## 1 Introduction  
The Chinese sturgeon, _Acipenser sinensis_, a large anadromous fish species, is predominantly distributed in the main stem of the Yangtze River and the continental shelf of the west Pacific [1,2]. Since the 1970s, the population of the fish has drastically declined because of habitat degeneration and overfishing [1,2]. Especially the migration route of its breeding population was completely blocked by the Gezhouba Dam (the first and lowermost dam in the Yangtze main stem) since 1981 [2,3]. In 1989, the fish was listed as one of the top-level protected animals in China, and, in 2010, the fish was upgraded as Critically endangered (CR) species on the International Union for Conservation of Nature (IUCN) red lists [4]. Moreover, its natural spawning activity, which has been observed continually more than 30 years (at least once a year), has been interrupted (i.e., no spawning in a year) since 2013 [5]. No filial generation was produced because of the degraded breeding population which implies that the natural population of the fish is on the edge of extinction. The Chinese sturgeon spends most of its life time at sea [1,2] (Figure 1). They mature when males are older than eight years old (approximately 150 kg and 250 cm) and females are older than 14 years old (about 250 kg and 350 cm). The adults approaching for spawning arrive at the Yangtze River estuary during June and July each year, and swim upstream along the Yangtze main stem to the spawning areas. Before the closure of the Gezhouba Dam in 1981, the spawning areas were reported in the low Jinsha and upper Yangtze River (approximately 2800 km from the Yangtze estuary) [1,6]. Two historic spawning areas had also been reported in the Pearl River [6]. Since 1981, only one regular spawning area has been reported, which is right below the Gezhouba Dam in the Yangtze River, around 7 km in total length (approximately 1700 km from the Yangtze estuary) [6,7]. Before spawning, they stay at somewhere in the middle Yangtze reach or near the spawning areas for approximately 18 months. In this period, their gonads gradually get matured. After spawning, the adults return to the sea immediately. The newly produced larvae drift along the water current and gradually arrive at the estuary, and grow up in the sea until they mature enough for spawning. Figure 1: Life history range of the anadromous Chinese sturgeon: the species lives in the continental shelf of the west Pacific and spaws in the rivers [1,2]. Historically (i.e., before the 1980s), the spawning areas were in the upper Yangtze and Pearl rivers, however, since 1981, only one remaining spawning area has been reported below the Gezhouba Dam in the main stem of Yangtze River [6]. Geographic data were from the National Geomatics Center of China ([[http://ngcc.sbsm.gov.cn](http://ngcc.sbsm.gov.cn)]([http://ngcc.sbsm.gov.cn](http://ngcc.sbsm.gov.cn))). Globally, 27 species of sturgeons and paddlefishes belong to the Acipenseriformes order [8]. Hydroacoustics, which uses active sound in water (e.g., sonar and echosounder) to study aquatic organisms (e.g., fish) [9], has been commonly used to study the Acipenseriformes species, such as American paddlefish [10], lake sturgeon [11], white sturgeon [12; 13], Atlantic sturgeon [14; 15] and green sturgeon [16]. The studies mainly use side-scan sonar [11; 13; 14; 15] or Dual-frequency IDentification SONar (DIDSON, so-called acoustic camera) [12; 16]. For Chinese sturgeon, the hydroacoustic methods have been widely used to study the spatial distribution [17; 18], the abundance estimation [19; 20; 21; 22], and spawning habitat characteristics [7; 23] in the Yangtze River. Since 1997, the numbers of the adult Chinese sturgeons below the Gezhouba Dam (in about 30 km reach) were assessed by echosounder surveys and the results were issued officially [24]. However, there was no acoustic target strength (TS) study on the species up to date.