## 4 Discussion
Indeed, the results of the PSD revealed that the low frequencies of the drag were obtained at 0.21, 0.033, and 0.016 Hz for the codends 1, 2, and 3, respectively, and they increased with the increasing catch weight. These temporal drag oscillations were synchronized with those of the coded transverse motions and transverse and streamwise flow velocities. That is why, because the 4P coded (coded 1) had a more agitated and intense transverse motion and streamwise flow velocity oscillation compared to the 2p coded, the oscillations of its drag are also intense. This trend was confirmed by [PERSON] et al. [17] who reported that 2p coded can have lower drag and a very lower motions compared with 4p coded. Moreover, the hypothesis that showed that the low frequency of temporal drag had a synchronization with those of the temporal flow velocity and coded motions was demonstrated by [PERSON] [16; 23] who studied the turbulence and tilt inside the codend of trawl, and found that the tension oscillation period was about 3-8 s and shortened with the increase in flow velocity. Furthermore, the results of the PSD obtained in the present study showed the frequencies of the drag are lower than those reported by [PERSON] et al. [39] for a square cylinder drag force. Concerning the codend motions, the coded 2 (4p coded) and the coded 3 (2p coded) showed more unstable behavior with more pendulum motions compared to the coded 1 (standard codend). This could be because the cross-section of these two codends could respond differently to the drag, resulting in instability. The higher motions of these two codends could impact the catch quality because the greater codend motion can push the fish to be rubbed by twine materials compared to the standard codend ([PERSON] et al., 2015). However, it was demonstrated that codends oscillations are synchronized with the drag force oscillations that showed that more codend resulted in an unstable behavior as the drag increased. This hypothesis was verified with the individual results obtained on each codon as a function of the catch weights showing that the drag and the transverse motions of the codend increase as the catch weight increase. While, when we spontaneously compared the tendency followed by the drag and the three coded motions, it was noted that the drag was not the only function of the coded motions but also of the twine area,which allows us to justify the lower drag obtained on codend 2 and codend 3 having more unstable movements unlike codend 1. We can therefore evoke a new hypothesis according to which the codend motions have more impact on the fish behavior inside the trawl net and selectivity as its drag which also depends on the warp motions and the design characteristic of the trawl [8; 10; 17]. In this study, the unstable behavior of the codend motions observed on the three codend can be caused by the greater pressure generated by the vortex shedding developing behind the codend and the unsteady turbulent flow inside the codend. This greater pressure could also increase the instability and the amplitude of the oscillatory motions of the codend due to the periodicity of the unsteady turbulent flow [15]. Indeed, the codend oscillations induce to a vortex flow generated inside the codend and behind the codend in the flume tank. This vortex flow increased with the codend motion and limited the flow passage through the codend by reducing the mesh opening. That is why the development of this vortex shedding was more important in the 2P codend compared to the 4P codend. This because velocity field observed inside and behind the 2P codend was lower than that observed on the 4P codend. The results showed that the PSD of transverse oscillations of the three codends are greater than that observed in the streamwise oscillations.