Codend and whole trawl selectivity of a shrimp beam trawl used in the North Sea

In preparation of a by-catch reduction program for the Belgian brown shrimp fishery, a study was carried out on the codend and whole trawl selectivity of the beam trawl widely used in this fishery. The codend cover technique was used to determine the codend selectivity, whereas the whole trawl selectivity was estimated using small mesh pockets attached to several positions on the body of the net. Codend selectivity for shrimp proved to be very variable and was significantly influenced by clogging of the meshes, catch volume and state of the sea. The L50 for shrimp, for all hauls combined, was 39.4 mm and the selection factor 1.82 for a mesh size of 21.7 mm. The selection range was 11.6 mm. The selectivity of the net body was quite important and allowed more shrimps to escape than the codend did. It was mainly the rounded lateral part of the belly that contributed to this selectivity. Due to the small mesh size, the codend selectivity for flatfish was very poor. The selectivity of the net body for flatfish was negligible