Size selectivity of flatfish in trawl codends

Flatfish constitute a substantial proportion of the catch in several demersal trawl fisheries across the globe. Therefore, knowledge on how to discriminate between the individuals that are to be captured or released, by size, is important for the sustainability of exploited stocks. Using European plaice (Pleuronectes platessa), flounder (Platichthys flesus), and dab (Limanda limanda) as case-study species, here we investigate how flatfish are selected in trawl codends by experimentally testing the influence of mesh geometry, and its variability, on size selection. Both diamond-mesh and square-mesh codends were tested, as well as three codends where the mesh shape was fixed to minimize its variation during fishing. The most discriminating size selectivity was found with fixed mesh geometry, revealing that variability in mesh openness negatively affects the selectivity of flatfish. Our results further demonstrate that the risk of retaining undersized flatfish tends to increase with increasing mesh opening angle in diamond-mesh codends. Our results also confirm that when fishing with codends of the same nominal mesh size, the square-mesh codend retains significantly higher proportions of undersized flatfish than the traditional diamond-mesh.