sea-Bird, SBE 911 plus). Deep Vision consists of a main body installed at the front of the codend of a mid-water trawl and a trawl deck network device. The main body consists of a stereo camera \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Trawl**} & \multirow{2}{*}{\begin{tabular}{c} **Date** \\ **(YYYY. MM. DD)** \\ \end{tabular} } & \multirow{2}{*}{\begin{tabular}{c} **Total** \\ **(Latitude/Longitude)** \\ \end{tabular} } & \multirow{2}{*}{\begin{tabular}{c} **End** \\ **(Latitude/Longitude)** \\ \end{tabular} } & \multirow{2}{*}{\begin{tabular}{c} **Mean** \\ **(Latitude/Longitude)** \\ \end{tabular} } & \multirow{2}{*}{\begin{tabular}{c} **Speed** \\ **(Lathitude/Longitude)** \\ \end{tabular} } & \multirow{2}{*}{\
\begin{tabular}{c} **Temperature (\({}^{\circ}\)C)** \\ **(Xnot)** \\ \end{tabular} } & \multirow{2}{*}{
\begin{tabular}{c} **Salinity (psu)** \\ **(Surface/Bottom)** \\ \end{tabular} } \\ \hline Haul 01 & 2020. 10. 19. & 32\({}^{\circ}\)53.8\({}^{\prime}\)/125\({}^{\circ}\)52.8\({}^{\prime}\) & 32\({}^{\circ}\)51.5\({}^{\prime}\)/125\({}^{\circ}\)52.4\({}^{\prime}\) & 104.2 & 4.4 & 22.08/16.92 & 32.74/34.40 \\ Haul 02 & 2020. 10. 21. & 33\({}^{\circ}\)04.8\({}^{\prime}\)/125\({}^{\circ}\)41.0\({}^{\prime}\) & 33\({}^{\circ}\)04.5\({}^{\prime}\)/125\({}^{\circ}\)44.4\({}^{\prime}\) & 107.6 & 5.1 & 22.87/16.74 & 33.72/34.55 \\ Haul 03 & 2021. 07. 17. & 33\({}^{\circ}\)15.0\({}^{\prime}\)/126\({}^{\circ}\)02.0\({}^{\prime}\) & 33\({}^{\circ}\)15.0\({}^{\prime}\)/125\({}^{\circ}\)58.6\({}^{\prime}\) & 110.3 & 4.6 & 28.30/14.93 & 27.38/34.05 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Details of the acoustic and mid-water trawl survey. Figure 1: Map showing the transect lines and trawling station in Western sea of Jeju Island. The black bold lines indicate the trawling locations; thin gray lines indicate the transect line for the acoustic survey.