Improvement of trawl selectivity in the NW Mediterranean demersal fishery by using a 40 mm square mesh codend

Abstract

Commercial fishing trials with 40 mm diamond (DM40) and square (SM40) mesh codends made of 5 mm diameter knotted PE-netting were conducted in July 2005 on the continental shelf (∼100 m) and upper slope (∼400 m) of the Catalan Sea (NW Mediterranean) to assess the size selectivity of European hake ( Merluccius merluccius), Norway lobster ( Nephrops norvegicus), poor cod ( Trisopterus minutus) and greater forkbeard ( Phycis blennoides) in the demersal multi-species trawl fishery. In total, 28 tows were done using the standard covered codend method. For all four species, the SM40 showed a significantly higher mean selection length ( L 50) than the DM40. For hake the L 50 was 16.0 cm in SM40 and 10.1 cm in DM40; the corresponding figures were 13.0 cm versus 9.2 cm for poor cod and 14.9 cm versus 9.8 cm for greater forkbeard. For Norway lobster, DM40 did not show any size-selectivity whereas SM40 produced a L 50 of 22 mm. The selection range (SR) between DM40 and SM40 was not significantly ( p > 0.05) different for any of the three fish species. Our trials indicate that substantial improvement in size-selectivity for these commercially important species is achieved by switching from the conventional 40 mm diamond mesh codend to a 40 mm square mesh codend. Our analysis also suggests that by using a 40 mm square mesh codend the short-term economic losses of commercial species, compared to the losses of 40 mm diamond mesh codend, will be low in the slope fishery (less than ∼5% of the total catch value), whereas in the continental shelf losses could be up to ∼30% of the total catch value.