## 4 Discussion  
According to fisheries statistical data, about 5839 tons of mullet were cultured in 2012, which was 7.7% of the total amount of fish cultured in Korea [18]. Redlip mullet is a valuable aquaculture species in the northwest Pacific Ocean that is mainly cultured along the coast of the Yellow Sea and the South Sea of Korea [3; 4; 19]. Until now, the traditional method of weighing was used to calculate the biomass of mullet on a fish farm [6]. This method requires a high cost, time, and human labor to estimate biomass. Using a hydro-acoustic method is an alternative way to overcome these limitations [7]. Research and development of an automatic fish-counter system, based on the hydroacoustic technique, are underway in Korea. One of the most important parameters for applying this system to a fish farm is to understand the acoustic \(TS\) characteristics of the target fish. The \(TS\) information of Redlip mullet was unknown before this study. In fisheries acoustics, the target strength (\(TS\)) functions are essentially required to convert volume backscattering strength (\(Sv\)) to absolute fish biomass. The accuracy of the \(TS\) information is a key parameter that determines the accuracy of the biomass estimate. In more detail, the \(Sv\) value within depth intervals can be changed to volume-backscattering area (\(S_{A}\)) for every one nautical mile of survey transect. By assuming that measurements of \(Sv\) represent the linear sum of echoes from individual fish within the sample volume, the numerical density of fish (number per unit area of sea surface) can be estimated by dividing \(S_{A}\) by the backscattering cross-section (\(\sigma_{bs}\), m\({}^{2}\)) of a single fish. Biomass density of fish (e, g m\({}^{-2}\)) can be estimated by multiplying the numerical density by the weight of a single fish [20]. The \(TS\) of individual fish is influenced by the tilt angle and the volume of the swim bladder. As the volume of the swim bladder in the body is difficult to calculate directly, the mean \(TS\) is expressed as a function of fish length or weight. Generally, the larger the fish length, the greater the swim bladder  
Figure 6: Relationship between the target strength (\(TS\)) and wet weight (\(W\)) at (**a**) 38, (**b**) 120, (**c**) 200, and (**d**) 420 kHz. Points represent the mean \(TS\) values, and solid lines represent least-squares regressions of mean \(TS\). volume, and hence the larger the \(TS\). The swim bladder is the most important factor affecting the \(TS\) of fish. In particular, it is known that more than 90% of the \(TS\) value is related to the size of the swim bladder [7]. Generally, a higher \(TS\) value is obtained when the tilt angle between the incident wave and the swim bladder area is perpendicular. After the \(TS\) experiments, the swim bladder shapes were obtained using radiography (M-1005; Softex, Mumbai, India). The lateral and dorsal views were acquired from X-ray images of frozen mullet (Figure 7). The mullet's head and back are flat-shaped and the body is wide, unlike other fish in the dorsal direction. In the case of Redlip mullet, the swim bladder has a positive inclination relative to the body axis and the tilt angle of the swim bladder was about \(8^{\circ}\) from horizontal. Figure 8 shows the relationships between mean \(TS\) and the tilt angle at 200 kHz for 19.4 cm and 35.0 cm \(FL\) values. The tilt angle was calculated by extracting the time-synchronized capture images from an underwater camera. At that time, the \(TS\) values could not be obtained at all angles. As a result of the relationship between the tilt angle and mean \(TS\), the \(TS\) values were about \(-35\) dB and \(-32\) dB for tilt angles of \(-10^{\circ}\) to \(0^{\circ}\), respectively. The closer the tilt angle of the swim bladder is to horizontal, the higher the \(TS\) value. The \(TS\) value tended to decrease for tilt angles less than \(-10^{\circ}\) or more than \(10^{\circ}\).