SIZE-SELECTIVITY OF DIAMOND AND SQUARE MESH CODENDS FOR 4 COMMERCIAL MEDITERRANEAN FISH SPECIES

In the present study, we compared the size selectivity of the 14 mm (k not-to-knot) diamond-shaped codend (14D), presently used in Hellenic w aters, with those of 20 mm square-shaped (20S) and 20 mm diamond-shape d (20D) codends for four commercially important demersal species (Merl uccius merluccius, Micromesistius poutassou, Trisopterus minutus capel anus and Lepidorhombus boscii). Codend selectivity was estimated using the covered codend method. Our results suggested that 14D was not sel ective for any of the target species and the proportion retained by 14 D ranged between 0.88 and 1. In addition, only in the case of M. merlu ccius did the results suggest that 20S is significantly more selective and retains fewer under-sized fish than 20D. With the exception of M. merluccius, the direct estimation of the 14D L(50) values was not pos sible, because of the small number of specimens retained by the cover codend of 14D. The L(50) value of M. merluccius for 14D was lower than its currently enforced minimum landing size (MLS) and length at 50% m aturity. In contrast, for 20S and/or 20D codends, even though the esti mated L(50) values were generally close to, or higher than, the specie s' MLS, the L(50) value of 20S for M. merluccius and that of 20D for L . boscii. were much smaller than their lengths al 50% maturity, a fact indicating that even the use of 20S and 20D, respectively, provides s mall opportunities for reproduction. Hence, the appropriate mesh sizes for these two species must be greater than 20 mm and their MLS must b e raised. In contrast, for T. m. capelanus and M. poutassou, the L(50) values of 20D (but not of 14D) were close to their lengths at 50% mat urity, a fact indicating that 20D is appropriate for these two species . The above mentioned facts clearly showed that: (a) 14D is harmful, i n terms of size of landed fish, for the Hellenic demersal stocks and h ence its use should be discontinued and (b) the use of a single approp riate mesh size for the Mediterranean trawl fishery as a whole is impo ssible because of its multispecies nature and the different body sizes of species involved. Since the majority of the individuals escaping t hrough the meshes of 20D or 20S are immature and under-sized (i.e. the ir length is smaller than MLS), the replacement of 14D by either 20D o r 20S, the former being the codend mesh size proposed by EU for the en tire Mediterranean, will be accompanied by increased yield per recruit . (C) 1997 International Council for the Exploration of the Sea.