Untitled - CNR

Fishery and Sea Resourcesbatus) or other species. Currently, Mediterraneanstocks are mainly managed and conservedby regulations by defining closed areasand seasons, Minimum Landing Sizes(MLS) and Minimum Mesh Sizes (MMS).Nevertheless it is recognised that in multispeciesfisheries a MMS in the codend appropriatefor one species could be unsuitablefor many others [1].Traditionally the Mediterranean bottomtrawl codend is made of small diamondmeshes (in some cases minimum meshopening less than 40 mm stretched) resultingin a very poor selective fishinggear [2, 3]. At the moment, in Italy theMMS of trawl codends is 40 mm stretchedmesh size, resulting in the capture of largeamounts of fish below the MLS. Improvingtrawl net selectivity is therefore of primeimportance. The use of square-mesh codendshas been specified as a conservationrequirement in the ICES regions [4].Relatively little scientific work, however,has been done to assess the selectivity ofsquare-mesh codend in the highly variablemulti-species conditions prevailing in theMediterranean demersal trawl fisheries [1].In Mediterranean, some works have assessedthe codend selectivity for someimportant species in demersal trawl fisheries;however, selectivity parameters havebeen traditionally estimated for pooled datawithin all hauls, while few studies used thegeneral framework, introduced by Fryer in1991 [5], for modelling codend selectivitydata in which between-haul variation ismodelled by allowing the selectivity curvesto vary randomly between hauls about amean selectivity curve with a given probabilitydistribution.The main goal of current study was undertakento analyse the effect, accordingto [5], of inserting a 40 mm square-meshcodend in a commercial Italian demersaltrawl, on selectivity of some commerciallyimportant fish species and some specieswith unknown selectivity. In fact, in orderto avoid further increases in mortalityrates for juveniles and to substantially reducethe amount of discards of dead marineorganisms by fishing vessels, CouncilRegulation (EC) No 1967/2006 [6], concerningmanagement measures for the sustainableexploitation of fishery resources inthe Mediterranean establishes that it is appropriateto provide for increases in meshsizes for trawl nets used for fishing for certainspecies of marine organisms and forthe mandatory use of square-meshed netting.2 Materials and methods2.1 Research vessel and trawlgearThe gear employed in the sea trials was atypical Italian commercial trawl as used inthe Central Adriatic, entirely made up ofknotless polyamide (PA) netting that wasapproximately 58 m long from the wingtips to the codend, with 600 meshes inthe top panel at the footrope level. It wasequipped with a 60.3 m footrope comprisinga 38 mm combined rope weighted with80 kg of leads over its whole length (Figure1). All rigging components of thegear were identical with those commonlyadopted in commercial practice in CentralAdriatic demersal trawl fisheries.Two codends were made using the samenetting, having meshes with ca. 40 mmnominal mesh size (ms), but in one codendthe netting was rigged in square-mesh configuration.In the current work, we used a diamondcodendwith 280 meshes (nm - number of1858