## 4 Discussion
In the near future, more than one frequency on the Chinese sturgeon survey should be applied to identify the species more accurately from other species. The combination with results of the KRM model in various frequencies and those from multi-frequency survey would give a better understanding on the Chinese sturgeon. Future hydroacoustic survey on Chinese sturgeon should be enhanced and improved. In recent years (2013, 2015, and 2017), the natural spawning activity of Chinese sturgeon was interrupted [5],  
Figure 7: Expected TS model of an adult Chinese sturgeon with the representative body length of 250.0 cm: (**a**) TS as a function of frequency; (**b**) TS as a function of tilt angle; and (**c**) TS as a function of frequency and tilt angle. which means that the sustaining of the natural population is facing a great challenge. In the past, a number of Chinese sturgeons could be captured on special occasions such as for scientific research, and they were accidentally caught by fishermen and caught sturgeon could be used to study or estimate its population conditions in the river [35; 36]. However, since 2018, the Chinese government has an ambitious plan that bans fishing for any fishes in the entire Yangtze main stem, meaning there will be no more incidental capture specimens. Therefore, only the hydroacoustic methods seem to be applicable to survey and study the adult population of Chinese sturgeon in the Yangtze River. On the other hand, future hydroacoustic surveys should not only be constructed in a zigzag mobile survey [17; 18; 19; 20; 21; 22] or a stationary point survey [37], but also long-time fixed cross section monitoring is highly recommended. As the fish is a migrating species, fixed cross section monitoring in its migration route could be highly efficient with low cost [38]. In addition, new and high-end acoustic technologies [39], such as wideband and multibeam echosounders as well as omni-directional sonars and imaging sonars, all could be combined to obtain a comprehensive understanding on the fish. To improve the survey efficiency, the multibeam echosounders with large detection range could be firstly used in the 1700 km middle and lower reaches of Yangtze River to ascertain the potential distribution areas of Chinese sturgeon, then the wideband echosounder and imaging sonars/acoustic cameras [40; 41] could be used to conduct more intensive surveys in these areas. We studied the TS of Chinese sturgeon by ex situ measurements and theoretical calculations (KRM model). The results from these two methods had good agreements. We further used the KRM method to predict the TS of an adult Chinese sturgeon with the representative body length of 250.0 cm, which could be employed in the case of the hydroacoustic survey. Again, the natural population of the fish is on the edge of extinction, thus hydroacoustic methods are the only feasible means to study the wild adults. The results in this study could provide a good basis for future hydroacoustic studies on the Chinese sturgeon as well as on endangered riverine fishes in the world. The following are available online at [[http://www.mdpi.com/2076-3417/8/12/2554/s1](http://www.mdpi.com/2076-3417/8/12/2554/s1)]([http://www.mdpi.com/2076-3417/8/12/2554/s1](http://www.mdpi.com/2076-3417/8/12/2554/s1)), Text File S1: 3D morphology data of a Chinese sturgeon, Table S1: Acoustic target strength of the Chinese sturgeon by KRM method. Conceptualization, H.Z., J.L., Q.W. and M.K. ; Formal analysis, H.Z., J.L. and C.W. ; Investigation, C.W., C.W., J.W. and H.D. ; Writing--original draft, H.Z. and J.L. ; and Writing--review and editing. Q.W. and M.K. This research was funded by the National Natural Science Foundation of China, grant number 31711540294 and 51641909. The authors give special thanks to [PERSON] in NOAA for help on the KRM model. The authors declare no conflict of interest.