Effects of the simulated catch on the hydrodynamic and oscillation characteristics of the codend

To investigate the relationship between hydrodynamic characteristics and catches inside the mid-water trawl codend, this study examined the effects of catch weights (0.00, 1.68, 3.36 and 5.04 kg) on the drag force, shape, and oscillation of codend under five different flow velocities (0.5, 0.6, 0.7, 0.8 and 0.9 m/s) using flume tank experiment. The results show that: (1) the drag force of the codend increased with increasing flow velocity and catch weight. At lower flow velocities, the drag force of the codend with a catch weight of 1.68 kg was 30% greater than that of empty codend; (2) the shape of the codend structure tended to constrict with increasing flow velocity, while it expanded with increasing catch weight. However, the fish guiding area tightened and took on a "waist drum"; (3) the codend volume was directly proportional to the catch weight. When the catch weight increased to 5.04 kg, the codend volume was approximately 0.8 times that of the empty codend, indicating that the empty codend has the greatest volume. Under different catch weights, the periodicity and the amplitude of codend position oscillations decreased with increasing flow velocity. The codend position oscillations were fitted using a third-order Fourier series, yielding satisfactory results. These findings are expected to contribute to the optimization of the codend structure and provide relevant information for improving the hydrodynamic characteristics, geometric shape, and selectivity of the mid-water trawl codend.