## 3 Results and Discussion
### Dial Vertical Migration  
Estimation of the vertical movement of the zooplankton in the scattering layer can be easily performed by using a scientific echosounder in general, but it is impossible to estimate the vertical movement speed in the water column. On the other hand, the ADCP method is a direct measurement of the vertical velocity component as long as the acoustic backscatter measurement and can measure a zooplankton behavior pattern using the vertical velocity component. Figure 8: The relationship between the mean volume backscattering strength (dB) around the SSL and zooplankton abundance (ind m\({}^{-3}\)) from net samples. The x-axis is the two dominant zooplankton species abundance (_Oithona_ sp. and _Paracalanus_ sp.) measured in the laboratory while the y-axis is the averaged MVBS from moored ADCP over 30 min around the biological sampling time. Figure 9 shows that the daily variability on echogram of the MVBS, vertical velocity, and current speed from April 4 to 8 were measured on the moored ADCP. In the case of the seabed installation, since the transducer is fixed to the bottom of the sea, the measurement of the vertical velocity with high accuracy can be expected. By integrating the MVBS at a long series of data, the diel vertical migration was confirmed on the SSL depth, and similar pattern with the vertical velocity profiles was found. At shallower depths until 11.5 m, lower vertical velocity value was detected during the observation. The vertical velocity value in this layer had much flatter variation compared to the SSL. Figure 9: Observed zooplankton’s DVM pattern from the comparison of **(a)** MVBS values with **(b)** vertical velocities and **(c)** current speed obtained from mooring ADCP. The graph line was mean vertical velocity in **(d)** surface to 11.5 m of depth and **(e)** in the sound scattering layer (11.5–16 m). The color in the vertical velocity profile expresses the zooplankton's swimming behavior. Red indicates that the upward movement and downward movement is indicated by blue. The migration stages of zooplankton consisted of the initial rise (from white to red), rapid rise (strong red), stable (white), initial descent (from red to white), and rapid sinking (blue). The sinking and rising speed of velocity component were detected from the maximum value of 4.9 mm s\({}^{-1}\) and 4.4 mm s\({}^{-1}\), respectively. These vertical velocities were much smaller than those obtained in a different area, especially in the sub-tropical and arctic which varied between 10-20 mm s\({}^{-1}\)[10; 30; 51]. It appears that zooplankton does not need to swim quickly to migrate because the vertical migration range is very short. The clear diel pattern captured was associated with low current speed (Figure 9a,c) on the nights of 4, 6, and 8 April, and correlated between the mean MVBS and current speed indicated by the similar pattern. When the current speed was high, at the same time the average MVBS value was lower compared to the same hour in another day. High current speeds of \(>\)100 cm s\({}^{-1}\) mainly affected the MVBS [10]. Zooplankton relies on water currents to move around. The upward and downward vertical movement ranging from the surface to 11.5 m (Figure 9d) at night and daytime might be caused by the tidal flow, as there was fluctuation on the mean sea level (MSL) over time and slowly affected the vertical movement of the particle in the water column, since the vertical velocity value was much lower compared to the SSL area. However, the vertical velocity changed significantly around the depth of 11.5-16 m, and it was confirmed as a sound scattering layer, with a pattern of the diurnal cycle of vertical movement. The peak of strong vertical velocity value in this deeper layer suggested that the diurnal cycle of the pattern was caused by something other than by the tides [31]. Such a phenomenon was often seen, indicated by the dashed line in the echogram. In Figure 10e, the clear peak of the MVBS and vertical velocity profiles reflects the DVM of the zooplankton.