## 2 Materials and Methods  
This study investigated the spatial and temporal distribution of adult and immature Pacific herring in Jinhae Bay, South Gyeongsang Province, which is a known spawning and habitat ground of the Pacific herring, by obtaining continuous acoustic data and performing a spatial survey using moored and ship-based echosounders and referring to fishing and environmental data. To identify the species, fish were caught twice in the waters near Neungpo Harbor, at the entrance of Jinhae Bay (34\({}^{\circ}\)53.450\({}^{\prime}\) N, 128\({}^{\circ}\)44.517\({}^{\prime}\) E) on January 23, 2018, the initial operation of the moored echosounder, and 25 April 2018, after the recovery (Figure 1). A stationary net with a length of 300 m and a width of 30 m was used as fishing gear, and the length and weight of every single Pacific herring caught for 24 h were measured. For the operation of the moored echosounder, an underwater mount (100 cm width \(\times\) 100 cm length \(\times\) 65 cm height) was made and installed in the waters near Jam Island in the bay (35\({}^{\circ}\)3.543\({}^{\prime}\) N, 128\({}^{\circ}\)40.613\({}^{\prime}\) E), which is part of the migratory path of the adult Pacific herring, in its movement to the spawning and habitat grounds of the larvae (Figure 1). This underwater mount was installed and fixed at a depth of 22 m by scuba diving, and the mooring split-beam wideband autonomous transceivers (WBATs) (SIMRAD AS) installed on the mount were operated by an internal battery (128 Ah) with split-beam transducers of 70 kHz (model ES70-7 CD) and 120 kHz (model ES120-7 CD) (Figure 2). With the sensors facing the surface layer, acoustic data were obtained from all water columns, from near the sea floor to the surface. The moored echosounder was operated for about 70 days from 21 January (winter) to 31 March (spring) 2018, when the Pacific herring returned for spawning. During the deployment, the WBATs were programmed to transmit an ensemble  
Figure 1: Study area in Jinhae Bay, Korea. The acoustic survey transects are shown as black dotted lines, and the locations of the echosounder mooring are indicated by red squares. The circle represents a stationary net station for fish catches. of 100 pings at the beginning of each hour at a ping interval of 3 s. The pulse width of the transmitted signals was set to 1.024 ms for both frequencies, and the Tx power was operated at 375 W and 200 W for 70 kHz and 120 kHz, respectively, depending on the water depth. A water temperature and depth logger (RBR T.D duet, RBR Ltd.) was installed on the underwater mount to supplement the acoustic data analysis. The water temperature data were collected at intervals of 2 min for the same period as the echosounder. The spatial acoustic survey in Jinhae Bay using the ship-based scientific echosounder of 38 kHz (model ES38-10) and 120 kHz (model ES120-7C) with a split-beam transducer type echosounder (EK60, SIMRAD AS) was conducted twice, in winter (20 January 2018) and in spring (3 April 2018) (Figure 1). The acoustic survey was performed in six transects, where Pacific herring were expected to pass through in Jinhae Bay. The ship-based echosounder was calibrated before the experiment, and the pulse width and ping interval of the transmitted signals during the operation were set to 0.512 ms and 1 s, respectively. The acoustic sensor was fixed vertically at a depth of 2 m on the side of the survey vessel, and the vessel speed was maintained within 6 knots for obtaining accurate acoustic data and sensor stability. The acoustic data acquired from the mooring- and ship-based echosounders were analyzed for volume backscattering strength (SV, dB re 1 m\({}^{-1}\)) using the virtual echogram technique of an echosounder data processor (Echoview Ver. 8.0; Echoview Pty. Ltd.) [19]. Although the SV data from the mooring-type echosounder were obtained for both 70 kHz and 120 kHz, this study analyzed only the 70 kHz data.