## 3 Results  
As a result of the survey of stationary net fishing performed twice in order to compare the data obtained from the echosounders in Jinhae Bay, 291 Pacific herring with a total weight of 42.18 kg were caught. During the first survey performed on 20 January 2018, 258 individuals were collected. The average fork length of the Pacific herring was 26.6 \(\pm\) 2.4 cm, and only adult Pacific herring longer than 22 cm were collected (Figure 3). Meanwhile, on 25 April 2018, which was after the spawning period of the Pacific herring, a total of 33 individuals (mean \(\pm\) standard deviation: 22.4 \(\pm\) 6.1 cm) were collected, which consisted of adult fish (26 individuals, 25.4 \(\pm\) 1.2 cm) and immature fish shorter than 20 cm (7 individuals, length distribution: 7.9-16.2 cm, mean \(\pm\) standard deviation: 11.2 \(\pm\) 2.8 cm). The bottom water temperature logged by the moored echosounder at a depth of 22 m was measured in the range of 5.11-11.68 \({}^{\circ}\)C (average, 7.47 \({}^{\circ}\)C) during the mooring period from 21 January to 31 March 2018 (Figure 4a). When the mooring was installed, the bottom water temperature was 7.9 \({}^{\circ}\)C on 21 January 2018, which decreased to below 5.5 \({}^{\circ}\)C by 15 February 2018. Since then, the water temperature had risen to 11 \({}^{\circ}\)C or higher by the end of March 2018. Figure 3: Pacific herring (_Clupea pallasiti_) length-frequency distributions in Jinhae Bay. Gray bars represent the length-frequency distribution in the two combined surveys. The black dashed and red solid lines indicate the length-frequency distribution during January and April 2018, respectively. High levels (\(>\)\(-\)60 dB) of SV mainly occurred from 5 m to 20 m between 7 March and 31 March. During winter (January to February), relatively low acoustic backscattering (\(-\)75 to \(-\)70 dB) was observed over the water column (Figure 4b). The fish signals were detected intermittently from 3 m to 15 m from January to February. From 7 March to 31 March 2018, however, the acoustic backscatter echo levels of small fish were measured at all water levels, and day and night movement patterns were also found. The data obtained from the moored echosounder showed that the average NASC to compare the relative fish density from the beginning of mooring was 14.60 \(\pm\) 5.66, which is a low level of acoustic backscatter (Figure 4c). After the fish school signals were detected from 7 March to 31 March 2018, the average NASC was 64.81 \(\pm\) 23.76, which is 4.44 times the average NASC (14.60 \(\pm\) 5.66) of the preceding 47 days (21 January to 6 March 2018). Figure 5 shows an example of acoustic data acquired from a ship-based echosounder by frequency. The survey of the transects obtained using the ship-based echosounder in January 2018, which is the winter season, detected many fish school echoes of \(-\)51 to \(-\)33 dB in the transects outside Jinhae Bay, at a low frequency of 38 kHz (Figure 5a). On the other hand, at a high frequency of 120 kHz, only weak signals of \(-\)60 dB or less were detected at the inner bay (Figure 5b). In spring, even at the outer bay, few acoustic signals from fish schools were detected at 38 kHz (Figure 5c), while acoustic signals of fish larvae and zooplankton in the range of \(-\)65 to \(-\)55 dB were received at 120 kHz at the inner bay, unlike the survey in winter (Figure 5d). Figure 4: Temporal variability in water temperature and acoustic backscatter at a mooring site: (**a**) bottom water temperature, (**b**) volume backscattering strength (SV) (dB re 1 m\({}^{-1}\)), and (**c**) nautical area scattering coefficient (NASC) (m\({}^{2}\) nmi\({}^{-2}\)). The light gray lines indicate the average of the 100-ping ensembles collected hourly; the black lines and circles show the daily averages in (**a**,**c**).