## Materials and Methods  
### The Ex Situ TS Measurements  
The ex situ TS measurements were conducted in a net cage base in the Three Gorges Reservoir, at the reservoir head near the Three Gorges Dam (approximately 1740 km from the Yangtze estuary). This base was specially built for culturing Chinese sturgeons. The specimens were randomly selected from the largest size group (around age 4) of fishes in the net cage base. Their length and weight were measured individually (Figure 2a) and then the specimens were put into a 5 m \(\times\) 5 m \(\times\) 10 m (length \(\times\) width \(\times\) depth) net cage (Figure 2b) for acoustic measurement. The TS of six specimens were measured one at a time. The individual specimen was freely swimming inside the net cage. A calibrated scientific echosounder system was used to conduct the acoustic measurement [9] (Table 1). The transducer was installed in a self-made buoy (Figure 2b) in the center of the net cage, and was looking down vertically with the transducer surface 0.3 m from the water surface. A laptop equipped with associated software was used to acquire and store the acoustic data (Figure 2c).  
The experiments were conducted during 1\(-\)21 April 2010, and the acoustic measurements on each fish was done for approximately 24 h. The body length and weight of the six specimens used in the experiments are shown in Table 2. Note that the beam angle of the transducer was 6.8\({}^{\circ}\), at 10 m water depth, and the footprint of the beam was 119 cm, which is approximately 30\(-\)50 cm larger than the body length of test specimens (74.0\(-\)92.6 cm); however, at 5 m depth, the footprint of the beam was 59 cm, which is approximately 20\(-\)30 cm shorter than the body length of the test specimens. Therefore, the entire fish was not always ensonified, especially in relatively shallow water depths.  
The acoustic data were examined and analyzed by Visual Analyzer 4.1 (BioSonics, Seattle, WA, USA) software [26]. The single traces, which could be visually identified, were selected and recognized  
\begin{table}
\begin{tabular}{c c} \hline
**Item/Parameter** & **Value** \\ \hline Instrument & BioSonics DT-X split-beam echosounder (Seattle, WA, USA) \\ Frequency & 199 kHz \\ Beam angle & 6.8\({}^{\circ}\) \\ Source level & 221 dB re \(\mu\)Pa at 1 m \\ Receiver sensitivity & \(-\)51.3 dB re Counts per \(\mu\)Pa \\ Water temperature & 15 \({}^{\circ}\)C \\ Sound speed & 1465.17 m/s \\ Absorption coefficient & 0.01028 dB/m \\ Data acquisition & Visual acquisition 5.0.3 (BioSonics, Seattle, WA, USA) \\ Pulse duration & 0.4 ms \\ Ping rate & 5 pings per second \\ Collection threshold & \(-\)80 dB \\ Data collection range & 1 to 25 m \\ \hline \end{tabular}
\end{table}
Table 1: Instrument and parameter settings for the ex situ target strength measurements.  
\begin{table}
\begin{tabular}{c c c c} \hline
**ID** & **Total Length (cm)** & **Body Length (cm)** & **Body Weight (kg)** \\ \hline
1 & 108.0 & 89.2 & 6.25 \\
2 & 102.0 & 85.0 & 4.50 \\
3 & 95.0 & 76.8 & 3.15 \\
4 & 107.4 & 91.2 & 6.80 \\
5 & 106.4 & 92.6 & 6.80 \\
6 & 89.3 & 74.0 & 5.50 \\ \hline \end{tabular}
\end{table}
Table 2: Six Chinese surgeons used for conducting the ex situ target strength measurements.  
Figure 2: The ex situ TS measurements of Chinese sturgeons in a net cage base in the Yangtze River: (**a**) the test specimen; (**b**) the net cage; and (**c**) the echogram from the acoustic measurements by Visual acquisition 6.0.7 (BioSonics, Seattle, WA, USA).  
by using the following parameters: single echo threshold \(-\)70 dB, correlation factor 0.9, minimum and maximum pulse width from 0.75 to 3, and end point criteria \(-\)12 dB. The identified traces were exported and then imported into Excel (Microsoft, Redmond, WA, USA) to estimate the ex situ TS. The Sigmaplot 12.0 (Systat, San Jose, CA, USA) was used to plot the figures.