## 4 Discussion
Therefore, it is highly likely that the echoes detected in the mooring system will be mainly from Pacific herring larvae. The TS of herring at 70 kHz from 22.0 to 37.7 cm in January was calculated as \(-\)39.94 to \(-\)35.26 dB, and TS of adults collected from 23.6 to 29.1 cm in April was \(-\)39.10 to  
Figure 9: Echograms from the mooring highlighting the change in fish signals at the site. The records start at 0000, 0100, 0200, 0300, 0400, and 0500 h local time on (**a**) 21 January, (**b**) 1 February, (**c**) 11 March and (**d**) 21 March 2018, indicated and last 600 s.  
\(-\)38.30 dB [23]. The TS range of the smaller herring from 7.9 to 16.2 cm was \(-\)48.84 to \(-\)42.60 dB. These echo levels were higher than the SV echoes detected in the mooring system. The net used during the survey mainly collected adults and large herring larvae. Thus, the detected acoustic echoes were judged to be of smaller herring larvae than the caught herring larvae. We obtained acoustic echoes with various acoustic frequencies to interpret the spatiotemporal distribution of Pacific herring in the bay [27]. The ship-type acoustic survey was conducted using 38 and 120 kHz, and the acoustic frequency of the mooring system was 70 kHz. All three frequencies are mainly used for acoustic surveys, and 38 and 120 kHz acoustic data are used to identify fish from zooplankton using the characteristic frequency [13,28]. Acoustic survey using 70 kHz frequency is mainly used for the detection of medium-size fish among general zooplankton such as Antarctic krill [29]. In data analysis, the MVBS technique was used to remove zooplankton echoes within the 38 kHz echoes [12], while fish echoes were removed from within the 120 kHz echoes [21]. The 70 kHz echoes obtained by the mooring system differed significantly from those of larval Pacific herring having swimbladders [23] and large copepods mainly distributed in Jinhae Bay [25], with the largest difference being more than 27 dB per single individual. Therefore, the purpose of acoustic data obtained by the ship-type acoustic survey is to detect adult herring moving to the spawning ground in Jinhae Bay and to identify the spatiotemporal distribution. The mooring-type survey focused on detecting the timing of herring larvae that hatched in the inner bay and moved into the open sea for migration. Although there are limitations in accurate length distribution and fish species identification by net and trawl survey, when using acoustic techniques, they can be interpreted through comparison with acoustic data, existing research results, and catch data. Although the average monthly surface water temperature increased with the transition of seasons, the water temperatures in March, April, and May 2018 were recorded to be 0.74-1.52 \({}^{\circ}\)C lower than that in the same period in 2016 and 2017 (Figure 8). This may have affected the spawning, growth, and migration time of larvae, including that of the Pacific herring. The results of the temporal and spatial distribution of organisms and fishing using the acoustic method were similar to those of the Pacific herring. Although previous studies found that larvae and immature fish would have already moved to the open sea by April, the results of the acoustic survey and collection data analysis indicated that the fish remained in Jinhae Bay. To investigate the timing of the complete movement of adult and immature Pacific herring to the open sea, it is necessary to carry out follow-up research and therefore, to keep the moored echosounder on the seabed for a longer duration. The spatiotemporal acoustic signals detected by the moored and ship-based echosounders during winter and spring in Jinhae Bay might not have been only from the Pacific herring but also from other cold-water fishes, including the Pacific cod. In addition, even an analysis using the difference in mean volume backscattering strength with multiple frequencies can produce some errors due to other fish species in the water layer.