## 3 Results and Discussion
### Theoretical TS using the DWBA Model  
Classification of zooplankton species by ADCP may encounter many challenges that cannot be resolved using MVBS analysis. The theoretical TS of dominant scatterers based on the biological sampling, _Oithona_ sp. and _Paracalanus_ sp. was used. From these samples, a photo from a biological sample was taken for digitizing the shape of each zooplankton. The digitized photo of dominant zooplankton was used to measure their 2D morphology (Figure 12). Figure 11: Daytime distribution on the MVBS at 13 m of depth vs. solar irradiance. The dashed lines mark major MVBS peaks and their correspondence in irradiance. Figure 12: Shape and length of _Oithona_ sp. (dorsal aspect) and _Paracalanus_ sp. (side aspect). The acoustic backscatter characteristic differences from different zooplankton also had a different response with various sizes and shape. Although other research [12] showed that there were some challenges to obtaining quantitative estimates of zooplankton abundance and biomass in mixed populations, by using acoustic modeling it was possible to identify which zooplankton was responsible for the acoustic backscatter collected. Figure 13 shows target strength (dB) calculated from body length (\(L\)) for two acoustic frequencies used in ADCP. Zooplankton could freely swim and changed their orientation in the water, for example during the diel vertical migration that could vary in TS value. Since all species were small, weak, and complicated in shape, the estimated TS were small and variable. Thus, they were compared to the TS from the theoretical model. Scattering from a different angle was modeled based on the orientation of zooplankton. TS values varied based on different angles at all 360\({}^{\circ}\). The 0\({}^{\circ}\) angle was the acoustic angle of incidence that affected the zooplankton head section, while the 180\({}^{\circ}\) angle scattered the tail part of the zooplankton. The difference in the acoustic incidence angle in both zooplankton also varies with the zooplankton orientation. As explained earlier, _Oithona_ sp. was digitized by the dorsal aspect, while _Paracalanus_ sp. was digitized with side aspects. All aspects were observed because  
Figure 13: Theoretical target strength (dB) vs. acoustic incidence angle of 0.518 mm _Oithona_ sp. and 0.201 mm _Paracalanus_ sp. based on distorted wave-Born approximation (DWBA) model. These zooplankton were modeled in two frequencies (307.2 and 750 kHz) according to the frequency used in ADCP. The width of the cylinder for _Oithona_ sp. and _Paracalanus_ sp. were 0.01 mm and 0.005 mm, respectively. The peak of TS occurred at dorsal (270\({}^{\circ}\)) and ventral (90\({}^{\circ}\)) aspect. small-sized zooplankton may roll in the water column depending on the water current strength [56]. Measurements at angles \(90^{\circ}\) and \(270^{\circ}\) in _Oithona_ sp. were from the left and right sides of the body, while at an angle \(90^{\circ}\) and \(270^{\circ}\) in _Paracalanus_ sp. were from the upper and lower sides of the zooplankton body. The difference in acoustic angle of orientation was observed to find out the three-dimensional acoustic responses in all zooplankton body parts. The results of these experiments indicate that either the dorsal or side aspect had the same theoretical TS results as that of the DWBA model. Theoretical TS value of the DWBA model for _Oithona_ sp. species ranged from \(-148.3\) to \(-148.19\) dB at the frequency \(307.2\) kHz and between \(-133.63\) to \(-132.92\) dB at the frequency of \(750\) kHz. Theoretical TS species of _Paracalanus_ sp. ranged from \(-161.9\) to \(-161.83\) dB at the frequencies of \(307.2\) kHz and \(-146.7\) to \(-146.4\) dB at a frequency of \(750\) kHz. The peak of the TS value was on the side aspect (\(90^{\circ}\) and \(270^{\circ}\)) of the _Oithona_ sp. body, whereas the TS peak was from dorsal (\(270^{\circ}\)) and ventral (\(90^{\circ}\)) aspects of the _Paracalanus_ sp. body.