## 4 Discussion
These increases in drag with the increase in the catch weight can be explained by the fact that the presence of the catch inside the codend reduces the flow field in the region in front of the catch due to the increase in codend motions. That modifies the meshes opening on the end-part of the codend, leading to a decrease in the flow passage through this part of the codend. Thus, creating the unsteady turbulent flow inside and behind the codend thus increasing the drag and influenced the physiology and behavior of fish (2013, 2015, 2017). Another reason is that when the catch weight rose, the codend diameter (the diameter of the sphere defining the catch shape) increased as well. As a result, the water flow field near this area of the codend decreased. By increasing the presence of the unsteady turbulent flow that affects the codend drag, this decrease in water flow will raise the water pressure on the codend, which will be especially critical on codends 1 and 2. The great presence of this vortex shedding and eddy flow inside the codend could affect the swimming energy, which influences fish escape by the fluttering net panel from the codend. In this case, because the motions of the 2p codend (codon 3) are greater than those of 4p codends (codends 1 and 2), velocity fluctuation inside this codend will be higher than those inside other codend (2010, 2013, 2017). This greater velocity fluctuation inside the 2p codend will facilitate the higher production of the turbulent kinetic energy, its redistribution, and its dissipation within the unsteady flow develop inside compared to the 4p codends (Figure 15). That will reduce the probability of the fish escaping inside this codend, which makes it the least selective codend unlike the others codends although it is the coded having a significant amount of drag (though smaller compared to the others). To solve this problem of selectivity on this codend (2p coded), it will be judicious to adapt the solutions proposed by [PERSON] [38] who recommended replacing the diamond mesh with the hexagonal mesh or that of [PERSON] et al. [17] having replaced the diamond mesh by the square mesh, which will thus make the 2p coded the most efficient coded. The reason for the lower drag on the coded 3 (2p coded) was also because its cross-section was lower than that of 4p coded (codends 1 and 2). Thereby, because the increase in drag affects the energy efficiency, it would be relevant to consider the fuel consumption as one of the improved parameters during coded design, although the codend drag is a lower proportion of the drag of the whole trawl net. That could justify the recommendation of the 2p coded, unlike the 4p coded. However, according to [PERSON] et al. [10], [PERSON] et al. [17], and [PERSON] et al. [6], the lower cross-section such as the one of 2p coded could allow the catch to build up in the forward direction thus increasing the vertical pressure acting on the coded. This increase in pressure could impact the catch quality. This might be a particular disadvantage for the 2P coded, which allows us to recommend the 4p coded (coded 2) due to the fact the difference between its drag and the one of 2p coded is lower than 4%. During the flume tank experiment, the three codends were attached to a circular rigid frame generating more intense oscillations than a codend attached to a trawl by [PERSON] et al. [22] and [PERSON] et al. [17]. These oscillations were synchronized with the drag and velocity field oscillations. Moreover, the temporal drag oscillations includes a weak wave oscillation and are caused by the unsteady turbulent flows developed inside and downstream to the codend in the wake zone, which gives rise to a transverse pressure on the coded [15; 17]. These oscillations were mainly of low frequency with amplitude increasing with increasing catch size and number of netting panels. Thereby, the drag oscillation of the 2P coded was linked to the low frequency small than those obtained on the temporal drag oscillations of the 4p codends.