## 3 Results
After the acoustic data obtained from the survey areas were separated into echoes of adult fish and larvae and zooplankton using the difference in mean volume backscattering strength, the extracted 38 kHz fish echoes and 120 kHz zooplankton echoes, including larvae, were converted into NASC and displayed on the transects (Figure 6). In January, few fish echoes were detected inside Jinhae Bay, while a relatively higher level of acoustic backscatter intensity was measured outside the bay (Figure 6a). On the one hand, larval and the 120 kHz echograms were detected inside Jinhae Bay, while the 120 kHz echograms were detected inside Jinhae Bay. The 120 kHz echograms were detected inside Jinhae Bay, while the 120 kHz echograms were detected inside Jinhae Bay. The 120 kHz echograms were detected inside Jinhae Bay, while the 120 kHz echograms were detected inside Jinhae Bay. The 120 kHz echograms were detected inside Jinhae Bay, while the 120 kHz echograms were detected inside Jinhae Bay. Figure 5: Examples of vertical echograms from the ship-type echosounder measured at 38 kHz (**a**) and 120 kHz (**b**) on 20 January 2018, and at 38 kHz (**c**) and 120 kHz (**d**) on 3 April 2018, in Jinhae Bay. In January, the 38 kHz echogram showed fish schools on the outside bay (**a**), and the 120 kHz echogram showed weak scatter in the inner bay (**b**). In April, the 38 kHz echogram showed no fish echoes in the outer bay (**c**), and the 120 kHz echogram showed dense scattering, indicating zooplankton and fish larvae in the inner bay. zooplankton signals were low across the bay (Figure 6b). In April, strong signals from adult fish were intermittently detected only outside the bay, and the frequency was relatively low compared to that obtained in winter (Figure 6c). On the other hand, larval and zooplankton signals were detected across the survey areas, unlike in winter, particularly strongly inside the bay (Figure 6d). Figure 6: _Cont._  
Figure 6: Acoustic backscatter observed in Jinhae Bay. The circles correspond to the observed fish aggregation backscatter using the nautical area scattering coefficient (NASC, m\({}^{2}\) nmi\({}^{-2}\)) data with a 0.1 nautical mile interval at 38 kHz (**a**) and 120 kHz (**b**) on 20 January 2018, and at 38 kHz (**c**) and 120 kHz (**d**) on 3 April 2018. The location of the echosounder mooring is indicated by a red square. A scale bar is provided at the bottom of the figure. The survey data and frequencies are listed on each panel. NASC: nautical area scattering coefficient. In February 2016, a group of Pacific herring larvae was photographed in Jinhae Bay (Figure 7). The adult Pacific herring usually spawns sinking eggs in areas where seaweed is distributed at a water depth of 5 m or less; adult Pacific herring schools move to shallow coasts or inner bays with seaweed communities and lay eggs on surfaces such as seaweed. This indirectly indicates that the Pacific herring spawns before February, and the larvae grow and move to the open sea. The ocean observation buoy in Jinhae Bay has provided data on monthly average water temperatures since September 2015 (Figure 8). The surface water temperature in the winter of December 2017, which is the major spawning period of the Pacific herring, was 11.87 \({}^{\circ}\)C, which was about 2 \({}^{\circ}\)C lower than 13.99 \({}^{\circ}\)C observed in 2015 and 13.84 \({}^{\circ}\)C observed in 2016. Although data for January and February 2018 were not available, the surface water temperature monthly averages for March, April, and May 2018 were 10.2 \({}^{\circ}\)C, 13.14 \({}^{\circ}\)C, and 16.02 \({}^{\circ}\)C, respectively. Figure 7: Photograph of the spawning ground of the Pacific herring (_Clupea pallassii_) during a scuba survey on the Nanpo-ri Coast in Jinhae Bay in February 2016.