Fixed mesh shape reduces variability in codend size selection

Diamond-mesh codends are the most commonly used in demersal trawls. However, mesh geometry tends to vary in these codends during fishing, which leads to a less well-defined size selection process. This leaves one questioning the rationality of regulating exploitation patterns based on mesh size when size selection and (or) variation between hauls is highly variable. While it has been speculated and theoretically investigated how much the variability in mesh geometry may contribute to the variability in size selection, it remained to be quantified experimentally. Therefore, we conducted field test comparing the size selectivity of a simple diamond-mesh codend, where meshes are subjected to variation in geometry, with a rigid diamond-mesh codend, where the geometry of the meshes were kept constant. For Atlantic cod (Gadus morhua), the simple diamond-mesh codend was found to have 45% more variation in size selection than the codend with fixed mesh geometry. This confirms theoretical predictions and may guide research toward codend designs with more well-defined size selection properties.

Automated summary

This study found that the commonly used diamond-mesh codends in demersal trawls tend to have varying mesh geometry during fishing, resulting in an unclear size selection process. By comparing experimental tests, it was discovered that rigid codends with constant mesh geometry have better stability in size selection for Atlantic cod. These findings can guide the design of more suitable codends in future research.