Diamond and Square-Mesh Codends Selectivity in Multi-Species Mediterranean Bottom Trawl Fisheries

Mediterranean demersal trawl fisheries traditionally operate using small diamondshape meshes in the codend, which tend to retain almost all animals. We investigated the effect of mesh configuration (diamond and square) on the size selectivity of nine commercial species commonly captured in the Mediterranean demersal trawls. Two codends having meshes with the same opening (ca. 39 mm, stretched) but different mesh configuration (DM38: diamond-mesh and SM38: square-mesh configuration) 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. Two other important external variables were identified: the trawling depth and the codend catch, which for some species influenced between-haul variation, but there was no evidence of a coherent effect on selectivity parameters. Current results demonstrated a substantial improvement in selectivity with squaremesh. The selectivity of 40mm diamond-mesh codend has been reported to be rather poor because a large proportion of the codend catch is composed by juveniles (i.e. virginal and early maturing) and smaller than the minimum landing size (MLS) or first maturity size. With the exception of the flatfish (Arnoglossus laterna), the effect of a change of mesh configuration from diamond- to square-mesh on size selectivity positively affected the retention length at 50% (L50). Square mesh codend appeared to protect individuals under the MLS of Nephrops norvegicus and Mullus barbatus even if for some other species such as Merluccius merluccius, the increase in L50 with square-mesh would not avoid the catch of juveniles.