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

Published source details Sala A. & Lucchetti A. (2011) Effect of mesh size and codend circumference on selectivity in the Mediterranean demersal trawl fisheries. Fisheries Research, 110, 252-258.
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This study is summarised as evidence for the following.

	Action	Category
	Decrease the circumference or diameter of the codend of a trawl net Action Link

Decrease the circumference or diameter of the codend of a trawl net

A replicated, controlled study in 2005 in two areas of seabed in the Adriatic Sea off Italy (Sala & Lucchetti 2011, same experimental set-up as Sala & Lucchetti 2010) found that diamond mesh bottom trawls with a smaller circumference codend had improved size-selectivity for European hake Merluccius merluccius and red mullet Mullus barbatus compared to a larger codend circumference, across two codend mesh sizes. For 56 mm mesh codends, the length at which fish had a 50% chance of escape was greater with a small (240 mesh) circumference codend than a larger (280 mesh) circumference for hake (small: 16.3 cm, large: 12.0 cm) and mullet (small: 12.8 cm, large: 10 cm). Similarly, for a 48 mm codend mesh size, the 50% escape length was greater with a small (280 mesh) circumference compared to a large (326 mesh) circumference (hake, small: 11.5 cm, large: 10.4 cm; mullet, small: 10.7 cm, large: 7.5 cm). In addition, an increase in mesh size alone resulted in an increase in size selectivity for both species. Gear trials (68 trawl deployments of 1 h duration) were carried out by research vessel in May and September 2005 on two fishing grounds (one 15–20 m depth, one 180–200 m depth) in the Central Adriatic. Four codends were tested combining small (240 and 280 meshes) or large (280 and 326 meshes) circumferences (or codend rigging ratio) with large (56 mm nominal) or small (48 mm nominal) mesh sizes (see original paper for full gear specifications). Small mesh (20 mm) covers over each codend collected catch escaping through the meshes. All catch was weighed by species, and fish lengths recorded.

(Summarised by: Chris Barrett)