## 4 Discussion  
The location where the underwater moored echosounder was installed is one of the sea areas where the eggs and larvae of the Pacific herring were collected at high densities in Jinhae Bay in 2015 and 2016 [22]. Analysis of the data from the moored echosounder revealed that the acoustic backscatter intensity after mid-March increased compared to the period from the end of January to the beginning of March, which was the initial mooring period (Figure 6b). Although it is difficult to make an absolute comparison of the acoustic data acquired from the moored and ship-based echosounders owing to the difference in frequencies, the results are similar to those of the spatial distribution survey conducted twice with the ship-based echosounder (Figure 5). The ship-based survey found strong signals, considered to be from fish alone, outside the bay in January, and signals matching the levels originating from small fish and zooplankton were detected inside the bay in April. The spatial distribution of the acoustic signals in the bay indirectly shows the general patterns and ecological characteristics of habitat changes according to the growth stage, where the adult Pacific herring migrates offshore for prey after spawning and hatched Pacific herring larvae grow and migrate from the coast to the open sea. As a result of sampling in April, the second fishing season, the catch was reduced to less than 15% of that in January, but both adult and immature fish were caught, unlike the case in January, when only adult fish were collected (Figure 3). The acoustic data and collection results indicate that the fish were likely to have already migrated further away from the mooring location after spawning, although the mooring was located in the middle of Jinhae Bay (Figure 1). Therefore, follow-up studies are required wherein the mooring location is moved or the mooring is performed in early winter. Pacific herring have a swimbladder [23], and echo data from fish with swimbladders may be distinguished from echo data from zooplankton or fish without swimbladders [24]. The length distribution of Pacific herring larvae collected from Jinhae Bay during winter was found to be 5-27 mm using an RN80 net in 2011 [9]. The acoustic target strength (TS; dB re 1 m\({}^{-1}\)) of Pacific herring at 70 kHz is between \(-\)72.8 and \(-\)58.2 dB from 5 to 27 mm, respectively [23]. On the other hand, the zooplankton community in Jinhae Bay is mainly composed of large copepods, _Acartia omorii_ and _Centropages abdominalis_, in winter and spring [25]. The adult prosome length of _A. omorii_ and _C. abdominalis_ was less than 2 mm in Jinhae Bay [25]. The contribution of these copepods to SV was negligible because of the low mean TS, which was reported to be below \(-\)100 dB for 2.57-mm-long copepods at 70 kHz [26]. Figure 8: Monthly annual variations of the averaged surface water temperature retrieved by the Korea Ocean Observing and Forecasting System of the Korea Hydrographic and Oceanographic Agency in Jinhae Bay from 2015 to 2018. Aggregation patterns typical of pre-spawning fish schools were evident in the mooring records, and there was evidence of rapid changes in local abundance (Figure 9). The observations at the mooring indicated that fish distributions were changing during the spawning season. Backscattering at the mooring was low on 21 January and 1 February (winter), increasing to high levels on 11 March and 21 March (spring). In a previous study in 2009, more than 80% of larvae were identified as herring from January to March as a result of monthly larval surveys in southwest Jinhae Bay [5]. The mooring area for the migration of the Pacific herring should be moved from the bay to the Pacific Ocean [1]. The acoustic level range of the detected echoes in the mooring is characteristic of fish, and typical acoustic echoes characteristic of zooplankton form scattering layers. In the case of fish, on the other hand, echoes up and down on the echogram suggest a sparse fish school.