## 3 Results and Discussion
### Relationship Between the MVBS and Zooplankton Abundance  
Measurements of zooplankton from both ADCP methods were correlated as shown in Figure 7. The relationship at sound scattered layers showed that MVBS from moored ADCP has a slightly higher signal compared to the MVBS from mobile ADCP. In the regression line, the deviations of 750 kHz relative to the 307.2 kHz in the SSL was \(y=x-5.64\) while the regression coefficient (\(r\)) was 0.9075. The MVBS differences between these two methods, \(\text{MVBS}_{750\text{kHz}}-\text{MVBS}_{307.2\text{kHz}}\), were the intercept of these linear equations (5.64 dB). The MVBS value from 750 kHz moored ADCP was higher than 307.2 kHz mobile ADCP. It was related to the frequency used in moored ADCP, which was higher compared to mobile ADCP. This MVBS difference happened because the acoustic method is very dependent on the acoustic frequency used, so the backscattered signal from the object will be different [15]. Based on the result, it is shown that these two methods can be applied for more detailed analysis for zooplankton detection.  
Figure 7: Relationship of MVBSs inside the SSL (11.5–16 m) determined from the selected ensemble from the four-beam averaged in two ADCPs.  
The acoustic wavelength given from the frequency of 750 kHz and 307.2 kHz ADCP instrument and sound speed set in the instruments were approximately 2 and 5 mm, respectively. However, the body size of zooplankton found in the observed area was smaller than the wavelength of the ADCP. The previous research found that if the zooplankton was dense and numerous enough at one layer, it could create strong backscatter signals as well [55]. So, it was important to find the relationship between the MVBS (dB) and the zooplankton abundance (ind m\({}^{-3}\)). The relationship between the MVBS and zooplankton abundance was done through analysis of two dominant species found that contributed to the observed MVBS averaged over depth and time interval with using regression analysis. The selected MVBS was only based on the SSL depth. Altogether, the most abundant zooplankton species from six biological sample were used to find the relationship with the average MVBS during the sampling time. The results of the linear regression of the MVBS and measured zooplankton abundance was summarized in Figure 8.  
The magnitudes of these coefficients are, among many other acoustical properties of the corresponding scatterers, related to zooplankton size (acoustic cross-sectional area). The linear regression of this relationship was explained, with the x-axis as zooplankton abundance and y-axis as averaged moored ADCP MVBS. The most representative species, _Oithona_ sp. and _Paracalanus_ sp., had high abundance compared to other species and were related to higher values of averaged MVBS. The MVBS variation significantly reflected the zooplankton abundance. In particular, the high MVBS value agreed with the highest abundance. Our research was found to be consistent with a similar result from other research [31; 55].  
However, the lack of biological samples made it difficult to find which zooplankton species had a strong influence on the MVBS variation. The zooplankton abundance could be used as a parameter by employing assumptions about the particular individual zooplankton found in the laboratory analysis to the MVBS value at the time of the sampling process. Such measurements of the MVBS only resulted in rough estimation; thus, it could not distinguish among different species of zooplankton which could differ in abundance as well as in the acoustic properties of the zooplankton species [50].