## 4 Discussion
The mean \(TS\) values were \(-41.4\) dB and \(-38.0\) dB when considering a tilt angle of \(\pm 20^{\circ}\), which is normal for fish swimming in a sea cage. Figure 8: Variation between the tilt angle and mean target strength (\(TS\); dB) of mullet at 200 kHz [(**a**) 19.4 cm fish total fish length (\(FL\)), and (**b**) 35.0 cm in \(FL\)]. Figure 7: X-ray images of the mullet’s swim bladder; (**a**) lateral aspect, and (**b**) dorsal aspect). The red lines is the boundary of the swim bladder. The swim bladder is generally inclined +15\({}^{\circ}\)-30\({}^{\circ}\) in the positive direction from the horizontal compared to other fish [21]. Therefore, the highest \(TS\) value occurred when the tilt angle of the fish body was between \(-30^{\circ}\) and \(-15^{\circ}\). Considering the characteristics of the acoustic signals transmitted in the vertical direction when the inclination angle of the swim bladder is high, the \(TS\) value of the fish also varied widely due to transmission and reflection fluctuations depending on the tilt angle at high frequency. In this study, the difference in the \(b_{20}\) value in the standard \(TS\) function at each frequency was within about 2 dB (Equations (9)-(12)). Generally, fish with large swim bladder angles in \(TS\) measurements have relatively high \(TS\) values at low frequencies. In the case of the mullet with small swim bladder angles, the difference of mean \(TS\), with frequency, was relatively small because the slope of the swim bladder is almost parallel with a horizontal line. These results show that the \(TS\) functions are consistent at all frequencies (Figures 5 and 6). However, the reason why the result at 120 kHz is lower than that of the other frequencies is that the coefficient of determination for the accurate beam axis was relatively low at the time of the \(TS\) experiment. Most fish in a sea-farm cage move predictably without sudden movement. The difference in the \(TS\) values between tilt angles of +20\({}^{\circ}\) and -20\({}^{\circ}\) was about 10 dB. Considering the relatively small fluctuation in the \(TS\) values at all frequencies in this study, the high frequency is valid to use when estimating fish biomass within a confined net at a fish farm. The results of this study will provide \(TS\) information for Redlip mullet and can be applied to estimate biomass. In future studies, we will attempt to calculate the theoretical \(TS\) using morphological parameters, such as the three-dimensional swim bladder and the body shapes and the physical properties of Redlip mullet for a numerical simulation with a scattering model. In addition, we will compare and analyze the measured \(TS\) data with theoretical \(TS\) data to improve the accuracy of the \(TS\) information. H.K., S.C., M.K. and J.P. performed the acoustic target strength (\(TS\)) experiments and analyzed the data; K.K. and D.K. discussed the \(TS\) measurement results. H.K. and D.K. wrote the manuscript and offered useful suggestions. This research was a part of the project titled \"Development of an automated fish-counter system and measurement of underwater farming-fish\", funded by the Ministry of Oceans and Fisheries, Korea (PM60560). We thank [PERSON] at KIOST MRC for his help with the \(TS\) experiments. The authors declare no conflict of interest.