## 3 Results  
In this experiment, the seawater temperature was 13 \({}^{\circ}\)C, and salinity was 32 psu, so the speed of sound was 1496.7 m/s [7]. The 16 live mullets were judged to be in good condition during the experiments. Their total fish length (_FL_, cm) ranged from 14.3 cm to 40.3 cm (mean _FL_ = 29.07 cm) and their wet weight (_W_) ranged from 28 g to 898 g (mean \(W\) = 335 g). The relationship between the _FL_ and \(W\) is: \(W\) = 0.0004825, _FL_\({}^{3.884}\) (_r_\({}^{2}\) = 0.97). Figure 3 shows a sample echogram of a mullets _TS_ measurement (_FL_: 35.0 cm, 200 kHz). In the echogram, the _TS_ data from the mullets were fully separated from unwanted surface targets, weight, and the bottom. The relatively low _TS_ value (about \(-\)50 dB) at the beginning of the measurement was likely to be due to the unstable condition of the mullet at the tethered depth. The higher _TS_ values (about \(-\)33 dB) show that the mullet was stable with an approximate \(\pm 5^{\circ}\) tilt angle. The mean _TS_ for mullet was calculated from the _TS_ data obtained between 14.26 and 14.38. Representative samples of the _TS_ distributions at 38, 120, 200, and 420 kHz are shown in Figure 4. Generally, the _TS_ of a fish with a large motion during _TS_ measurements have a typical Rayleigh distribution, and thus the deviation of the mean _TS_ tends to be large. However, the _TS_ data measured in this study show similarity with the Gaussian distribution, and thus show relatively stable mean _TS_ values. The numbers of valid _TS_ data (745 at 38 kHz, 656 at 120 kHz, 454 at 200 kHz, and 460 at  
Figure 3: A sample echogram of the Redlip mullet (_FL_: 35.0 cm) from the _ex situ_ target strength measurements. 420 kHz) were considered to be sufficient to calculate a mean _TS_ value for mullet. The _TS_ values for mullet, ranged from \(-52.4\) dB to \(-30.6\) dB at 38 kHz, \(-58.4\) dB to \(-28.6\) dB at 120 kHz, \(-57.0\) dB to \(-26.4\) dB at 200 kHz, and \(-59.2\) dB to \(-27.0\) dB at 420 kHz. The mean _TS_ values from the data were \(-37.4\) dB at 38 kHz, \(-37.4\) dB at 120 kHz, \(-34.6\) dB at 200 kHz, and \(-36.6\) dB at 420 kHz. Figure 5 shows the relationships between the mean _TS_ and the _FL_ at each frequency. The black points are the mean _TS_ values from the 16 mullet, which ranged from \(-47.6\) dB to \(-34.4\) dB at 38 kHz, \(-50.1\) dB to \(-34.5\) dB at 120 kHz, \(-49.4\) dB to \(-33.8\) dB at 200 kHz, and \(-49.9\) dB to \(-34.4\) dB at 420 kHz. Considering the mean _TS_ values for all frequencies, the difference in the maximum mean _TS_ was within 1 dB and the difference in minimum mean _TS_ was 2.5 dB. These results indicate that the frequency dependence of the mean _TS_ value was relatively low. From Equation (2) using the mean _TS_ data, it is determined by the least-squares regressions of mean _TS_ with _FL_. The _TS_ functions at all frequencies were fitted for Redlip mullet (Figure 5),  
\[TS_{\text{38 kHz}}=21.6\cdot\log_{10}(FL)-69.3 \tag{5}\] \[\big{(}95\%\text{ CI}:15.1\text{ to }28.1\text{ and }-78.8\text{ to }-59.8\text{; }r^{2}=0.78\big{)},\] (6) \[TS_{\text{120 kHz}}=22.9\cdot\log_{10}(FL)-72.4\] \[\big{(}95\%\text{ CI}:13.0\text{ to }32.7\text{ and }-86.7\text{ to }-58.1\text{; }r^{2}=0.64\big{)},\] (7) \[TS_{\text{200 kHz}}=26.9\cdot\log_{10}(FL)-76.2\] \[\big{(}95\%\text{ CI}:19.7\text{ to }34.0\text{ and }-86.6\text{ to }-65.8\text{; }r^{2}=0.82\big{)},\] \[TS_{\text{420 kHz}}=26.2\cdot\log_{10}(FL)-77.5\] \[\big{(}95\%\text{ CI}:20.0\text{ to }32.5\text{ and }-86.5\text{ to }-68.5\text{; }r^{2}=0.85\big{)}, \tag{8}\]  
Figure 3:0 Representative target strength (_TS_) histograms of mullet (_FL_: 35.0 cm) at (**a**) 38, (**b**) 120, (**c**) 200, and (**d**) 420 kHz. Index, \(n\) is the sample size. where \(r^{2}\) is the coefficient of determination, and CI is the confidence interval for \(m\) and \(n\). The mean \(TS\) values for each frequency were calculated from the standard fits of the \(TS\) function (see Equation (3)).