Flow field measurements behind a rigid cod-end and a moving bottom trawl are conducted using Time Resolved PIV method. This method is based on an image acquisition rate allowing the time resolved sampling of the vortex shedding phenomenon. Based on such measurements, the main flow characteristics around fishing net structure are investigated. Moreover, the vortex shedding frequency phenomenon associated with both the rigid cod-end and the moving bottom trawl is analyzed. We then present the analysis of the instability of the shear layer separating a moving trawl having 3 degrees of freedom. Using also the Proper Orthogonal Decomposition (POD) for such investigation, it is demonstrated the effectiveness of this procedure to extract not only the large scale spatial flow structure but also their associated frequencies. POD is then shown to be robust to act as a filter for the frequency analysis.