Selectivity experiments in the NE Mediterranean: the effect of trawl codend mesh size on species diversity and discards

Abstract

In the present study we compare the overall performance of the 14 mm (from knot-to-knot) diamond-shaped codend (14D), presently used in Hellenic waters, with those of 20 mm square-shaped (20S) and 20 mm diamond-shaped (20D) trawl codends. The experiments took place at 12 stations in the western Aegean Sea and were based on the covered codend method (cover codend: 10 mm from knot-to-knot, 10D). The results of applying multivariate techniques, to the different station/codend combinations suggest that they form groups which differ from each other in terms of species composition, species diversity, and catch numbers/weights per hour of fishing. In general, the 10D covers of 20D and 20S were separated from the 10D covers of 14D. The 20D and 20S codends were generally grouped together and separated from those of 14D and 10D. Finally, the 14D codend either form groups by themselves or group together with the 10D cover codends. In addition, the results indicated that the 20D and 20S codends, when compared with 14D, allow significantly more individuals and number of species to escape through the meshes and are characterised by considerably higher commerical/non-commercial ratios in terms of both weight and number. In absolute numbers and weights 14D caught a very large quantity of non-commercial species whereas the weight of the commercial species caught was among the lowest. The commercial/non-commercial ratios of 20D were higher than those of 20S and, in absolute numbers and weights, 20S caught relatively more non-commercial species than 20D. These facts clearly indicate that 14D is harmful in terms of species diversity for the Hellenic demersal stocks and hence its use should be prohibited.