The coupled dynamics of the codend fluttering motion and its hydrodynamics behavior are investigated based on PIV measurements carried out around a 1/10 scaled model codend. The mean wake flow characteristics are firstly analyzed demonstrating the great effect of the codend motion. The differences in flow contour between fluttering motion and no-fluttering codend are highlighted by analyzing the classical mean characteristics of a body wake flow. In presence of fluttering motion, the classical results are recovered only if one considers the vertical extent of the codends oscillations as a reference length scale. Secondly, the frequency content of codend oscillations is analyzed and it is linked to the physical mechanisms. It is observed that the motion of the codend is mainly of a low-frequency activity and of another component related to the vortex shedding street. It is then demonstrated a complex fluid-structure interaction where the near wake hydrodynamics of the moving codend structure are an intricate interplay between the codend fluttering motion and the shear layer instabilities and large scale vortex shedding. The knowledge of such flow instabilities is of great importance to improve the understanding of the force acting on a trawl and for the implementation of some selected device to drive fish near the codend and then to reduce juvenile by-catch.