## 1 Introduction  
Redlip mullet (_Chelon haematocheilus_) is a euryhaline species that inhabits both marine coastal water and freshwater environments [1; 2]. The species is a commercial aquaculture fish in Korea and has an ecological niche in the marine ecosystem [2; 3]. It is widely distributed around the coastal waters of Korea, Japan, Taiwan, and China in the North-Western Pacific Ocean [4]. The young mullet feeds on phytoplankton or small organic sediments in coastal waters and the adult mullet becomes omnivorous [5].  
The traditional netting method has been used to measure the abundance of mullet in coastal waters [6]. In addition, the biomass of mullet in aquaculture nets has been estimated by the total wet weight (_W_) using a scale. As these methods are labor intensive and costly, it is necessary to consider more efficient methods. Since 2000, a hydroacoustic method using the acoustic equipment of fisheries has been implemented to estimate the distribution and abundance of fish in marine and freshwaterenvironments [7]. Hydroacoustic methods are considered to be an efficient way to overcome the limitations of traditional methods [8; 9].  
The acoustic target strength (\(TS\)) of individual fish and volume-backscattering strength (\(S_{v}\)) from fish schools must be obtained using well-calibrated acoustic equipment to determine an accurate biomass of a target fish. The \(TS\) information is required in order to convert \(S_{v}\) to absolute fish biomass. The accuracy of the \(TS\) information is a key parameter that determines the accuracy of the biomass estimate. Previous studies have reported that the \(TS\) information can cause up to 40% of the error in the biomass calculation [7; 10].  
\(TS\) is generally measured directly in in situ or _ex situ_ experiments and indirectly use an acoustic scattering model [11]. The in situ method has the advantage of obtaining \(TS\) data in the natural state for the target fish, but it is difficult to distinguish the \(TS\) data due to other fish or unwanted fish [12]. The _ex situ_ method uses tethered or caged live fish and has some advantages, such as species identification, tilt angle, and fish information (length and weight), but there is a possibility for error in the \(TS\) measurements due to unnatural movements from the artificial control [13]. The acoustic scattering model, based on physical morphology, estimates \(TS\) information at various frequencies and fish lengths, but it depends heavily on internal physical characteristic data (body or swim bladder shape, density gradient, and sound speed gradient) of the target fish [14]. There has been a tendency to use all three of these methods together to obtain more accurate \(TS\) data.  
Considering labor and costs, acoustic methods that use high frequencies are a very efficient methods for obtaining rapid biomass estimates of cultured Redlip mullet (_Chelon haematocheilus_). Unfortunately, there is very little information about \(TS\) measurements and related information on this mullet species.  
The purpose of this study was to measure the \(TS\) of cultured Redlip mullet, which is the key parameter for estimating biomass using the acoustic technique. As a result of this study, we derived the relationship between the \(TS\) function and the total fish length (\(FL\), cm) or wet weight (\(W\), g) at frequencies of 38, 120, 200, and 420 kHz. Furthermore, we tried to understand \(TS\) characteristics based on the swimming angle.