Anàlisi dels descartaments efectuats per la flota d'arrossegament en el Golf de Lleó

Resumen

Multispecies Mediterranean trawl fisheries are carried out in a wide range of depths and affect different bottoms and communities. The demersal resources distributed in the trawl fishing grounds off the south-western Gulf of Lions were studied. The aim of this study is to evaluate qualitatively and quantitatively the amount of discards associated to demersal trawl fisheries in this area and to improve the selectivity of bottom trawl nets introducing square mesh codends. A weekly sampling was conducted with a chartered commercial trawler (length 23 m; 600 HP) between march 1998 to march 2000. All the samples took place during daylight at depths ranging between 50 and 500 m. A total of 68 hauls were analyzed and 307 species were identified. The main taxonomic group are fishes with 133 species followed by crustaceans, cephalopods, echinoderm and other invertebrates. Leptometra phalangium is the most abundant species in number of individuals and Sardina pilchardus and Merluccius merluccius smiridus are the most abundant in biomass. From the total catch discards represented 71% in number of individuals and 36% in biomass. The main reason to discard is the compliance with current legislation in relation to minimum landing size and TACs. Seasonal discards were important in number of individuals during spring due to recruitment of some commercial species. In summer, large amounts of Sardina pilchardus were discarded due to legal regulations that established TACs to small pelagic species. Discards decreased with depth. We obtain majors discards in the continental shelf and less in the slope. We realized that square mesh design in codends is better than diamond mesh design. The percentage of individuals and biomass escaped increase in square mesh codends and the fraction discarded decrease. The estimated fifty percent retention length (L50) and selection factors were higher for square mesh codends than diamond mesh codends in the most cases except for Lepidorhombus boscii. Also the L50 obtained with square mesh codends are very close to or coincide with the legal size for the majority of the species studied. This results may be explained by the different body shapes and behaviour of the individuals species. We have observed that square mesh codends contribute to reduce the impact of fisheries in immature and undersized individuals from the stock, especially in species with a round shape body like Merluccius merluccius smiridus and Scomber scombrus. Moreover, the square mesh codends assured that landings are made up of marketable lengths.