## 4 Discussion  
Changes in the codend design by modifying its geometrical shape and the number of netting panels and catch weights may be some of the solutions to improve the selective properties of the codend by increasing the likelihood of fish escape, the engineering performance of the whole trawl net, and to better understand the fish behavior inside the trawl net. Indeed, this study demonstrates that the modification of the geometrical shape of the codend and catch weight are directly related at the origin of the intense fluttering motions of the codend, which influence flow passage through the codend and will create  
Figure 15: Time evolution of the TKE (**a**) inside and (**c**) behind the different codends linked to the normalized frequency spectrum energy (left in a log-log scale) (**b**) inside and (**d**) behind the different codends. the unsteady turbulent flow, in particular, the vortex shedding inside and behind the codend. The development of this turbulent flow affects overall trawl performance, fish swimming speed, and maneuverability of the fish in the codend. However, the results of this study did not only identify the dependence of the codend deformation and the nature of the codend motions on the codend design and the maximum cross-sectional of the catch (catch weight), but also to demonstrate the dependence of the codend drag on the complex interaction between the codend structure and flow field. Therefore, it has been shown in this study that the use of 4p codend such as codend 2 and 2p codend such as codend 3 certainly increases the codend motions and TKE, and decreases the velocity field through the codend compared to the standard codend (codon 1) due to their instability but, allows it to considerably reduce the codend drag. The findings of this study confirmed the trend followed by [PERSON] et al. (2010), [PERSON] et al. (2017), [PERSON] and [PERSON] (2018), and [PERSON] and Germain (2015), who modified the geometrical shape of the codend (mesh shape, twine diameter, and mesh size) and the catch weights to better understand the codend behavior. In contrast to the investigation of [PERSON] et al. (2010), [PERSON] et al. (2014), [PERSON] (2016, 2013), [PERSON] et al. (2017), [PERSON] and [PERSON] (2015), and [PERSON] et al. (2016), the present study showed how changing the gear design and catch weight can strongly affect the codend drag and turbulent flow developing inside and behind the codend. However, the results obtained experimentally in the present study showed that the use of the 2p codend instead of 4p codends led to a decrease in drag force \(\sim\)4.7% and \(\sim\)2.12% compared to those of codends 1 and 2, respectively. Furthermore, it was found that at the empty stage, the 2p codend had a drag force higher than those of 4p codends. This difference in drag was attributed to the netting friction, another plausible explanation is that the decrease in the netting panel number during the design made it more unstable thus, decreasing the water flow velocity and an increase of the vertical pressure acting on the empty coded free surface (2010, 36; 37). This increase in vertical pressure allows the 2p codend to oscillate more than 4p codend that decreases its mesh opening, limiting the flow passage, and thus increasing its drag. This hypothesis is supported by the study of [PERSON] et al. (2016) who demonstrated that the 4p coded did not oscillate significantly under the condition of an empty codend that which could make its drag weaker unlike the 2p codend (2017). This study also demonstrated that the increase in catch weight led to an increase in drag due to the intense codend motions caused by the presence of catch inside the codend. Indeed, the statistical analysis indicated that the variation in drag force was about 18.73%, 17.175, and 15.91% between the lowest catch weight and the greater catch weight for the codends 1, 2, and 3, respectively.