Understanding and predicting codend size selection for flatfish species

In several demersal trawl fisheries flatfish constitute a large part of the catch, but often a significant fraction of these flatfish are unwanted species or sizes. To design trawls that are capable of addressing this problem, it is necessary to understand and quantify the size selection of flatfish species in trawls. Therefore, based on laboratory experiments, we tested the ability of different sizes of three flatfish species, plaice (Pleuronectes platessa), flounder (Platichthys flesus) and dab (Limanda limanda) to physically pass through diamond meshes of different sizes and opening angles. By combining these results with results from fishing trials, we were able to determine that the angle at which fish contact the meshes during an escape attempt plays a major role in the size selection of flatfish species. Contrarily, wiggling movement and the ability to squeeze through the mesh during escape attempts have a very limited effect. On the basis of these insights, we produced guidelines for the sizes and shapes of meshes in trawl codends to match the desired harvesting pattern of the three flatfish species investigated.