## 4 Discussion  
In this study, the ex situ measurements and theoretical models were combined to evaluate the TS of Chinese sturgeon. As the small beam angle, the beam could not cover the whole body of the  
Figure 5: The comparison of measured TS and calculated TS at 199 kHz: (**a**) predicted TS as a function of the body length by the KRM model, where the blue dot with bar indicates the mean TS and its standard deviation of the ex situ measurement from the six specimens (Table 4); and (**b**) calculated TS model for the 74.0 cm specimen as a function of tilt angle (Figure 3). fish because of the shallow water depth. Figure 4 indicates that the TS did not evidently increase with the water depth. To cover the 74.0 cm specimen completely, at least 6.23 m water depth was required. Meanwhile, it is well known that most reflected energy of a fish is from the swimbladder; thus, if the swimbladder was detected, the reflected energy (i.e., TS) would be very close to that from the whole fish. A study has demonstrated that more than 90% of reflected energy of three gadoid species and mackerel is from the swimbladder [32]. In this study, only six specimens were measured because the Chinese sturgeon is one of the top-level protected animals in China. Thus, it was extremely difficult to obtain even six specimens. Furthermore, no adult was measured since very few adults were cultured in the artificial environment. They were too precious to be used for the field experiment. Hence, the ex situ TS measurement for an adult Chinese sturgeon (250 cm) was not conducted. In addition, at least \(\sim\)21 m water depth would be needed to measure the whole fish, thus it is very hard to create an appropriate experimental condition in inland waters. However, the TS of Chinese sturgeon from the measurements and theoretical calculations had a high degree of agreement. It can be mentioned that the theoretical TS models based on frequency, tilt angle, and body length can be used in a future hydroacoustic work for targeting the Chinese sturgeon. Figure 6: Estimated TS of Chinese sturgeon as a function of body lengths in six acoustic frequencies: (a) 38 kHz; (b) 70 kHz; (c) 120 kHz; (d) 200 kHz; (e) 333 kHz; and (f) 420 kHz. The acoustic identification method of Chinese sturgeon from echograms should be improved. Currently, the main ground for the identification is that the adult Chinese sturgeon is large, thus its TS should be stronger than the TS values of common river fishes [17; 18; 19; 20; 21; 22]. In fact, other large body fishes in the Yangtze River, such as the four major Chinese carps, can grow up to 131.1 cm in body length [33]. When the TS values of these carp species are similar to those of Chinese sturgeon, it would be terribly difficult for the Chinese sturgeon to be distinguished. Therefore, ascertaining the acoustic backscattering characteristics of not only Chinese sturgeon but also various fish species in the Yangtze River should be acquired for better fish resources management including accurate identification of the sturgeon. Furthermore, the TS was in general used to estimate the length of the target based on the traditional TS and length formula, such as Love or other empirical formulas [9; 34]. The traditional empirical formulas to estimate the body length of the potential target of Chinese sturgeon need to be improved. Meanwhile, this study presented that the size and body posture of the fish had great impacts on the TS. The Chinese sturgeon with the body length of 200\(-\)400 cm could have a smaller TS than that of sturgeon shorter than 200 cm (Figure 6). For example, in 200 kHz (Figure 6d), the estimated TS for the 337.6 cm Chinese sturgeon was at a low peak value (\(-\)38.9 dB). Therefore, only TS value without consideration of tilt angle (swimming behavior) obtained from a single frequency was not enough to judge whether the echoes were from the Chinese sturgeon. In addition, the 200 kHz echosounder is the most popular frequency in freshwater environments in the world [28].