## 5 Conclusions  
This study was carried out to experimentally evaluate the impact of gear design and catch weight on the hydrodynamic performances, ECVM flow field inside and behind the codend, cross-section, and codend motions with the main of better understanding of the fluid-structure interaction of the codend in order to predict trawl drag, the fish behavior, and the trawl selectivity. Three 1/15 th scale codend models were designed and tested in the flume tank. The power spectra density (PSD) method was used to analyze the non-stationary time series of the drag, coded motions, flow velocity field, and the ensemble-averaged turbulence indicator. The following main conclusions can be drawn from this study:  
1. The drag forces, codend motions, coded deformation, and Reynolds number increased as the catch weight, number of netting panels, and flow velocity increased, while the drag coefficient decreased as the catch weight and number of netting panels increased.
2. The circumference of the catch regions of the 2P coded was significantly smaller than that of the 4p codons. As a consequence, this codend had lower drag but it is less selective and can impact the catch quality than the 4p codons.
3. The analysis of temporal flow velocities presents an oscillation of quasi-periodic nature and clearly showed that inside and behind the 4P and 2P codons, the flow velocity field recovered between 0.48-0.79 \(u_{0}\) and 0.45-0.75 \(u_{0}\), respectively. In addition, the analysis of TKE has underlined that the TKE is mainly produced inside the codend (about 60% greater) compared to those obtained behind the codend, and this turbulent kinetic energy is greater inside and behind the 2P coded compare to that obtained on 4P coded. Thus, the distribution of the turbulent fluctuation inside and behind the 2P coded was more important.
4. The spectra content obtained on the motions of the 2P coded exhibited a dominant low frequency component compared to those obtained on the motions of the 4p codons. Moreover, it was found that the vortex frequency of the 2P coded structure are greater than that of the 4P codend structure. Therefore, the gear design and catch weight greatly influenced the codend behavior.
* These basic results from the measurements of flow field, drag forces, and fluttering motions could help with understanding the complex fluid-structure interactions that induce large deformation and oscillation of the codend which modifies the drag force instantaneously, the mouth opening, and herding response or escape behavior of fish due to the towing velocity. In this case, it can be concluded that 2p codend has very good performance in terms of drag force, but considering the improvement in fish behavior and selectivity we recommend the codend 2 (4p codend).  
Conceptualization, B.T.N.N., H.T., J.Z. and F.H.; Methodology, B.T.N.N.; Software, B.T.N.N. and W.L.; Validation, B.T.N.N., H.T. and J.Z.; Formal analysis, B.T.N.N. and F.H.; Investigation, B.T.N.N., H.T., J.Z. and W.L.; Resources, B.T.N.N., H.T. and F.H.; Data curation, B.T.N.N., J.Z., W.L. and F.H.; Writing--original draft, B.T.N.N.; Writing--review & editing, H.T., L.X. and F.H.; Visualization, B.T.N.N.; Supervision, H.T., J.Z. and F.H.; Project administration, L.X.; Funding acquisition, H.T. and L.X. All authors have read and agreed to the published version of the manuscript.  
This study was financially sponsored by the National Natural Science Foundation of China (Grant No. 31902426), Shanghai Sailing Program (19 YF1419800), and a Special project for the exploitation and utilization of Antarctic biological resources of Ministry of Agriculture and Rural Affairs (D-8002-18-0097).  
This manuscript does not contain any studies with human participants or animals performed by any of the authors.  
Data Availability StatementData will be available upon request by the corresponding author [PERSON].  
The authors declare no conflict of interest.