<p>This study experimentally investigates the hydrodynamic characteristics, geometric configurations, fluttering motions of the codend, and the instantaneous flow fields inside and around the codend, with and without a liner, under varying catch sizes and inflow velocities. A proper orthogonal decomposition method is employed to extract phase-averaged mean properties of unsteady turbulent flows from flow measurement data obtained using an electromagnetic current velocity meter inside and around the codend. The results reveal that as catch size increases, the drag force, codend motion, Reynolds number, and codend volume increase while the drag coefficient decreases. Owing to the codend shape and pronounced motion, a complex fluid-structure interaction occurs, demonstrating a strong correlation between drag force and codend volume. The oscillation amplitudes of the hydrodynamic forces and codend motions increase with increasing catch size, and their oscillations mainly involve low-frequency activity. A significant reduction in the flow field occurs inside and around the unlined codend without a catch. The flow field is 5.81%, 14.39%, and 27.01% lower than the unlined codend with a catch, the codend with a liner but without a catch, and the codend with both a liner and a catch, respectively. Fourier analysis reveals that the codend motions and hydrodynamic forces are mainly characterized by low-frequency activity and are synchronized with the unsteady turbulent flow street. Furthermore, the proper orthogonal decomposition results reveal the development of unsteady turbulent flow inside and around the codend, driven by flow passage blockage caused by the presence of the liner, intense codend motions, and the catch. Understanding the hydrodynamic characteristics and flow instabilities inside and around the codend, particularly those associated with its fluttering motions, is crucial for optimizing trawl design and improving trawl selectivity.</p>