Effect of mesh size and codend circumference on selectivity in the Mediterranean demersal trawl fisheries

The effects of mesh size and codend circumference on selectivity of some commercially important species were evaluated. Four different codends were used in the sea trials. The nominal mesh sizes tested were 48 mm and 56 mm and two different nominal circumferences were associated with each mesh size. Selectivity experiments were carried out in the Central Adriatic Sea, in the course of two different periods of the year (May and September 2005) on two different fishing grounds normally exploited by local fishermen. Reliable selectivity results were obtained for red mullet (Mullus barbatus), little squid (Alloteuthis media) and European hake (Merluccius merluccius). For all the species, the codend circumference plays a role which is as important as mesh size. The positive effect of increasing mesh size, from 48 mm to 56 mm, could be made fruitless by an increase in codend circumference. According to species, the change only in the mesh size from 48 mm to 56 mm significantly increases L50 by +15-42%, while by increasing only the codend circumference of around 13-17%, L50 decreases by 9-41%, thus being effective as by increasing mesh size. As a consequence the selection factor (i.e. SF is the ratio of L50 and the codend mesh size) depends on the codend circumference and does not result mesh-size-independent. We confirm that it is not sufficient to regulate selectivity in multispecies fisheries just by setting a minimum mesh size and that there is a need in mixed fisheries to improve selectivity by using additionally mechanisms. Possible solution would be to design policies and management programmes that seek to minimise discards, which include active rather than static management, obligations to land all catches, and no MLS and reduced benefits from landings of juvenile fish.