Size selectivity of diamond and square mesh codends for four commercial Mediterranean fish species

Abstract

In the present study, we compared the size selectivity of the 14 mm (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) codends for four commercially important demersal species (Merluccius merluccius, Micromesistius poutassou, Trisopterus minutus capelanus and Lepidorhombus boscii ). Codend selectivity was estimated using the covered codend method. Our results suggested that 14D was not selective for any of the target species and the proportion retained by 14D ranged between 0.88 and 1. In addition, only in the case of M. merluccius 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 L₅₀values was not possible, because of the small number of specimens retained by the cover codend of 14D. The L₅₀value of M. merluccius for 14D was lower than its currently enforced minimum landing size (MLS) and length at 50% maturity. In contrast, for 20S and/or 20D codends, even though the estimated L₅₀values were generally close to, or higher than, the species' MLS, the L₅₀value of 20S for M. merluccius and that of 20D for L. boscii, were much smaller than their lengths at 50% maturity, a fact indicating that even the use of 20S and 20D, respectively, provides small opportunities for reproduction. Hence, the appropriate mesh sizes for these two species must be greater than 20 mm and their MLS must be raised. In contrast, for T. m. capelanus and M. poutassou, the L₅₀values of 20D (but not of 14D) were close to their lengths at 50% maturity, a fact indicating that 20D is appropriate for these two species.

The above mentioned facts clearly showed that: (a) 14D is harmful, in terms of size of landed fish, for the Hellenic demersal stocks and hence its use should be discontinued and (b) the use of a single appropriate mesh size for the Mediterranean trawl fishery as a whole is impossible because of its multispecies nature and the different body sizes of species involved. Since the majority of the individuals escaping through the meshes of 20D or 20S are immature and under-sized (i.e. their length is smaller than MLS), the replacement of 14D by either 20D or 20S, the former being the codend mesh size proposed by EU for the entire Mediterranean, will be accompanied by increased yield per recruit.