Estimating selectivity of experimental diamond (T0) and turned mesh (T90) codends in multi-species Mediterranean bottom trawl

This paper evaluates the effect of changing from a diamond mesh codend (T0) to a 90° turned mesh codend (T90) on thesize selectivity of seven commercially important species in the Mediterranean bottom trawl fishery. In sea trials conducted in thenorth-western Adriatic, two experimental codends made of 54 mm nominal mesh size netting and differing only in mesh configurationwere alternately mounted on the same trawl. Overall, the T90 mesh significantly improved codend size selection for all thespecies analysed. The difference in the predicted average L50 values between the T90 and T0 codend was particularly marked inEuropean hake (Merluccius merluccius, 21.26 vs 11.26 cm total length), common squid (Loligo vulgaris, 12.06 vs 7.88 cm mantlelength) and mantis shrimp (Squilla mantis, 20.78 vs 13.35 mm carapace length). Both codends showed an excessive size selectivity,which involves a commercial loss, especially for red mullet (Mullus barbatus), Mediterranean horse mackerel (Trachurus mediterraneus)and whiting (Merlangius merlangus). These findings demonstrate the efficiency of the T90 configuration in excludingundersized specimens, especially of hake, whose average L50 was above the minimum conservation reference size of 20 cm. Theadoption of this practical and inexpensive solution can contribute to improve the management of the demersal resources targetedby the Mediterranean bottom trawl fishery.