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

Abstract

Commercial fishing trials with 40mm diamond (DM40) and square (SM40) mesh codends made of 5mm 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 (L50) than the DM40. For hake the L50 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 L50 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 40mm diamond mesh codend to a 40mm square mesh codend. Our analysis also suggests that by using a 40mm square mesh codend the short-term economic losses of commercial species, compared to the losses of 40mm 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.