Lateral Target Strength (TS) Estimation of Free-Swimming Nile Tilapia (<em>Oreochromis niloticus</em>) in Ponds Using a 200 kHz Single-Beam Echosounder

As global aquaculture continues to expand there is an increasing interest for sustainable and non-invasive tools to monitor fish growth. Nile tilapia (Oreochromis niloticus) is one of the most farmed species worldwide. Its biomass estimation often relies on manual sampling or stereo-cameras systems limited by water turbidity. This study establishes a robust relationship between lateral Target Strength (TS) and the total length (TL) and weight (W) of Nile tilapia using a cost-effective 201 kHz single-beam echosounder. Measurements were conducted with free-swimming fish in a controlled pond environment (TL range 13–44 cm). The results show a strong linear correlation between acoustic and biometric data. Specifically, the relationship for mean TS was defined as TSmean = 20.1log(TL) − 66.7 (R2=0.91) and TSmean = 6.3log(W) − 55.4 (R2=0.96), proving the system’s accuracy for biomass estimation. Furthermore, the Method of Fundamental Solutions (MFS) was employed for numerical validation based on X-ray morphometry of the swim bladder. A very good agreement was observed between experimental data and numerical simulations, reinforcing the validity of the acoustic models despite the inherent complexity of biological targets. These findings demonstrate that calibrated single-beam acoustic systems provide a viable, non-intrusive tool for real-time monitoring in aquaculture ponds.