The influence of twine thickness on the size selectivity of polyamide codends in a Mediterranean bottom trawl

We investigated the effect of twine thickness on the codend selectivity of Mediterranean bottom trawls. Two codends having meshes with the same opening (ca. 44 mm) but different twine thicknesses (Light PA: R3644tex, O 2.38 mm and Heavy PA: R5312tex, O 2.89 mm) were fished daily and alternately on the same trawl. Selectivity was measured using the covered codend technique, the cover being supported by circular hoops. The results were analysed taking into account the between-haul variation in selectivity. They demonstrated a substantial reduction in selectivity with increased twine size. For all the species considered, the lengths at 50% retention (L50) indicate that an increase in twine thickness from 2.38 turn (Rtex 3644) to 2.89 turn (Rtex 5312) reduces selectivity by 20-31%. Therefore, the twine thickness of the codend netting played an important role in selectivity. It is thus essential to include this factor in EU legislation aimed at improving codend selectivity in Mediterranean bottom trawl fisheries, where the recent trend towards using thicker twines threatens to cancel the benefit of the effect of mesh size increases or that of the square-mesh codend, allowing the present high rate of discarding small demersal fish to continue.